So, I have an 86 Trans Am since 1993 and I have done a lot of mods on it over the years. When I was younger, I thouight I would turn it into a strictly drag racing car and a 383 was put in. I removed the main computer like 25 years ago and while I sure it isn't hard to replace, will that take care of the tach and speedo not working? All other gauges work fine but the tachhomoter and speedometer do not.

I had run an Auto Meter tach in it but that was a simple hookup now I just want the stock back and working in the dash.

Thoughts?

 

If you pulled the ECM harness out of your 86 than you also removed the ground for the speedometer VSS Buffer Assembly. If you cut splice a ground wire into the C207 Dash connector,... ( PIN 'M' I think) this will re-apply ground the the VSS Buffer ( as well as anything else on that circuit ) and the speedo should work again. (as well as anything else on that ground circuit)

You could also just cut/splice the BLACK/WHITE ground wire directly at the VSS Buffer add a length of wire, and screw the new ground wire to the dash metal brace or body somewhere convenient.


Not sure about your Tach issues; Tach wiring is part of the ENGINE side 'C100" firewall harness. I'd follow the current aftermarket tachometer wiring to figure out what was changed before looking elsewhere for that problem.

 

Much appreciate this info however, this was a carburated L69 305 originally so I don'ty know if this info is going to work. I can install a new ECM but I am, unsure how to wire it and which pin. I will dig around.

Thanks again

 

It will work. ( If you removed the ECM harness ) .

The PIN you need is PIN "M" ( BLACK/WHITE wire ) on plug C207.

You do NOT need to reinstall the ECM harness.

You do not need to reinstall the ECM.

This schematic is for an 88 Firebird,.... Yours should be (pretty much) the same. You can see that the Buffer Ground (circuit 450) passes from the back of the head (G118) & thru the ECM harness thru the fender and under the passenger side of the dash @ the Dash-to-ECM harness plugs (C207).

There is a factory Splice on that Ground wire (S217) and any item/accessory on that circuit will have also lost it's ground. (Neutral Safety Switch maybe ? I don't have that schematic) This is probably irrelevant - but it's a good argument for re-connecting Ground @ C207 rather than directly at the Buffer.

Once the Speedo Buffer Assembly gets Ground again it will 'come alive' and begin to accept and report the tranny mounted Speed Sensor info to Cruise Control, and the Speedometer. ( as well as the ECM - that you don't have/need. ) A good idea to take a minute to look under the car to be sure that the Tranny Speedo Sending unit is connected./plugged-in !

Once you remove the plastic lower "hush panel" and the lower "dash trim" panel on the passenger side you will expose the C207 plugs and the VSS Buffer (yellow plastic case ). Find PIN "M" ( BLACK/WHITE wire) on C207 and splice it to Ground; quick and easy 10 minute & 1-wire fix.