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 transmisison. I originally tryed this with a 92 PCM controlling the 3800 Series I SC engine I installed into a Fiero with a 125-C trans. While this mod worked for the non-electronic auto trans, it should work with manual transmission equipped vehicles as well.

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 this thread is general in nature, most GM PCMs work on the same principles but all may not be compatible with this mod. Be that the case, this mod is a cheap attempt to working around an expensive OBDII reprogram if that is what you are working with.

The Shift and TCC Solenoids

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

He is a typical diagram of the PRNDL position (gear selector range) switch used on the FWD type electronic transmissions.

As you can see, there are 4 main switches 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 fasion:
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 supplys 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 3800 pcm's 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. Another problem is the 3800 PCM is looking for an analog VSS signal whereas the stock getrag is a digital signal which most PCM's are not compatible with on that particular input. I have not ever attempted to make the stock PCM programming compatible with the getrag signal but it may work. It also may need an adapter to work, one similar to the diagram I have on my website at http://dtcc.cz28.com/fiero/fmods.htm . While you don't really need an accurate signal if you have a manual trans, the PCM would like to see that the car is moving or it is stopped for drivability and idle issues. If you are using a non-electronic auto and still want the PCM to control the TCC normally, you will need to get this signal pretty close otherwise the PCM will assume you have a slipping TCC issue or incorrect vehicle speed and will disable TCC operation.

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

I have not attempted to bypass these switchs so I don't know how the PCM would react if it didn't see them operating. Honesly, I think they are meant for the transmission controlling part of the PCM and probably don't play a role when it comes to the REV limiter and idle characteristics of the engine control side. Of course not hooking these up will cause a few trouble codes.

Hope this information is helpful to a few. 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 and even if it doesn't work, you are not out that much money.

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1987 Fiero Coupe 3800 Series II Intercooled Turbo (12.79et @ 106.6mph)
1987 Trans Am GTA 5.7L Superram 4L60-E (13.1et @ 109mph)
1985 Fiero SE 2.8 (soon to have an L36)

Fiero-related Conversions Performed:
1985 SE 3800 Series 1 SC 4T60-E
1987 Coupe 3800 Series II Turbocharged 4T60-E
1987 SE 3.4 TDC 5-speed
1984 Coupe SBC V8 non-OD to 4T60 OD swap, electric power steering install
1985 SE 3800 Series II Supercharged 4T60-E

http://dtcc.cz28.com

very interesting information. so what would it take to bypass the 1-2 shift so the computer wouldnt think anything was wrong, and set up a push button on my steering wheel (like the n2o switches on the ricers in 2f2f) to control the shift. Sometimes I have a problem with hitting the rev limiter on the 1-2 shift. When i drive my car all grandma like for a few hours, the pcm learns to shift slow and smooth. So when i get on it, and i boost the modulator (btw its the 4t60e) it hits the rev limiter 3 or 4 times before it shifts. Id rather tell the tranny when to shift, and then let it do the shifting for 2-3 and 3-4. any ideas? thanks

Nice write up! Thanks a bunch! There are still some of us plugging along with the old stuff.

Mine is a 92 SSEi w/M17 4 speed (86 GT). I have started to get some things up but it's still pretty rough.
http://www.geocities.com/fierobuzz/ser1.html

There are listings for the Bonnie PCM connectors which may help someone.

There are also some wiring details for SII from fieroflyer's swap.

Great description. It should be noted that this will only definately work for pre-97 (OBD I) computers. It might work for a 97-98 depending on the transmission and absolutely will not work for 99 and newer PCMs. The newer ones also look at transmission input shaft speed compared to output shaft speed and it will give you errors no matter how you lie to the computer.

quote Most 3800 pcm's 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 want the Getrag to provide the PCM the proper pulses per mile, the GM part # for the reluctor ring is 8672178 and the new speed sensor is #25074656. Both are still available from GM.

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www.fieroexcitement.com
Rate me if I've been helpful! :)

I was told to just ground the "drive" and "park" wires. Seems to work fine on my ODB II.

Good info though Darth!! I will be printing it soon

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98 3800SC GTP motor, 5sp.
custom hood,trunk,side scoops,IRM front spoiler, dk.Shadow Grey

For 92/93, why not just clear the F31 bit an set it to a 125?

quote Originally posted by fieroX: very interesting information. so what would it take to bypass the 1-2 shift so the computer wouldnt think anything was wrong, and set up a push button on my steering wheel (like the n2o switches on the ricers in 2f2f) to control the shift. Sometimes I have a problem with hitting the rev limiter on the 1-2 shift. When i drive my car all grandma like for a few hours, the pcm learns to shift slow and smooth. So when i get on it, and i boost the modulator (btw its the 4t60e) it hits the rev limiter 3 or 4 times before it shifts. Id rather tell the tranny when to shift, and then let it do the shifting for 2-3 and 3-4. any ideas? thanks

Hey FieroX, I used to control the shift by toggle switches in my 86 Monte Carlo when it had a 4L60-E. Here is a run down of how the transmission selects gears.

Gear / Shift Sol A / Shift Sol B
1 / ON / ON
2 / OFF / ON
3 / OFF / OFF
4 / ON / OFF

So, to shift into second gear earlier you need to basically cut the shift solenoid A wire between the PCM and the trans. This can be accomplished by wiring in a toggle switch. You will leave the toggle "on" until you want to shift then flip the toggle to turn it "off" which will cut the ground to shift solenoid A thus making the trans shift into second. After the shift completes and you are part way thru second gear you can turn the toggle back on to let the PCM continue to control the transmission.

[This message has been edited by Darth Fiero (edited 08-25-2003).]