My 94' Suburban is running a 91' Speed-Density TPI from a 5.7L GTA.

The engine runs $8D using a 727 ECM. The 4L60E is still controlled by the PCM which is $OD. The PCM still uses input signals from the TPS, distributor and MAP sensor via jumpers.

Recently, the transmission has been acting up and I think it may be TPS related. It had been running fine for approximately 35k miles. But it now gets an intermittent case of shifting problems. Mostly a herky-jerk when pulling away from a stop, late-hard upshifts, and sometimes the converter will not lock-up. I can often stop, turn off the ignition, restart and it will run fine.

TPS voltage read from the 727 ECM is .50v with throttle plate closed. However, the voltage read from the truck's PCM is only.47v. I am not sure why there is a difference.

The TPS is a new piece from Carquest. The 727 ECM shows no codes. But periodically the PCM will give a low TPS code.

I have three questions:
1. I have read and verified that a speed-density TPS is not adjustable. The ECM will reset the starting point each time you start the car. Hence, speed-density TPS is not slotted. So why is it only reading .50 volts closed from the 727 ECM when it should be .54v?

2. Why is there a TPS voltage disparity between the ECM and PCM?

3. I have read that when it comes to sensors, it is best to keep with genuine GM parts. Perhaps I need to try another TPS, this time from GM. Is this true, or is it a myth. A new AC Delco TPS is pretty pricey to experiment on a hunch. I say a hunch since the 727 ECM is not showing any codes.

Any feedback would be appreciated.

Thanks, Ed

 

1. A TPS is just a potientometer. Any GM TPS can be adjustable if modified slightly. You can adjust the voltage one of two ways. The best way is to take a Dermel tool and slot the mounting holes. The next way is to VERY carefully bend the wiper arm but you risk damaging the sensor this way. Not knowing how your setup is wired, the voltage difference could be due to two different causes. Most likely the difference is due to grounding issues. Depending on where the ground wires are tied to a common point, voltage drops due to the wires resistance can become substantial, moreso as current draw increases. 30mV, in your case is not a large drop but is significant in the application. Signal grounds should be tied to a common point with seperate wires to eliminate "ground loops". The second, least likely, cause is due to a difference in the measuring circuitry in the different ECM/PCM's. Double and tripple check the TPS wiring.

2. See answer to question 1.

3. A genuine GM sensor is always recommended as the systems were designed around them. Aftermarket replacement sensors are "clones" of the GM parts and are generally "good enough" although this does not hold true when referring to oxygen sensors. Having said this, a genuine part is always the safest bet but not absloutely critical. I always use a genuine AC/Delco O2 sensor but I have been known to stray when it comes to other sensors in a pinch.

Lastly, you know that the original PCM in your Suburban has been successfully converted to run a TPI or any MPFI setup for that matter as well as the electronic tranny. See this thread for more info. https://www.thirdgen.org/forums/diy-...-its-time.html . I have never been a big fan of piggy-back ECM's which is why I set out to use the TBI PCM fpr TPI. Anyway, something to consider.

As for your tranny problem, 90% of the calibrations involve either the TPS or VSS or both. A few need a MAP and RPM input. Datalogging the transmission datastream should verify your TPS suspissions. If you have Datamaster $0D, you're all set. If you are using TunerProRT, it is the A218 datastream. HTH