96 GMC K2500 Suburban Restoration (with a couple of mods)

[South VA]

I enjoyed your post, and consider myself fully briefed & up to speed on your project.
If knowing that there are several of us watching over your shoulder gives you that
wee bit of extra juice to divide & conquer the problems on your project, PERFECT.

Speaking for myself, if I have an idea & never say it in public, there's a good chance it won't happen.
On the other hand, IF I say something in public and others hear it, then I have no choice but to make it so.

Anyway, be sure to download that NASA .pdf file, and treat that Suburban like it's a
college-level hands-on course on real-world High Reliability wiring harness fabrication.
(Man, think what the tuition would be for that? The mind boggles... :0)

I'm going to join the folks already following your project & will be patiently waiting for updates.

But no pressure. No rushing.

Remember, "Quality is the Constant, and Time is the Variable."

Good Luck --

That is one nice looking Cobra replica! My nephew is building one of these kits with his son, which is pretty cool.

I downloaded and perused the NASA pdf (well, not the WHOLE thing). Thanks for that! As I work through my wiring issues, it has already come in handy.

And I’m finding out that there is a whole lot more to making wiring connections than I had realized. Which is interesting, because my father was into electronics, and taught me how to splice and solder when I was quite young. At least the basics. So I thought I had a pretty good idea of how that sort of thing works, and really didn’t need to spend much time reading about something that I already know. Right?

Wrong.

So, as I alluded to a few posts ago, I’m trying to embrace a ‘beginner’s mind’ with regard to the subject, and have been reading and watching youtube videos. As a result both my crimped and my soldered joints have improved substantially. At least it seems so.

It may sound odd, but electrical stuff makes me nervous. I’ve been stranded (no pun intended) by wiring issues twice, and nearly a third time, in the past couple of months. I really dislike that, and my gf likes it even less, because she just wants the Suburban to do its job and get us and our camping trailer to wherever we’re going.

I’ve done some minor wrenching, including rebuilding a few engines as a lad, that actually started and ran afterwards. Replacing a timing belt in an old Audi, where you have to remove the front bumper, radiator, and so forth. I worked at several gas stations through and after high school. Nothing serious, but at least I am reasonably comfortable with tools, and have a decent set. But when it comes to electrical issues, particularly with this vehicle, I have a nagging concern that I’m just not going to fix it, that one of my connections won’t be quite good enough, or I’ll miss something obvious, and we’ll wind up being towed once again.

Which is why, after tightening that battery ground cable to the block yesterday, I was left with a bit of unease. It just didn’t seem quite right. So at 6:30 this morning I crawled under it just to make sure it was right.

That damned packrat did SO much damage under the hood that I’m in awe that this thing runs at all. When I bought it, I flew out to Arizona and blithely drove it back to Virginia, feeling pretty good about it because it had been ‘checked out’ by a shop, the belts and hoses checked, oil changed, and the A/C charged. Sheer dumb luck that I made it is one explanation. I wonder about the others.

Oh, and as for being checked out, during my crawling around under this beast, I discovered that the zerks haven’t been greased in quite awhile. Which is interesting, because it’s been to two shops since I drove it back: one time for an oil change while they were charging the A/C system; another when they changed the pitman arm. Both shops should have caught it. Nope.

Going forward, I’m back to doing my own wrenching to the extent possible. Which is a good thing, from a budgetary perspective. Paying to get work done, typically under duress, and then finding out that they missed a thing or two, doesn’t cut it. I’m too old for that ****, and no longer have the income to support it.

Did I mention that I really want a garage?

So to your point, it really helps knowing that you and several others are here on this excellent forum, keeping an eye over my shoulder. I have learned so much already, and enjoy reading what you and others have posted here and on your own projects. I joined gmt400 because it was recommended. I stay because it‘s way better than I expected, and keeps delivering. Thank you.

 

[South VA]

As our camping trip scheduled for the 7th (a week from tomorrow) looms ever closer, I decided to revisit my priorities.

It comes down to a matter of time/energy, and it seems I’m running out of both. If I could devote more time to this project, I’d probably possibly get it all done this week. But between my other projects and commitments, plus limited daylight work hours due to the heat and humidity, I cannot. So it goes. And, once again, I get to learn the lesson of being realistic in my expectations. Or maybe I’m just a wimp.

We could have postponed the trip, but finding and booking another decent campsite within the next few weeks isn’t realistic. Besides, I booked the trip for the 7th as a motivator, and it has worked. Just not the way I expected it to.

So we’re sticking with the plan, and I’m focused on what I need to do to make this Sub roadworthy - which is to run and tow reliably - and put aside, for now, those things that aren‘t mission-critical. The bits that are on pause are the sound system, backup camera, and the driver’s door hinge and weatherstripping.

I thought I had the wire routing for the camera sorted out, but after spending more time digging into it I’m still not sure how best to route it. I suspect I’m overthinking it. I also know from experience that the answer will come, but likely not in time for this trip. As for the sound system, routing the amp and sub wiring is also proving more of a challenge than I had anticipated. Again, I’ll figure it out, but not in time for this trip. The door hinges can wait too.

So I started reassembly last night, and continued early this morning. I cleaned and organized my tools. I’m feeling some relief, and actually enjoying the process more.

Of course everything takes longer than I think it will, especially when I’m the one doing it. One small example from early this morning is that I carefully put the steering wheel shrouds back on, and the upper half took me 5 or 6 tries to get that damned 6 mm torx socket drive screw in next to the ignition switch before I finally succeeded. The screw kept falling off the screwdriver bit and lodging in the ignition switch. A magnet on a stalk came to the rescue, but it ate up time.

Then I had to remove the shrouds so that I could put in the dash panel that I forgot to do first. Sigh.

However, apparently some learning had taken place, as this time I managed to get the 6mm screw back in on the first try. Using a headlamp helped.

Once the dash and associated stuff are back in place, I can reinstall the battery, hook up the new ground cables, install the newish air intake assembly, and see if it plays. If it does, I’ll spend the next few days driving it* to determine if it really is fixed. And then go camping.

This is not very exciting stuff, I realize, compared with some of the amazing build threads I’m following on this forum. But as an old guy that has not done much wrenching for decades, as my build progresses (however slowly) I’m enjoying it much more than I’d anticipated. As I’ve said before, I’m learning a lot, some of which I thought I already knew. That said, at the moment I’m excited at the prospect of simply driving this beast again!



* With my fingers crossed.

 

[HotWheelsBurban]

As our camping trip scheduled for the 7th (a week from tomorrow) looms ever closer, I decided to revisit my priorities.

It comes down to a matter of time/energy, and it seems I’m running out of both. If I could devote more time to this project, I’d probably possibly get it all done this week. But between my other projects and commitments, plus limited daylight work hours due to the heat and humidity, I cannot. So it goes. And, once again, I get to learn the lesson of being realistic in my expectations. Or maybe I’m just a wimp.

We could have postponed the trip, but finding and booking another decent campsite within the next few weeks isn’t realistic. Besides, I booked the trip for the 7th as a motivator, and it has worked. Just not the way I expected it to.

So we’re sticking with the plan, and I’m focused on what I need to do to make this Sub roadworthy - which is to run and tow reliably - and put aside, for now, those things that aren‘t mission-critical. The bits that are on pause are the sound system, backup camera, and the driver’s door hinge and weatherstripping.

I thought I had the wire routing for the camera sorted out, but after spending more time digging into it I’m still not sure how best to route it. I suspect I’m overthinking it. I also know from experience that the answer will come, but likely not in time for this trip. As for the sound system, routing the amp and sub wiring is also proving more of a challenge than I had anticipated. Again, I’ll figure it out, but not in time for this trip. The door hinges can wait too.

So I started reassembly last night, and continued early this morning. I cleaned and organized my tools. I’m feeling some relief, and actually enjoying the process more.

Of course everything takes longer than I think it will, especially when I’m the one doing it. One small example from early this morning is that I carefully put the steering wheel shrouds back on, and the upper half took me 5 or 6 tries to get that damned 6 mm torx socket drive screw in next to the ignition switch before I finally succeeded. The screw kept falling off the screwdriver bit and lodging in the ignition switch. A magnet on a stalk came to the rescue, but it ate up time.

Then I had to remove the shrouds so that I could put in the dash panel that I forgot to do first. Sigh.

However, apparently some learning had taken place, as this time I managed to get the 6mm screw back in on the first try. Using a headlamp helped.

Once the dash and associated stuff are back in place, I can reinstall the battery, hook up the new ground cables, install the newish air intake assembly, and see if it plays. If it does, I’ll spend the next few days driving it* to determine if it really is fixed. And then go camping.

This is not very exciting stuff, I realize, compared with some of the amazing build threads I’m following on this forum. But as an old guy that has not done much wrenching for decades, as my build progresses (however slowly) I’m enjoying it much more than I’d anticipated. As I’ve said before, I’m learning a lot, some of which I thought I already knew. That said, at the moment I’m excited at the prospect of simply driving this beast again!



* With my fingers crossed.

I had a similar experience in September and October 2018. My Burb had a bad caliper and brake hose, that collapsed internally and locked the brake up. Caught something on fire and melted the ABS sensor thingy on the backing plate.
So once it got towed to my shop building, I got it in the bay, put the fan on and got to disassemble everything. One side at a time anyway. I was replacing both calipers and brake hoses and installing a new set of disc pads,and the ABS sensor and backing plate.
Once I got it apart, I saw why we'd worn out the front tires on the edge: the upper ball joints were toast! So I gotta replace them too, another $100. And another week or so added to the job.....
I'm working on this truck while working 10-12 hours a day at work, so I'm only getting a couple days a week for wrenching. And the deadline is the middle of October, for a show in Lafayette, Louisiana that's already been booked.
So finally get the ball joints beat out of the spindle after unbolting them, and get the new ones in. All the parts went back together okay, but a lot of this is stuff I haven't done by myself before. And once everything is back together, time to bleed the brakes! I had a power bleeder from HF, bought on a mechanic friend's recommendation (he also works solo).
Got it back together and bled, we're going to take it into town to get an alignment and new tires. Going down the driveway and our short street, pedal hose to the floor..... finally on the fourth stop, it firms up. So we go on to the tire and alignment shop.....and wait all day!
Got everything handled, but it did come down to the wire. I've had to do rush work, to be able to go to work later that morning, and I hate to. Too easy to forget stuff that you need to do. I make checklists and that also helps stay on track and on task.
And the feeling of accomplishment, on getting it roadworthy...and the trip going great, was worth it!

 

[South VA]

I had a similar experience in September and October 2018. My Burb had a bad caliper and brake hose, that collapsed internally and locked the brake up. Caught something on fire and melted the ABS sensor thingy on the backing plate.
So once it got towed to my shop building, I got it in the bay, put the fan on and got to disassemble everything. One side at a time anyway. I was replacing both calipers and brake hoses and installing a new set of disc pads,and the ABS sensor and backing plate.
Once I got it apart, I saw why we'd worn out the front tires on the edge: the upper ball joints were toast! So I gotta replace them too, another $100. And another week or so added to the job.....
I'm working on this truck while working 10-12 hours a day at work, so I'm only getting a couple days a week for wrenching. And the deadline is the middle of October, for a show in Lafayette, Louisiana that's already been booked.
So finally get the ball joints beat out of the spindle after unbolting them, and get the new ones in. All the parts went back together okay, but a lot of this is stuff I haven't done by myself before. And once everything is back together, time to bleed the brakes! I had a power bleeder from HF, bought on a mechanic friend's recommendation (he also works solo).
Got it back together and bled, we're going to take it into town to get an alignment and new tires. Going down the driveway and our short street, pedal hose to the floor..... finally on the fourth stop, it firms up. So we go on to the tire and alignment shop.....and wait all day!
Got everything handled, but it did come down to the wire. I've had to do rush work, to be able to go to work later that morning, and I hate to. Too easy to forget stuff that you need to do. I make checklists and that also helps stay on track and on task.
And the feeling of accomplishment, on getting it roadworthy...and the trip going great, was worth it!

What a great story! I’ll bet you had a feeling of accomplishment!

Checklists are a great idea, and I’ve been using one for this project to remind me what needs done. However, I don’t have it to the level of telling me to do A before B, or C before A, etc. I’m thinking I need to up my checklist game.

I was reminded of this just now when I was looking in the engine compartment, searching for a missing roll of Tesa tape (found it) and checking yet again the rivnut ground lug on the body, and noted that I really should grease the Pitman arm, since the zerk will be really hard to get to from above once the air intake assembly is installed. That little item alone just saved me time and aggravation!

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Speaking of grounds: without the battery installed I checked the resistance between my battery to body ground cable (attached to the aforementioned rivnut) and the main battery ground cable, and came up with 0.2 ohms. The resistance between the main ground cable and a spot on the fender well where the paint came off from something like battery acid reads 0.2 ohms. Likewise between the alternator case and the main battery ground cable. The resistance between the crimped lug against the rivnut and the alternator housing is zero. It appears that
there’s 0.2 ohms of resistance between the main battery ground and the body. I’m wondering if that amount of resistance is within acceptable limits, or not. I’ll read up on it, of course, including @Road Trip ’s thoughtfully provided NASA spec. If you or someone else here has any insight on this, please feel free to share it.

Anyway, thanks again for your excellent post!

 

[Erik the Awful]

It comes down to a matter of time/energy, and it seems I’m running out of both. If I could devote more time to this project, I’d probably possibly get it all done this week. But between my other projects and commitments, plus limited daylight work hours due to the heat and humidity, I cannot. So it goes. And, once again, I get to learn the lesson of being realistic in my expectations. Or maybe I’m just a wimp.

I've been to the sandbox, and when it comes to wrenching in the heat, it's not a matter of "powering through". Do that and you'll end up in the hospital. Hydrate before, during, and after. Take your time and work at a steady, measured pace. Stay hydrated. If you're feeling wore out and your focus is going, throw the tools back in the garage, shut the door, and cool down. Hydrate some more. Take care of yourself.

 

[South VA]

I've been to the sandbox, and when it comes to wrenching in the heat, it's not a matter of "powering through". Do that and you'll end up in the hospital. Hydrate before, during, and after. Take your time and work at a steady, measured pace. Stay hydrated. If you're feeling wore out and your focus is going, throw the tools back in the garage, shut the door, and cool down. Hydrate some more. Take care of yourself.

Solid advice, thank you! I drank a fair amount of water early this morning, and do my best to remember to drink water throughout the day, even when I'm not thirsty. Still, at the moment I'm dripping wet, as the sun is full on the Burb, shining on the driver's side. Which is where all the final assembly took place, of course. But it's done now, and all I have left is to put away the tools and various parts, hook up the negative battery terminal, and start test driving. Woo Hoo!!

 

[South VA]

I'll admit, it was with some trepidation this morning that I started the Burb up (with finger firmly on the alarm disable button) and let it idle. But start it did, and apart from what my ear said was a less than a perfectly smooth idle, it ran fine.

I managed a short drive of 25 miles on rural roads. I was encouraged that the dash voltmeter stayed at or very close to 14v the whole time. The needle did not move noticeably when turning on the blinkers or emergency flashers, or applying the brakes. The transmission shifted smoothly, compared with its hard shifting on a number of other occasions. It didn't get stuck in gear. I was waiting for it to do something, but it didn't. Except run quite nicely. I started to feel optimistic. Not giddy, though. That may come later.

Thus emboldened, tomorrow I'll go for a hundred miles or so, with some of it on interstates. I don't know if its intermittent refusal to upshift an hour and a half into the last camping trip was correlated with temperature, time, vibration, or a transmission solenoid going bad. Maybe some combination. I still don't know. The best way I can think of to test it is to get the temperature up and keep it there for awhile, and see if the Burb misbehaves again. After that, we'll try it with the trailer in tow.

Btw, except for one of it being loaded onto a flatbed for the last time (hopefully), I haven't posted any pictures of the Burb for awhile. Here are two:

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These pictures are from a stop on my drive, which was at a house that I'm helping to renovate. I wanted to have a look at the riprap that was installed at the driveway culvert this morning. Hence the rocks.

Clearly there is some room for improvement (and fun to be had) with this installation, but for now it is good enough. I can't wait to get my hands on it when the weather cools down in late fall.

By the way, is anyone here into building dry-stack walls?

More to follow.

 

[Orpedcrow]

A successful test drive! Heck yea!

I have no interest in BUILDING a dry stack wall, but do enjoy watching other build them

I might be more inclined to build a Gabion (?) wall if we actually had rocks and stones naturally occurring in my part of Texas.

 

[South VA]

A successful test drive! Heck yea!

I have no interest in BUILDING a dry stack wall, but do enjoy watching other build them

I might be more inclined to build a Gabion (?) wall if we actually had rocks and stones naturally occurring in my part of Texas.

Thanks! Hopefully tomorrow's drive will be more of the same.

As for building dry stack walls, it's something that I've done a few times, which has given me more of an appreciation of some of the walls I've seen, particularly in some mining towns in Colorado. It can be really hard work. I admire people that do it well. I was curious if others here had done any of that.

I've occasionally toyed with the idea of having a half truckload delivered for building some low landscaping walls around the house and in the yard. You know, another project! Although it just might take a bit longer now than it did 15 or so years ago when I built the last one.

We don't have much in the way of rocks or stones on the surface here either. This riprap had to be quarried.

A Gabion wall... made with chain link fencing? That makes for a serious wall. Takes equipment though.

 

[Road Trip]

That is one nice looking Cobra replica! My nephew is building one of these kits with his son, which is pretty cool.

Thanks! Back in '00, the kit that the brothers running Factory Five offered just
seemed to have as much focus on the Go as the Show. And it's
held up despite 20+ seasons of enthusiastic usage. As for your
nephew & his son, they will be making memories in addition to the
car. For example, we had Paul's son install & torque the cylinder
heads to the engine. Now every time we finish a ride I'm always sure
to tell him, "Good Job torquing those heads -- they didn't blow
clean off the motor -- Again!" We always chuckle over that. :0)

Paul's son bringing up the torque in stages. Good focus, nice sense of accomplishment after.

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So, as I alluded to a few posts ago, I’m trying to embrace a ‘beginner’s mind’ with regard to the subject, and have been reading and watching youtube videos. As a result both my crimped and my soldered joints have improved substantially. At least it seems so.

It may sound odd, but electrical stuff makes me nervous. I’ve been stranded (no pun intended) by wiring issues twice, and nearly a third time, in the past couple of months. I really dislike that, and my gf likes it even less, because she just wants the Suburban to do its job and get us and our camping trailer to wherever we’re going.

Automotive electrical can seem to be a bit overwhelming, if for no
other reason than the sheer quantity of circuits & associated wiring.

One thing that may help is to divide & conquer. By that I mean that
you should divide all the wiring into, say 3 groups:

* Group (A) = showstopper circuits. (ie: The ones where your gf will
absolutely know if it were to fail. The circuits that will make you have
to walk home when it fails.) Wiring for the starter, engine support,
the electric fuel pump, etc.

* Group (B) = Nice to Have circuits. Losing the air conditioner or
fuel quantity gauge won't keep you from returning home, but it's nice to have.

* Group (c) = all the misc. stuff like a heated seat, one of the rear speakers
not playing, dome light doesn't work, etc. NOTE: You would be surprised
just how much of the wiring harness is for stuff like a remote starter
or other non-essential functions.

Saying it another way, if you figure out which circuits are the essential ones,
and then carefully inspect only those wires & correct anything that isn't up
to original specification, then you can truthfully say that you have a '99
Suburban shell with a 2023 wiring harness. Might sound crazy, but we're
only talking 5-10% of the entire harness. And IF you do this, when your
gf asks you is the 'burban going to make it without fail, you can answer
confidently that it will.

Keep one more thing in mind -- all the wiring harness that never moves
never really ages. 99% of all wiring harness trouble is either in the
connector and/or the first couple of inches closest to the connector. (!)

So in reality let's say that you identify 20 wires that will put you on the
side of the road. You only need to locate and clean/inspect/verify
each end of those 20 wires right in the vicinity of the connectors.
(Salvage title cars excluded -- you can find weird stuff like logically
unrelated yet physically adjacent wires shorted to each other because
somebody's drill went too deep and stabbed a harness while attempting
to repair collision damage...) And of course if the vehicle sat & critters
started chewing the wires then a full length harness visual inspection is
your only recourse.

I believe this approach works because for years we have been driving
down south and bringing back our next DD. (I'm getting too old to
fix any more rust buckets. :0) A couple of times we got a great
deal on a rust-free ride because the seller disclosed that the car had
electrical issues making the car unreliable. (Which kills the value for most
folks.)

Instead of waiting for the electrical gremlins to put us on the side of the
road while commuting, or going to the hospital, etc., we would instead work
methodically through the showstopper circuits & end up with a great
reliable DD for cheap. And once we trusted the car, then we could tinker with
the radio that quits playing when you hit the railroad tracks at our leisure.

One more thing about this approach. Troubleshooting wiring when it's
failed is super stressful, for you don't know if you are going to find
the problem in a reasonable amount of time.

On the other hand, if the vehicle is working, and you are taking what you
found in the wiring diagrams and tracing/inspecting that circuit in the
real world, it's 99% less stressful. If your 'burban was 2 years old this
might be in the crazy prepper category. But once a vehicle is 22+ years
old, it's just prudent if you are going to drive something further away
from the house than you are willing to walk. :0)

Speaking of grounds: without the battery installed I checked the resistance between my battery to body ground cable (attached to the aforementioned rivnut) and the main battery ground cable, and came up with 0.2 ohms. The resistance between the main ground cable and a spot on the fender well where the paint came off from something like battery acid reads 0.2 ohms. Likewise between the alternator case and the main battery ground cable. The resistance between the crimped lug against the rivnut and the alternator housing is zero. It appears that
there’s 0.2 ohms of resistance between the main battery ground and the body. I’m wondering if that amount of resistance is within acceptable limits, or not. I’ll read up on it, of course, including @Road Trip ’s thoughtfully provided NASA spec. If you or someone else here has any insight on this, please feel free to share it.

Anyway, thanks again for your excellent post!

With normal consumer-grade multimeters, the gross electrical malfunctions
are easy to find/fix. But when you get close to the limits of their resolution
(and it's getting hard to get a repeatable reading) ...then that's when electricity
seems like pure voodoo.

For example, if you need to tell if a sensor's resistance is either 40 ohms vs
400 ohms, no problem. But when you are asking this same meter to measure
0.0 ohms vs 0.2 ohms, you are at/near the limit of the meter's ability to
discern the difference repeatably.

Big picture? In a home we have 120/240 volts to work with. If the grid
sags a handful of volts you probably won't even notice the difference.

Conversely, in our vehicles we only have a grand total of ~12 volts to work with.
We can't afford to lose hardly *anything* without weird behavior
commencing.

And get this, all of our engine sensors communicate with the ECU with
less than half of battery voltage! Regulated 5.0 volt power is sent
out to the sensors, and then the sensors will vary the
voltage back to the computer.

A good example is the TPS (Throttle Position Sensor) where
(round numbers) ~0.5 volt represents a closed throttle, and
~5 volts is wide open throttle. When you only have a few volts
to work with, again we can afford to lose precious little before
wonky behavior sets in. (!)

This is why all the experts in this forum are always emphasizing
clean, conductive grounds in new condition IF you wish to have
a vehicle as reliable as it was when it was first built. (!) Same
goes for the positive side of the circuit -- whether it's hot to
the battery, starter, alternator, or power distribution box(es).

****

So when your affordable meter is asked to do more than it
can deliver, you the troubleshooter may start to believe what
others say about electricity being incomprehensible.

But working with electricity is just like working on mechanical
stuff. Trying to work on your car with cheap/too small hand tools is a
sure-fire recipe for frustration instead of a good fix.

Same goes for electricity/electronics test equipment - check
this out:

The first step up from a consumer grade multimeter is to invest
in a Fluke like the pros use. Check out the Tip at the bottom
of this screen snap, and see if it sounds familiar to you?

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(credit: Fluke.com (LINK)

But even if you spend $449 for a Fluke 87-V, the maximum
resolution for resistance is 0.1 ohms. (Link)

But there are dedicated meters to measure very small resistances,
like the high current connections on our vehicles. If you had to
rely on the measurements for a living, you would purchase
this Amprobe -- no problem resolving down to 0.1 milliohms.
(100 micro-ohms) This is 1000x the resolution of the gold
standard Fluke multimeter!

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For mere mortals / advanced hobbyists, this $120 milliohmeter might be
the best bang/buck: (Link)

The last item to look at is where Analog Devices shows you how to brew up
your very own milliohm meter using a breadboard and a handful of their chips.
(Very cool - link)

Believe it or not, once you spend some time in the neighborhood
of very low resistance measurements, you stop talking about
Ohms and instead talk about Mhos. (NOTE: I was taught Mhos,
but they have since changed the name -- nope, I find that Mhos
as backwards Ohms is a great mnemonic. (Link)

****

The only reason I know about this stuff is that all of our Avionics
black boxes had to be 'bonded' to the aircraft in order to share
a common ground plane -- otherwise, the resulting RFI
(Radio Frequency Interference) from poor grounding/bonding would
impact the noise floor of all the magic bits in functionally unrelated
boxes. (ie: The box searching for the tiny GPS satellite signal can't
do it's job because the leaky radar package is drowning it out.)

Bottom line, the aircraft would not perform to it's full potential.
Worse, attempting to unwind undesirable interactions like this
could be counterintuitive & stimulate the use of a large bore
parts cannon, adding unknowns to the mix & making a
further mess of the situation.

I have fixed many hangar queens by careful inspection of wiring
harnesses at the connectors, pin retention tests of the connectors
themselves, and then the truly weird stuff with re-bonding all my
finicky black boxes. And there's a parallel here, in that we were
flying 20-25 year old military fighters as if they were new, and I
think that they age in dog years due to the 9G stresses.

Given all this, asking an older GMT400's electrical subsystem
to work reliably is not out of the question. And it isn't that
hard. If you can get a 0.0 ohm reading out of your meter
on a high current path (like you did) then 0.2 ohm is not your friend.

****

I sincerely apologize for the length, but after all this if I ask you
to get as close to 0.0 ohms as you can on any high current
connection in our limited 12-volt world, I am serious about that...
for I don't want your gf to want you to get rid of your GMT400 and
get into something newer. (Heck, plenty of newer stuff breaks
too! :0)

Remember, keep the mission-critical circuits in new condition &
you will gain the reliability AND lose the fear of the aging
wiring harness.

Enjoy your upcoming adventure --