'90 Firebird T/A no Speedo after replacing ECM
 

I have a 1990 Trans Am.

A while back we replaced the stock 305 with a modified 350. I still have the stock TPI. I opted to replace the stock ECM with the Accel DFI Thruster VII. Upon replacing I have found the speedometer in the dash no longer works. I found a few previous threads that indicate there should have been a buffer box but I do not see the box nor the wires to connect one.

I have tried running the Stock ECM along side the new one, and it did not work?

Has anyone found a fix for this or know of a work around?

I should note I am very Green at this, so any input would be beneficial.

Thanks in advance.

Re: '90 Firebird T/A no Speedo after replacing ECM
 

The '90 - '92 TPI & MPFI ECM ('7730) buffers the VSS signal and sends it on to the speedometer and cruise control.

Find a '90 - '92 TBI Camaro/Firebird and look for the yellow 4-OUT BUFFER box. Under the dash on the passenger side. Other vehicles have that same box, such as some earlier Firebirds and TBI Caprices.

Wire it in to buffer the VSS. There are several posts/threads here on wiring it.

RBob.

Re: '90 Firebird T/A no Speedo after replacing ECM
 

Please Pardon my ignorance.

So the '90 did not come with a buffer box unless it was TBI and I need to find one?

Re: '90 Firebird T/A no Speedo after replacing ECM
 

https://cimg7.ibsrv.net/gimg/www.thi...bdaa53d19d.jpg

https://cimg8.ibsrv.net/gimg/www.thi...36254a7fee.jpg

https://cimg2.ibsrv.net/gimg/www.thi...cffea5abea.jpg

Yes if it's a TPI, the 7730 ECM drives the speedo. Check C1Black, B9 & B10 (VSS), B11 (Speedo) and C2Black C1 (cruise control). You could find a way to have the old ECU powered up in parallel when ignition on (12V hot in run) so it can drive the speedo. That would be C1Black A6 (12V Ignition) + A12 (Ground) and C2Black D1 (Ground). Otherwise you'll have to find the yellow buffer box RBob has mentioned (I have included a diagram) or you could check http://www.dakotadigital.com/index.c...rod/prd126.htm . Not sure about the cruise control function though as it might take into account other input from the ECM.


Best of luck!

Re: '90 Firebird T/A no Speedo after replacing ECM
 

Thank you for all the helpful info. I will see if I can get the old ECU running, I'll let you know how it goes.

Edit, so after finally tracing all the wires it turns out all the wires leading to the passenger side kick plate have been removed by the previous Mechanic. I will attempt to run new wires and install the Digital Dakota SGI-5E.

Thanks again for all the assistance.


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Update
Thank you SBFormula for the advice.
I was able to install the Dakota Digital SGI-5E without issue, following Application 2 in the the provided installation guide.