92TAConv

I have already swapped everything and run the car, but I would like to do away with the ECM for good. It looks to me like the only thing still using the ECM is the speedometer and cruise control to convert pulses from the sending unit. Is there some way around this?

86TpiTransAm

Actually, you can remove the ECM completely!! My friend has an '89 formula that was fuel injected and he did the swap, removed the ECM and his speedo still works just fine....and yes, it is an electronic speedo and not a cable speedo! As a matter of fact, his ECM is sitting right here in the floor next to me right now! LOL!! At this point, I believe the only thing your ECM is useful for is the lock-up on your tranny and that's if you're using the 700R4 tranny which I'm sure you are!! You can wire a switch up to the ALDL (diagnostics) box below the steering wheel and switch the lock up on and off manually therefore completely eliminating your ECM! There's an article in the tech article section here on thirdgen about how to do it!!

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1986 Trans AM
305 TPI
200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Soon to upgrade to a 355 converted from TPI to Carb, Edelbrock 600CFM Carb, Edelbrock Performer RPM Intake, Hedman Shorty Headers, Heads (I can't make up my damn mind), XE268 Cam, Moroso HEI ignition kit with external MSD Blaster II Coil and an MSD 6-AL Box!!
Current project: Keeping my 305 running until I get my income tax returns!

92TAConv

Thanks for the info. I'm running a T5, so the trans is not a problem. In my service manual it looks like the speedometer is wired to an output on the ECM and the cruise is wired to another. I may have looked at something wrong. I'll have to check again. Does your buddy's cruise work?

86TpiTransAm

I'm not sure if his cruise works or not...I'll have to ask him!! It's "possible" that the wiring of the speedo is different between the 700R4 and the T5 but I really doubt it!! What you were seeing in that diagram was probably the wire from the Speed Sensor going to the ECM! The speed sensor "talks" to the computer and tells it how fast you're going and then cuts off the fuel supply when you hit the "limit" speed (usually 120mph)!! Since you have a carb now there's no fuel supply for the computer to cut off so therefore the Speed Sensor is useless!! The speedo and the ECM aren't really connected in any way! Even though the speedo is electronic it is still run off gears!! There's a gear that turns in the tranny and they somehow send an electical signal through the wire up to the speedo and that's how the speedo works!!

A little extra tip for ya!! DON'T try to cut out the part of the wiring harness that you're not using!! My friend did that this weekend.....his speedo worked before without the ECM....now the speedo doesn't work at all!! His alternator quit charging too....I had to read the electrical diagram and re-wire it for him so it would charge again! The harness may be a pain but I guarantee it's less of a pain than what would happen if you tried cutting it out!!



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1986 Trans AM
305 TPI
200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Soon to upgrade to a 383 converted from TPI to Carb, Edelbrock 600CFM Carb, Edelbrock Performer RPM Intake, Hedman Shorty Headers, AFR 195's (if I can afford them), XE268 Cam, Moroso HEI ignition kit with external MSD Blaster II Coil and an MSD 6-AL Box!!
Current project: Keeping my 305 running until I get my income tax returns!

Zepher

iTrader: (1)

I made the mistake of cutting the harness off in the engine compartment. Speedo stopped working. Pete posted a diagram of what wires go from the tranny to the VSS Buffer, that is located next to the ECM, and then to the speedo head. There were only two wires to hook back up.

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Zepher
'86 Pontiac Trans Am WS6, LG4 305, 5 Speed, CF DF Clutch,
Edelbrock 1406 Carb, Edelbrock Performer RPM 7101 Intake Manifold.
Edelbrock Open Element Air Cleaner: Flowmaster Exhaust, 160* Thermostat,
Grant GT Steering Wheel, MSD HEI Coil, Taylor 8mm Wires, Leather 96 Camaro SS Seats,
Alpine CD+Changer,Alpine V12 Amps, Premier TS-1040C Subs, Polk Audio highs/mids.
Best ET: 14.94@91.67
High Speed Run: 157mph
WS6 Trans Am

cfabe

I'm not sure where the years break on this, but on some systems there was a yellow plastic box about the size of a pack of cards that did all the speed sensor signal buffering and etc. There was an output from that box that went to the ecm. If you get one of these boxes ( I got mine from an 86 t/a that i bought for parts ) you will be able to wire it up to drive your speedo and cruise. The buffer box is usually found around the ecm, i think mine was actually mounted to it. If you have this seperate box, you can pull the ecm no problems, just be sure you dont cut the wires to the buffer box. http://www.angelfire.com/mt/psh/speedo.htm That website has complete info about swapping in a speed buffer box. Also, there have been discussions in the electronics forum about this, try searching archives there.


There were also some setups that had the vss buffer built into the ECM. If you pull the ecm in one of these systems your speed and cruise are gone. If your car has this setup you should be able to swap in a buffer box and use that to drive your equipment.

One question, tho. If your car is working properly now, why do you want to bother taking out the ecm. If youre getting an SES light, just pull the bulb and be done with it. Leavt it in there and dont worry about breaking your speedo or whatever.