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...
NEUTRAL
BLK/WHT - OPEN
Yellow - GROUND
Grey - OPEN
White - GROUND
DRIVE 3
BLK/WHT - 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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