I'm wanting to convert my 1985 trans am to use the 140mph cluster that I have from an 89 formula. I have found all the wirinig changes that need to be done but I still have questions about the speed sender. Car is carbureted and has no ecu anymore. If I buy the factory vss do I have to run the buffer box? If so, is there an aftermarket vss in that sends the right signal to run the speedo? How many pulses per mile does the stock speedo look for?

 

My 84 Berlinetta was a factory Carb and I had pulled the entire ECM and harness from the car and still had a perfectly functional Speedo. I'm fairly certain that my 84 and your 89 cluster require the same type of tranny VSS. The signal from that VSS is then sent to the VSS Buffer and then the Buffer sends signals to the ECM, Cruise, and Speedo.

**I THINK the 140 Speedo expects 2000PPM. -------> Call it an educated guess 'cause I'm not SURE that's right !

You need to be SURE that you get the correct VSS

You need to be SURE that you but the correct Speedo Gears

Yes; You need a VSS Buffer.



When my ride was a Carb/ Auto I used all factory parts. Then I changed to an aftermarket speedo and kept using the factory VSS and Buffer. ( adjustments for gears & tires was done thru the aftermarket speedo head. ) I Then converted from 700R4 to T56 ( with T56 tranny VSS ) and the factory Buffer wouldn't work, so I bought a Dakota Digital signal converter and installed it in place of the factory Buffer. You can use all factory parts, you could use aftermarket parts,... The choice is yours to make.

 

After some more digging and more learning I've just been left with more questions! Depending on which post you read, the speedo needs either 4k ppm or 2k ppm and the ECU and cruise use whichever of the 2 the speedo doesn't. My research shows that the buffer box is only used to split and modify the VSS so it can be understood by the components that need it. So in theory since my car doesn't have an ECU anymore and I don't care about cruise, could I use either the 2 or 4 pulse sensor from JTR and wire directly to the speedo without the need for a buffer box? In my head this makes perfect sense but I would like someone to at least double check if I'm heading in the right direction. Lol

 

Not all Thirdgen Electric Speedo systems are built the same. Beginning with the 90 the fuel injected cars had the buffer removed from the system and in those cars the VSS sent the signal directly to the ECM. ( 90+ TBI cars still used the Buffer like all other previous Thirdgens with electric Speedo )

** Be advised,.... 84-89 electric Vehicle Speed Sensors are different than the 90-92 electric Vehicle Speed Sensors. I have always used the proper VSS based on the car I was building and don't know if they are interchangeable.



Do what you want to do based on the 86 Firebird Vehicle Speed Sensor diagram below. ( VSS to Buffer,..... then from Buffer to ECM, Speedo, & Cruise)






For anyone reading this that needs the 90+ fuel injected VSS data; ( VSS to ECM,.... then from ECM to Speedo & Cruise)





Why not,... Here's the 90+ TBI VSS data too: ( same as earlier Speedo configuration; VSS to Buffer,..... then from Buffer to ECM, Speedo, & Cruise)

 

P.S.

1) VSS are also designed to work with a limited number of Speedo Gears......... The sensor ( & gears) you will need will depend on what your rear-end gear ratio is.

2) Electric Speedo Gears are specific to the sending unit they are installed on. Electric Speedo gears and Cable Speedo Gears are NOT the same.

3) Automatic Speed Sensors and Gears are not compatible with T5.