HOW-TO bypass the auto transmission controls in electronic auto transmission-compatible PCMs

I have gotten a couple of questions on how I was able to bypass the auto trans controls on a stock 3800 PCM in order to prevent early rev limiters and other drivability issues it had when I was not using an electronic automatic transmission. I originally tried this with a 92 PCM controlling the 3800 Series I SC engine I installed into a Fiero with a 125-C trans. I have since used the methods explained below in other applications using manual transmissions.

First, let me explain why you will have a problem with a stock PCM when there is no electronic automatic transmission hooked to it. The GM PCM looks at a couple of sensor feeds as well as looks to see if there are shift solenoids hooked up to it. If it finds a fault, the PCM will most likely take action to prevent what it thinks is a failing transmission and reduce power output or cause other drivability issues. While the information contained in this page is general in nature, most GM PCMs work on the same principles but not all may be compatible with this mod.


The Shift and TCC Solenoids (4T60-E, 4T65-E, 4T80-E)

While the PCM grounds the shift and TCC solenoids in order to control their operation, it also looks to these same circuits while they are not activated to see if there is a return signal (ie: B+ 12v feedback from the ignition circuit). If it does not see a return signal, a fault code will set and the PCM may take actions to prevent "damage" to the transmission it thinks is there.

To bypass the shift and TCC solenoids, you simply need to connect standard 194 light bulbs to these individual circuits coming from the PCM and then connect the other side of the bulbs to ignition B+. The light bulbs will then serve as a "load" which will simulate the solenoids. Be sure you keep these bulbs away from any low temperature plastics as they will turn on and off at various times as the PCM "thinks" it is shifting the trans.


The Pressure Control Solenoid (4T65-E & 4T80-E only)

The PCS is a solenoid that the PCM uses to regulate line pressure in the 65-E and 80-E transaxles. The same rules apply here as they did with the other solenoid circuits with the only difference being that this circuit needs to be loaded differently. In a TRANS-GO vacuum modulator conversion kit for the 4L60-E they supply you with a 6 ohm, wire wound resistor to take the place of the PCS and fool the PCM into thinking there is a solenoid there. While a resistor would work I suggest finding a 12v light bulb of a similar resistance as the resister would get very hot after a while as it is intended to remain immersed in trans fluid. The only purpose of this mod is to prevent PCS related trouble codes.


The Transmission Range Position Switch

The transmission gear position range switch has 4 wires coming out of it that normally go to the PCM.  These wires are colored Yellow, White, Black w/ white stripe (BLK/WHT), and Grey.  Basic operation of this switch routes GROUND and OPEN signals to the following wires depending on gear switch position...



Yellow -        GROUND

Grey -           OPEN

White -          GROUND



Yellow -        GROUND

Grey -           GROUND

White -         GROUND


As you can see above, the YELLOW and WHITE wires will always need to be grounded.  The BLK/WHT and GREY wires are the ones that are going to need to change state from OPEN to GROUND in order to indicate a shift into gear to the PCM.

As you can see, there are 4 main switch circuits besides the standard park/neutral switch that the PCM uses to determine transmission range selection. When I bypassed the 3800ISC 92 PCM for use with the 125-C, I simply grounded the yellow and white wires going to the PCM and then hooked the blk/wht and grey wires to the NC (normally closed) side of a relay that would ground them anytime I wanted the PCM to think the car was in gear and would cut ground from them any time I wanted the PCM to think the car was in neutral or park. This gave the illusion to the PCM that it was either in Neutral or Drive3.

Using a non-electronic automatic trans with a P/N grounding position switch (like what the Fiero's 125-C has stock) I would hook a standard relay up in this fashion:
Relay term -- connect to
85 -- IGN B+
86 -- P/N wire from trans switch (ORG/BLK)
30 -- Ground
87 -- not used
87A - BLK/WHT & GRY wires going to PCM

If you don't have an automatic, you can wire this relay to the brake or clutch switch in one of the following ways:

For a switch that supplys B+ when pedal is depressed:

85 -- Switched B+ from pedal
86 -- Ground
30 -- Ground
87 -- not used
87A - BLK/WHT & GRY wires going to PCM

For a switch that supplies B+ when the pedal is NOT depressed, simply wire the relay as follows:

85 -- Switched B+ from pedal
86 -- Ground
30 -- Ground
87 -- BLK/WHT & GRY wires going to PCM
87A - not used


The VSS or speed sensor

Here is where it gets tricky. Most electronic-auto transmission compatible PCMs like to see a 29-31 pulses per tire revolution signal from the VSS (depending on reluctor count). Using the standard tire size from a 3800 SC equipped vehicle, this works out to about 24,000 pulses per mile; which is about 6 times the amount that the stock reluctor from a getrag gives out. If you are going to use one of these auto-trans compatible PCMs with a manual trans that is using the older, 4000 pulse per mile VSS; you will need to have the PCM reprogrammed to work in this application.  The PCM would like to see that the car is moving or it is stopped for drivability and idle issues.

The Trans Gear Pressure Switch (4T65-E & 4T80-E only)

I have done many swaps using OBD-2 PCMs that were supposed to only work with 4T65-E transmissions but instead used them in applications where a manual-trans was present.  As long as the PCM has been reprogrammed to disable the trouble codes for this device, it will control the engine just fine.

I hope this information is helpful to you. While it is meant to bypass the trans control part of the PCM, I cannot guarantee that it is compatible will all PCM's and engine/transmission platforms. What it does do is gives you a cheap workaround to try.

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