GM OBD1 Question

1995 Chevy 3500HD with 7.4 gas. It came here for exhaust work and a few other issues. Idling way high with a check engine light when I pulled it in so I scanned it and had 5 or 6 codes. I got rid of all but 2.

It still has a 32 EGR malfunction and 39 Torque converter clutch stuck off. I changed out egr to no avail. Also the tcc does actually lock up, so I'm not sure why the computer is unhappy. The thing that's weird is they both show up at the same time every time. Is it possible they are related somehow? Maybe share a common ground somewhere? Just for kicks I also changed out the brake light switch as it shuts off tcc when brakes applied. I found a list of codes online showing 39 as brake pedal position switch stuck off and this truck had issues with that switch in the past. Scanner is showing it cycling when brake pedal applied, but still have the codes.

I don't have a service manual as its not my truck and haven't been able to find anything online necessarily linking these 2 codes but my gut tells me its the same issue causing them both. Just wondering if anybody has any ideas?

I cant really spend much more time on it guessing or throwing parts at it, but I really hate seeing that check engine light. I thought maybe somebody here has spent some time working on something similar and have some idea where to look. If not, I guess I'll just have to call it done.

Thanks, Jeff

Have you tried removing and reinstalling all connectors to the computer?
Sometimes it is just a slightly dirty connection.

Code 32 doesn't always mean the valve is bad. Could be clogged passages on either the intake or the exhaust side. If scanner is bi-directional you can operate the egr valve and see if its actually working while engine is running. Also make sure the wiring to the valve is good.

On my 2000 GMC sierra the check engine light was on for several months. One day it would not start, and I found a slightly loose battery cable. Tightened it up and no more check engine light also. Sure surprised me .

I think this is the diagnostic flow chart for code 32 Its not hard to do.




If its not the correct one I googled gm code 32 diagnostic flow chart and clicked on images. The best I remember that one will work on a GM V8 TBI engine... You will need to jump the ALDL connector A to B.





Positive backpressure EGR valves



Negative backpressure EGR valves

https://www.aa1car.com/library/egr.htm

General rule of thumb. If multiple codes, clear and drive again. The 'hard' codes will come back. Start with the lowest number. Fix that one first and you might stop the domino effect.
Code 32 might just be the EGR Solenoid. That usually is just a vacuum gauge to check input/output.
Usually the way to force a solenoid to open is by power braking it. That tells the computer you are in gear and with a load.
It should make the solenoid open, just be sure to do this with a warm engine. The temp sensor might over ride the solenoid command by the ECM.
No, I don't know what I am doing.
Good Luck!

95 is still a TBI I believe. The book says to check for vacuum leaks first,the biggest problem that was common with the TB is where the two halfs bolt together. With the air filter off try to turn the throttle body side to side,if it moves take out (I think it was )4 bolts and replace the gasket. It will have a massive vacuum leak if the gasket is bad.

Had one do that. Took off throttle body and all the passages for egr in throttle body were plugged. Cleaned new gaskets ran great no codes.

Been a while since I played with TBI trucks so if I am off slightly don't shoot me.

TBI and high idle, throttle body gaskets are a common problem.

A sticky PCV valve or cracked/disconnected hose to vapor cannister will give you a high idle condition as well.

On a 95 the computer is not able to tell much about the EGR valve, all it does is wait until the engine is warm and when the driving and RPM parameters are met it switches the solenoid to control the vacuum to the valve, it then wants to see a change in what the oxygen sensors readings are and the corresponding fuel adjustments.

If it does not see a change it assumes there is an EGR fault.

A vacuum leak can prevent the valve from cycling the computer will not see a change so it reports a fault.

In addition when a large vacuum leak exists the computer will be trying to control the high RPM's by closing the IAC as much as it can in an attempt to stabilize the air fuel ratio, the readings it is getting from the sensors at this point are not representative of what parameters it is expecting to see and the list of codes grow and grow.

Thanks for the info guys. This gives me a few relatively easy things to look at. This truck actually has an electronic egr valve. That's why I changed it out. I guess i figured there wasn't much else to be at fault. Obviously, that's not the case.

I may just pull the throttle body off to change gasket and check egr passages. This truck had a backfire issue in the past that fried the injector harnesses on the throttle body, so entirely possible it compromised the gasket.

I would never have dreamed it would have that type on it but I have not seen many 7.4 in that truck... You will need a scan tool with by-directional controls...

I spec it best you stay with the work you are good at and leave the bones to someone else...

I think it's a matter of principle to
figure it out now.....

I messed with it a bit today before work.
Verified passages were open by starting
it with the egr valve off. Crude, but
effective . Throttle body tight to
manifold.

I found a troubleshooting guide and
determined no 12v to egr. Found an ecm
fuse one slot over from where it was
supposed to be. So my tcc code went
away, but egr is still there.

Didn't have any more time to mess with
it, but have a couple more ideas before I
admit defeat.

If it means that much to ya invest in some good information... If I am working on a tractor I am not familiar with I will buy a good repair manual and past the cost on to the customer...

Its called the cost of doing businesses...