New GMT400 Owner - Build/Restore Updates & Questions

[Smafugula]

Hey Everyone,

I have recently acquired a 1990 C1500 5.7l GMC truck. It has been neglected for awhile but does run and drive. I have learned that these trucks are hard to kill and this one is a good example of that. I am in the process of cleaning it up and getting it to run/function as it should.

Have a couple questions:

1. Do these trucks have issues with ECM's getting messed up internally? Particularly with capacitors or weak points internally. If so, who is a good resource to send off for rebuilding? I've learned from Ford's I've worked on the ECM can be a hidden MFer that gets overlooked.

2. How are codes read for these OBD1 trucks? I have a Ford code reader and am used to running those types of basic codes from the 30 buck reader, is it the same kinda process for these trucks/

3. For the torque converter connector on the side of the transmission of a 700r4, are there any wire color matching I need to worry about when replacing the connector? The one on it now is completely exposed, all the wires are bare copper and touching... new one in the mail.

4. Truck runs but has some erratic idling, up and down, not even going to ask you all what that could be as it could be a million different things, best bet is to run codes right?

5. I guess lastly, anything I should take a look at or replace in regards to the engine? I am doing the basics, more concerned about weak points in the speed density sensor system and what those would be.

Thats all for now. I'm sure I will have a few more, great forum you all have here. Look forward to y'alls responses! I'll post some pics up of the rig at some point.

 

[toolmakerson]

welcome from nj.

i'm no expert, but i'd say, regarding the questions:

1) some experts here might know of control module typical failures, etc.
2) a decent scan tool (someone else might know of specific threads here for recommendations for your year). i have a used MT25000 snap on i got off ebay, but i have a 97 (vortec) k1500.
3) shop manuals are here on a sticky thread; correct wiring i'd look for in shop manual
4) scan tool will show live data as it runs; shop manual has a starting point for normal values; scan tool of course also would show stored codes if any
5) manual also has diagnostic steps for specific customer complaints, might start there.

others here may have much more to say...

 

[fancyTBI]

Welcome!

1. Failures are not unheard of but I would say they aren’t common points of failure. As far as rebuilders, I don’t know of any nor have I heard of any. Most times people source one from a forum member, eBay, or get lucky at the yard.

2. OBD1 codes can be read with a Tech 1, Snap On MT-G2500, Solus, or a laptop with an ALDL cable and the appropriate software. There isn’t really a cheaper solution than the ALDL/computer method. Several here have done this. @Scooterwrench & @PlayingWithTBI come to mind.

3. Can’t help there. I’d position the connector like factory and splice in the new wires to be in the same spot as the old one.

4. Basic tune up will help with establishing a good baseline. Might need an IAC relearn, might need some vacuum lines repaired, might be the TBI gasket… I always rebuild my TBIs on a new purchase. They get gummed up and the throttle shaft can bind sometimes from sitting. It’s best to not mess with the idle adjustment screw. Live data will be your friend here, like you said. @Schurkey will likely chime in with his great “usual three four” things to check when something isn’t acting right.

5. I’d just go with a basic tune up using AC Delco, Standard Motor Products, or Delphi and then go from there. The O2 sensor is likely so worn out and lazy by now. I’d as that to the list of things to replace off the bat. The distributor has a magnet on the main shaft and they are known to crack. Clean and inspect yours (can see it with the cap off) and if it’s cracked or degrading, replace that.

Again, welcome. The TBIs are good engines. Not fast, hard to kill, easy to maintain. Please share some photos!

 

[Smafugula]

Welcome!

1. Failures are not unheard of but I would say they aren’t common points of failure. As far as rebuilders, I don’t know of any nor have I heard of any. Most times people source one from a forum member, eBay, or get lucky at the yard.

2. OBD1 codes can be read with a Tech 1, Snap On MT-G2500, Solus, or a laptop with an ALDL cable and the appropriate software. There isn’t really a cheaper solution than the ALDL/computer method. Several here have done this. @Scooterwrench & @PlayingWithTBI come to mind.

3. Can’t help there. I’d position the connector like factory and splice in the new wires to be in the same spot as the old one.

4. Basic tune up will help with establishing a good baseline. Might need an IAC relearn, might need some vacuum lines repaired, might be the TBI gasket… I always rebuild my TBIs on a new purchase. They get gummed up and the throttle shaft can bind sometimes from sitting. It’s best to not mess with the idle adjustment screw. Live data will be your friend here, like you said. @Schurkey will likely chime in with his great “usual three four” things to check when something isn’t acting right.

5. I’d just go with a basic tune up using AC Delco, Standard Motor Products, or Delphi and then go from there. The O2 sensor is likely so worn out and lazy by now. I’d as that to the list of things to replace off the bat. The distributor has a magnet on the main shaft and they are known to crack. Clean and inspect yours (can see it with the cap off) and if it’s cracked or degrading, replace that.

Again, welcome. The TBIs are good engines. Not fast, hard to kill, easy to maintain. Please share some photos!

Sweet, thanks for the response! I've actually got a smoke machine for finding vacuum leaks, I'll probably hook it up and see if I can find some leaks. I will look into also replacing the 02 sensor.

Is the magnet on the dizzy on the outside or inside? Just curious what I should be looking at. Would the crack be on the dizzy housing basically?

 

[fancyTBI]

Is the magnet on the dizzy on the outside or inside? Just curious what I should be looking at. Would the crack be on the dizzy housing basically?

The magnet is on the main shaft. It’s below the rotor inside a shield. It’s also riveted to a piece of metal that looks like a star. Here’s a great thread showing a crack. Seems like they crack by the rivets.

Pick up coil or ICM?

Hey, 1991 2500 5.7 had a a head crack, did the rebuild and fired. Had a bad FF and wouldn't stay running, replaced ... then it had a no start with fire at the coil but not at dizzy, replaced cap and rotor and now I'm getting a weak intermittent spark. My pick up coil is ohming at ~800+/-. I can...

www.gmt400.com

And here’s a photo of mine. They can get real disgusting with age. No doubt this was ready for replacement.

 

[Smafugula]

The magnet is on the main shaft. It’s below the rotor inside a shield. It’s also riveted to a piece of metal that looks like a star. Here’s a great thread showing a crack. Seems like they crack by the rivets.

Pick up coil or ICM?

Hey, 1991 2500 5.7 had a a head crack, did the rebuild and fired. Had a bad FF and wouldn't stay running, replaced ... then it had a no start with fire at the coil but not at dizzy, replaced cap and rotor and now I'm getting a weak intermittent spark. My pick up coil is ohming at ~800+/-. I can...

www.gmt400.com

And here’s a photo of mine. They can get real disgusting with age. No doubt this was ready for replacement.

Got it, thanks. Any brand preference on replacing distributor? I know Cardone is trash.

 

[fancyTBI]

Got it, thanks. Any brand preference on replacing distributor? I know Cardone is trash.

I can’t say for certain on brands. If your magnet is fine then you shouldn’t need to replace unless the teeth are excessively worn.

 

[Smafugula]

Learned today that not all bumpers are the same, my bracket doesn't fit the newer bumper, I think it came off a 95 or so. The holes don't line up on either side at all. Maybe I am missing something not sure. Tried taking the newer bumper bracket and mounting it on the truck, no dice. Ran out of sunlight so had to end it for the day. I guess maybe I could drill new holes if I line everything up right...

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[evilunclegrimace]

I can’t say for certain on brands. If your magnet is fine then you shouldn’t need to replace unless the teeth are excessively worn.

The teeth don't wear as they do not touch anything.

 

[fancyTBI]

The teeth don't wear as they do not touch anything.

I should’ve been more clear. The teeth on the distributor lower main shaft that are driven by the cam gear.