Any ideas:replacement PCM causing incorrect speedometer readings and reverse difficul

I recently bought a replacement PCM from my local GM dealer after 'frying' my original PCM during an LTI Edit upload stopped by a power interruption.

After I installed the replacement PCM and took the car for a ride, it was difficult to get the car in reverse and the speedometer was reading too low. Other than that everything else seemed normal. I subsequently swapped the replacement GM PCM for the one I had previously used from PCMforless. The car easily went in reverse and the speedometer was reading correctly.

Any ideas as to what is causing the speedometer and reverse issues. Could the dealer have flashed the replacement PCM with the wrong data even though they had the VIN?

I don't want to continue the LT1 Edit tuning until I get these issues corrected.

Thanks for your help.

The car is a 1995 Formula 6 Speed.

The PCM is not effecting the tranny, I don't see how it could as the only input rom the PCM to the M6 is CAGS which has nothing to do with reverse. You could disconnect or disable it to be sure it has nothing to do with it though. The replacement PCM is probably programmed for a different rear end ratio and that is throwing your speedo off.

Rich

maybe you have a pcm for an auto car with 3.23/2.73 gears

When you say its hard to get into reverse, do you mean its hard to push the shifter to the side and get it into the gate, or that it goes into the gate OK, but is hard to shift into reverse?

The PCM could affect putting it into reverse if it wasn't controlling the reverse lockout solenoid properly.

thanks guys,

I called the dealer and he said "oh, that PCM has to be programmed in the car" (not sure why he did let me know that when he sold it to me over the counter).

Therefore, there must be some programming that the dealer must do to set up the speedometer reading and reverse lockout solenoid.

Hopefully, this dealer programming won't negatively affect the rest of the PCM as a result of my existing modifications (Kooks long tube headers and dual cats, Corsa cat back, 1.6 Crane roller rockers and H-11 valve springs, March under drive/overdrive pulley swap, lower thermostat, K&N cold air induction)

The PCM is not effecting the tranny, I don't see how it could as the only input rom the PCM to the M6 is CAGS which has nothing to do with reverse. You could disconnect or disable it to be sure it has nothing to do with it though. The replacement PCM is probably programmed for a different rear end ratio and that is throwing your speedo off.

Rich

Seems odd Rich, but when i got my 1st tuned PCM, i got it done by TPIS. When i got it back, it was always hard as hell to get into reverse, which didn't make sense to me. Sometime later, i got another PCM and started my own tuning, then used the TPIS PCM as a back up. Everytime i used that one, it was hard to get into reverse, until i threw in my own tune.

Ken R.

thanks guys,

I called the dealer and he said "oh, that PCM has to be programmed in the car" (not sure why he did let me know that when he sold it to me over the counter).

Therefore, there must be some programming that the dealer must do to set up the speedometer reading and reverse lockout solenoid.

Hopefully, this dealer programming won't negatively affect the rest of the PCM as a result of my existing modifications (Kooks long tube headers and dual cats, Corsa cat back, 1.6 Crane roller rockers and H-11 valve springs, March under drive/overdrive pulley swap, lower thermostat, K&N cold air induction)

It is odd that you are using LT1edit and don't understand this.

Program it with the tune you already have.

Used PCMs run $50-75. How much did the dealer rape you for?

In addition to posting here I also contacted LT1 Edit about this issue.

Dave at LT1 Edit advises that new PCMs require both engine programming and transmission programming. He believes that the engine programming must have been done in the dealer shop whereas the transmission programming requires the PCM be in the car.

As to why I didn't buy a used PCM, a comedy of errors surrounding my tuning this the car over the last 4 months led me to try to pursue the least problematic route (or so I thought) and buy a new one. The cost was $185:(

In addition to posting here I also contacted LT1 Edit about this issue.

Dave at LT1 Edit advises that new PCMs require both engine programming and transmission programming. He believes that the engine programming must have been done in the dealer shop whereas the transmission programming requires the PCM be in the car.

As to why I didn't buy a used PCM, a comedy of errors surrounding my tuning this the car over the last 4 months led me to try to pursue the least problematic route (or so I thought) and buy a new one. The cost was $185:(

There is no difference in programming your PCM in the car vs. out of the car. He must be extrapolating from some other kind of PCM or something.

Regardless, at this point you just need to upload whatever program you previously had that worked in your car.

Pretty sure that is a used not new PCM they sold you.