I have a strange fault I'm struggling with.

My '8625 PCM is reporting a high MAP DTC, dumping fuel and showing 100kPa constantly in Tunerpro.
However the MAP sensor itself is working properly and I'm measuring the correct voltage range at the MAP input pin on the PCM when pulling a vacuum on the sensor with a hand pump, yet the three indicators above suggest the PCM is actually using a full scale measurement for the MAP sensor all the time.

I don't know why the PCM is indicating the sensor to be at full range ~5v when the voltage on the pin at the PCM is changing correctly with vacuum applied to the sensor.
Even with the sensor unplugged it still shows full scale on Tunerpro when it should show 0kPa (0v)......

Has anyone seen this before? I have tried my 2 other spare PCMs with the same outcome.


The back story is that it has been running fine with the PCM for years. Last week I killed the 700r4 and swapped in a 4l60e. The wiring for the 4L60e has been in place since the PCM was installed in 2004, I only had to do a few small mods related to the TCC and VSS (DRAC) so I don't think the wiring changes created this problem.
The be sure and eliminate the possibility of the recent changes causing this problem, I tested the MAP sensor again using and old .bin that I ran prior to the 4L60e installation (in case I screwed up something in the .bin) and disconnected the connector at the trans, yet the MAP sensor still shows full scale when I pull a vacuum and measure 1-4v at the input pin. This is as closely as I can replicate the previous, working configuration, yet the fault continues to exist in this scenario.

I really don't think the trans swap has caused this, but the timing of the fault is hard to ignore......

There was one ground wire on a bellhousing bolt that I did not forget about and I didn't pinch any wires between the bellhousing and block when bolting up the transmission.

I think I may have to run the PCM on the bench with Tunerpro dataloging it and keep adding inputs until the MAP goes bad?

Any suggestions? I'm running out of ideas...

 

If the sensor is provably good...and three ECMs produce the same results...I'd think it has to be in the wiring between sensor and ECM.

But where ECM re-programming is concerned, I'm clueless.

 

OK, I solved this.. I feel stupid, but I'll post it for anyone with the same issue who may find this thread during a search..

The MAP sensor does not work when the motor is not running on the later PCMs.....
Maybe during this time it's used for baro measurement, I don't know, but it does not show movement when logging the ALDL like the TPS etc does.

So in my case, there was nothing wrong with it and I was scratching my head testing it for an hour for no reason

 

So you turn on the ignition, but don't start the engine. You use a vacuum pump to vary the vacuum applied to the sensor...and the readings DON'T CHANGE as the vacuum changes???

 

Yes, that's exactly what happens.
The sensor is still putting it's 0-5v signal on the PCM input pin, but it is ignored.


It's been a while, but I was sure the 1st gen 7747 & 8746 etc work differently.

 

I'd have never guessed that. Thanks for the update.

 

Is the torque converter different in your 4L60E than the old 700r4? Tighter converter will make the vacuum drop off and the MAP reading higher. Might need to adjust the KPA that code 34 flags with running a larger than stock cam. Generally as these ECMs or PCMs got newer, the emissions windows became tighter making the system more sensitive for error codes to generate.