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

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South VA

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A low (or otherwise anomalous) TPS voltage signal to the ECM will impact transmission performance/function as the ECM relies on TPS data, among other sensor data flows, to control the transmission. If the TPS was replaced with a non-OEM part, there's always higher risk of it being DOA. If the new part is working correctly, then there's likely a short somewhere between TPS and ECM (including the ECM). I'm not super-informed as to how TPS actually work but my understanding is that they will send a variable voltage signal to the ECM based on throttle position. If the ECM thinks it's not seeing enough voltage at any given point it will store a DTC (usually P0122). Late, harsh or otherwise erratic shifting patterns are most commonly reported and this is when the transmission was previously functioning normally with no previous drivability symptoms.
Thanks Nick.

Given that the replacement TPS I used is an ACDelco part (Gold), let’s assume for the moment that it’s ok.

Add to that my guy’s recollection of the ‘96 being the first year for the VCM, which combined the ECU and TCU, and that same year having shorting issues within the VCM, I have to wonder, as Road Trip has (see below), if the VCM isn’t in fact the culprit.

It also seems highly suggestive that I’m getting ABS codes and lights while this is going on. This is a new occurrence, and roughly corresponds in time to the hard shifting. ABS is also controlled by the VCM.

So, the previous 5 paragraphs are written from the perspective of the Throttle Position Sensor
as being the perpetrator of the error codes being thrown. As a good troubleshooter, I need to
consider the alternate universe, where the TPS sensor is a victim of a flaky/intermittent +5V reference,
Ground, or the To/From wiring interconnecting the TPS to the engine ground and also the Black Box.
I agree. It seems to fit the observed facts. I’ll run this past my guy when I talk with him tomorrow, but it seems like as good a working hypothesis as any.

Because of its intermittency, the simplest way to test it, I suppose, would be to install a new VCM. They appear to be available and reasonably priced - around $150. That would require flashing the new VCM so that it will function with the Subdivision. I don’t know if my guy can do it, or if my scan tool is capable of doing it. He may have to refer me to a shop that can do it. Whatever. It will be a topic of conversation.

In any case, I’ll talk with my guy tomorrow and see what he thinks.

This discussion has really been helpful.
Thanks again!
 

South VA

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Suffice it to say that next week’s camping trip isn’t happening. Even if I could get the VCM swapped out this week, there wouldn’t be enough time left for test driving to ensure that it’s ready for prime time.

So it goes.

Nevertheless, for the first time in a long while, I feel that we’re on to something here. And look forward to pursuing the matter to its logical conclusion.

In the scheme of things, I’ll happily trade a missed camping trip or three for a Road Trip Ready Subdivision.
 

NickTransmissions

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Thanks Nick.

Given that the replacement TPS I used is an ACDelco part (Gold), let’s assume for the moment that it’s ok.

Add to that my guy’s recollection of the ‘96 being the first year for the VCM, which combined the ECU and TCU, and that same year having shorting issues within the VCM, I have to wonder, as Road Trip has (see below), if the VCM isn’t in fact the culprit.
You're welcome. It could be the VCM but I'd just be guessing from here. I would think that shop should be able to diagnose and rule in or out a bad VCM, TPS or ground/electrical problem(s) for sure before you spend any money, potentially unnecessarily. As for programming/flashing a new VCM, I believe that can be done w/a TECH2 but not 100%.

As far as testing the TPS, here's a decent article covering the process and procedures.
 

Orpedcrow

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I wonder if someone like blackbear can provide a (stock) pre-tuned vcm. (Maybe with a core charge?) They could also adjust transmission stuff if you wanted.
 

South VA

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You're welcome. It could be the VCM but I'd just be guessing from here. I would think that shop should be able to diagnose and rule in or out a bad VCM, TPS or ground/electrical problem(s) for sure before you spend any money, potentially unnecessarily. As for programming/flashing a new VCM, I believe that can be done w/a TECH2 but not 100%.

As far as testing the TPS, here's a decent article covering the process and procedures.
Thanks. That’s an informative article!

Yes, I’m thinking my guy is pretty savvy when it comes to these vehicles, and will have more information today. We’ll see.
 

South VA

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Trent, my transmission guy, calls again and tells me that it continues to shift normally. And other than a slight miss at idle, it seems to run quite well. No codes since he cleared those from the day before. Hmmm.

After listening to my hypothesis that the VCM is the culprit, he suggested that maybe there should first be a bit more troubleshooting done to truly narrow it down. This advice sounded oddly familiar (ahem: @NickTransmissions). It’s almost as if he had done this before.

So much for my enthusiasm over replacing the VCM as a primary diagnostic move.

He referred me to the very shop that had referred me to him. Apparently that shop has a guy, the owner’s son, that understands problems like I’m experiencing.

So I call the guy, whose name is Bubba, I kid you not, and he seems refreshingly able to listen.

After I finished my tale, he said ok, bring it in next Monday. Which also happens to be the former first day of our now-cancelled camping trip. Could this be auspicious timing?

Anyway, the appointment was sooner than I expected, which is good thing. He then became a bit more conversational, and by the time the call was over I believed, and my gut confirmed that the Subdivision is going to be in capable hands. Just as it was, and is, with Trent the transmission guy.

Edit: Bubba also said that he’d call Trent and hear from him what he encountered while he had it in his possession.

So we’re picking it up tomorrow. Then I’m to drive it as much as possible between now and Monday morning, so that it can generate and store as many codes as possible. Which in turn will aid him in his role in debugging this beast. My only concern is that the hard shifting could possibly damage the transmission. Is that realistic, @NickTransmissions, or am I being overly worried?

About that emotional roller coaster that @GrimsterGMC mentioned a few posts back: Now I’m excited.

Why? Because Bubba, according to Trent, understands and actively practices diagnosis and repair of the electrical and electronic systems of vehicles like mine. He’s been doing it for a number of years. And judging by his reviews online, his customers are overwhelmingly satisfied with the shop’s, and therefore his, work.

Given what we have been through, I’m more than pleased that someone like Bubba will be spending some quality time with the Subdivision. I can’t think of a better opportunity to figure out exactly what is (or what are) causing its issues.

So yes, I’m excited.
 
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South VA

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Yesterday I drove it home from Trent’s shop. Roughly 70 miles including a couple of errands and getting gas.

Speaking of gas ($3.15 at Costco!!), this last tank was 11.6 mpg, based on mixed driving. A couple of mpg better than I had been getting and therefore a pleasant surprise.

Back to the drive: The Subdivision performed nearly flawlessly.

In those 70 odd miles it didn’t have a single hiccup. The shifts were smooth as butter. I only say nearly because of the now-persistent ABS light and the less than perfect idle.

I wish I could say that I really enjoyed that drive, but I did not. I kept waiting for the other shoe to drop. I figured it would at some point.

And so, today after lunch, I reluctantly started it up, ready to head out, put it in gear, and knew immediately that we were back in it.

It shifted hard, and kind of bogged down when I eased on the gas. I made it about a hundred yards before turning around and parking it.

I hooked up the scan tool and, sure enough, a code appeared: P-1122-18 Throttle position circuit intermittent low voltage.

I called Bubba and told him. Based on the code and the fact that the TPS was recently changed, he suspects 1) the actual wiring to the VCM, or 2) the VCM itself. And said that a faulty TPS seems unlikely. He mentioned someone having had three TPSs installed by a dealership before they started looking at the VCM. Seems this set of symptoms is not unheard of.

He is also now aware of the previous inactivity of this Suburban and its resident pack rat a few years back. He’s used to such things; evidently folks in and around his area have problems with chipmunks chewing their car wiring harnesses. One hapless owner incurred $4700 worth of such damage to a new Camry. Which was a real eye-opener, and which somewhat tempered my excitement.

But I don’t really think we’re talking about that kind of carnage here. The electronics are far less complex than in newer vehicles, and I sense that that Camry was ravaged beyond what mine has endured.

Whatever. However small or subtle the problem turns out to be, it is enough to stop the Subdivision from doing its job. It has to be found and fixed. Sorted. And sorted it will be, because it’s going to the right shop.

And then we can get back to towing this camper, and actually camping in it. I don’t think that’s too much to ask.

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South VA

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Looking back at the previous post, it may sound like I’m bummed out. I’m not.

All things considered, the above is actually a good outcome.

Debugging has, I believe, sufficiently narrowed the scope of this search to where subsequent steps will be much more productive than past efforts. It seems that we’re finally closing in on what ails this beast.

And that’s a very good thing.
 

62barsoom

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South VA,

I know exactly the sensation that you just described. At first I thought that it was simply the feeling
that you were getting away with something for nothing, like when you are in the middle of
eating the free food samples at Costco?

But the feeling runs deeper than that. At the risk of paraphrasing Robert Pirsig**
poorly, our level of satisfaction with something is tied to how much we are a Participant
versus an Observer in it's creation.

Using a food analogy, you work at a job, you get paid, and then you take some of
that pay and buy some french fries. And enjoy them. The transaction that allowed
you to obtain those fries was a financial abstraction. Ergo, you are for the most part
an observer.

On the other hand, early in the season you pick out your favorite version of Tomato
seeds. And you start the process of growing those Tomatoes from seed. 60 to 100
days of tending to and anticipating the finished product. As the tomatoes ripen, you
are even reminded of your favorite Little Feat album cover from back in the day.
(Waiting for Columbus)

And then finally, you pick the first ones & slice them up. Whether in a salad or
on a burger fresh off the grill, you share them with your friends, and everyone
marvels at how much more amazing real tomatoes are compared to what we are
used to from the supermarket.

Both the french fries and the Tomatoes provides calories & sustenance. But
we enjoy those 'from scratch' Tomatoes way more than the fries. Is it because
of all the synapses firing from all the interaction with creating this food from
seed as a direct Participant? Is it the Sense of Accomplishment, or the chance
to share the bounty of your focus/effort with your friends, magician-like?
Or is it just better eating?

****

IMHO I think that driving old/something you fixed is kinda like that. So much of
modern life is just one abstraction piled on top of another. What does it mean
when someone goes to a restaurant, takes a picture of a meal before eating it
...and then posts that pic on FB for *you* to see?

Sure, the picture is a form of communication...but it pales in comparison with
sharing a slice of a real Tomato from your garden while breaking bread with
that same individual.

That's the best I can do. But I can report that the chore truck seems to be
spreading good vibes here in Salt City. Whether stopping to buy gas or
parking at the local Wegmans, people strike up conversations about it all the
time. Always fun. And I am actually surprised how many of them comment on the
color-matched wheels -- I guess they really stand out in a sea of same same
aging alloy wheels (slightly discolored by rusted brake dust) on today's daily drivers
here in the rust belt?

So, yeah, I think there's definitely a feeling about driving old/driving different.

Keep on keeping on -- you will eventually finish getting the Subdivision fully sorted...




**Author of a book about Quality: "Zen & the Art of Motorcycle Maintenance"
I was fortunate to see Lowell and crew at Lisner Auditorium in 77. What a show.
 

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