94 GPX GT: Multiple Codes, First Gear Only, No Reverse.

Howdy all,

A friend of mine couldn't pay his labor on his other car, so he handed me keys and title for a 94' Grand Prix GT Sedan. 3.8L, 4T60E. Car was driving for about three days without an issue and suddenly just downshifted to first gear and that's all she wrote. I thought maybe an electrical issue, as I've heard that if first works, reverse should also as a general rule... (I'm not a trans guy so I really don't know that much about it honestly, just basic knowledge gained from past experiences.)

Computer pukes out a mess of codes, they are as follows:
12 - TEST ACTIVE
26 - QUAD DRIVER MODULE A ERROR
39 - CLUTCH SWITCH CIRCUIT ERROR
53 - SYS VOLTAGE TOO HIGH
54 - FUEL PUMP CIRCUIT LOW VOLTAGE
67 - AC PRESSURE SENSOR CIRCUIT ERROR
65 - RIGHT BANK 0XYGEN SENSOR EXHAUST RICH INDICATED
69 - AC HIGH PRESSURE or AC CLUTCH RELAY FAIL

That's all I've been able to pull out of the computer. This car has had some wiring harness modifications, but they appear to have been done professionally, has had an aftermarket intake and fuel injection system installed, Spectre cold air intake, and Leece-Neville alternator. The car was used for a while as a stereo competition car, and has been driven very little in it's life (111k miles shows accurate according to the DMV.) :screwy:

Anywho, any help in diagnosing the transmission issue would be great, I'm going to go do some further research on these codes. I'm honestly not much of a trans guy, but hey, when you've got less than a hundred bucks into a car, why not use it as a learning experience? :iceslolan Worst case scenario, we'll find out how long it can run underwater. :iceslolan

Best Regards,

Bill S.
R.I.P. Eagle's "Last Chance Service"
Closing for good - 8/31/13

Unfortunately I believe a new transmission is in order. Mine dumped the transmission in the parking lot and first and reverse wouldn't work but very little. $2400 later had a newly rebuilt transmission.

Unfortunately I believe a new transmission is in order. Mine dumped the transmission in the parking lot and first and reverse wouldn't work but very little. $2400 later had a newly rebuilt transmission.

Ok, since my last post, I know more about the car, and the transmission....

First of all, the trans is good, I can actuate the solenoids by hand with switches and it will shift as necessary.

Second, this car was originally equipped with the DOHC 3.4.... NOW it's equipped with a 3.8 out of a Camaro. Thanks to the previous owner seeing it at my shop for providing that info.

He cannot remember who modified the wiring harness for him, but it's a trainwreck. He said the wiring was a problem from day one, there was always something having an electrical failure. The wiring wasn't run through tubing or conduit, nor was it properly lengthened in places like it should have been... (like to fit over/around the 260-amp Leece-Neville generator some stereo-head installed)... they just pulled it until everything was stretched tight.

I'm not sure if anyone has a clue what a wiring schematic would look like for this engine swap.... if anyone could give me any details, I'd greatly appreciate it.

Bill S.

Well, obviously, you have to get a schematic for for both engines to see what the differences are.....I personally would go to a junkyard, and take a complete wiring harness out of a GP with a 3.8 in it....they all go to a bulk connector at the firewall....

Another thing is the computer....does this have eraseable flash memory or a PROM? Chances are, they use the same number PCM(computer), but if the PROM was not replaced, or the computer was not programmed to the present engine, that will also cause problems....

Any time you have a harness that is tight, and does not take into account the engine flexing forward and backwards will always cause wiring problems eventually.....

Well, obviously, you have to get a schematic for for both engines to see what the differences are.....I personally would go to a junkyard, and take a complete wiring harness out of a GP with a 3.8 in it....they all go to a bulk connector at the firewall....

Another thing is the computer....does this have eraseable flash memory or a PROM? Chances are, they use the same number PCM(computer), but if the PROM was not replaced, or the computer was not programmed to the present engine, that will also cause problems....

Any time you have a harness that is tight, and does not take into account the engine flexing forward and backwards will always cause wiring problems eventually.....

I have ordered a detailed schematic of the wiring, should be in my e-mail in the morning. I was told by the engine-swap guru I managed to touch base with for a minute that this body of car (don't quote me on this, I'm not a Pontiac guy) never came with a 3.8 so this required an extensive wiring harness modification. He did say that it was the original computer in the car though. As I said, IDK, I'm not a Pontiac guru.

Yeah... the dummies butchered this harness bad. Electrical harness fabrication was one of the things I was good at in college, and seeing this kind of a mess just frustrates the OCD part of me. lol.

He is correct....this came with a 3.1 or 3.4(double overhead cam).....3.4 used in a GXP.....the 3.8 was not available for this year....not until 1997.....that computer is for a 1997 is OBDII, the computer in your car is OBDI....

So this creates a PCM problem, as far as programming goes....that PCM is programmed for a 3.4, not a 3.8.....

Probably need to talk to a speed shop that specializes in engine conversions, to see what needs to be done, computer wise....

Gear ratio of the tranny has to be matched to the engine in the PCM, as far as programming/correct PROM goes........

He is correct....this came with a 3.1 or 3.4(double overhead cam).....3.4 used in a GXP.....the 3.8 was not available for this year....not until 1997.....that computer is for a 1997 is OBDII, the computer in your car is OBDI....

So this creates a PCM problem, as far as programming goes....that PCM is programmed for a 3.4, not a 3.8.....

Probably need to talk to a speed shop that specializes in engine conversions, to see what needs to be done, computer wise....

Gear ratio of the tranny has to be matched to the engine in the PCM, as far as programming/correct PROM goes........

Thanks for the info guys. I'm going to try to find a speed shop near here, but I'm not a hotrodder so I'm not real familiar with the high-performance places around here.... I'll get looking. This is a project I see lots of posts about online, but not a lot of details, regrettably. Maybe, being a shop myself, I can get them to cough up some info without too much cost.

Best Regards, Bill S.

Anyone want to see pics of this crazy car? Grandprixforums.net has requested photos, thought I'd ask if I should post some here too.