So I have this 87 Firebird GTA

[348tripower]

In the shop.
Someone has pulled the TPI engine and replaced it with a 350 carb unit. The speedometer doesn't work. Would it have to run thru the missing computer to work or is it just hooked to a hall effect type sensor? I know the TPI set up I have in the COE runs to the computer but I have a VDO speedo.
Thanks,
Don

[enjenjo]

A Hall effects sensor will run it, but it has to be a square wave sensor to make it work.  Last one I needed, I got from Painless through Autozone. Get the Painless number, and have the guy at Autozone look it up in their Vendor Catalog.

[wayne petty]

perhaps this will help...

http://www.jagsthatrun.com/V8-chapters/V8-tpi-Speed-Sensors.pdf

my manuals are boxed right now.. so i cannot grab one without unstacking.. hauling them out and opening each of 20 boxes or so... all i have out is my corvette book.. and the corvette uses a cluster that sends VSS signal to the ECM/PCM

[348tripower]

Thanks Will the stock vss work without the computer?

[wayne petty]

hmm.... will it blend..... oh wait.. thats a late night show..

the digital electronics in the cluster...

http://i1207.photobucket.com/albums/bb476/waynep712/90firebirdclusterdiagram_zps0f568f62.jpg



how the VSS seems to be wired into the ECM/PCM

http://i1207.photobucket.com/albums/bb476/waynep712/90firebirdVSSwiringdiagram_zpsb3e37b3b.jpg

the later buffer...

http://i1207.photobucket.com/albums/bb476/waynep712/speed_buffer_schematic_zps9af45854.jpg



in looking at the diagrams above... and the JTR article..

your dash uses the 2000 count speed buffer signal...

but your VSS if it is transmission mounted is the 2000 count VSS...

but the yellow 4 output speed buffer needs the 4x signal as an input...

but... wait.. there is a 4000 count output on the 4 output buffer... if you are only putting in half the counts needed..

perhaps you could drive the speedo with the 4000 count output...




i cannot say if it will work. as i have NOT TRIED it.. and don't know what the actual signals look like.. but it is the closest i can get without experimentation..

in second thought.. i wonder if this is going to work.. probably worth a try if you can get one cheep...

does the 4x vss put out 2000 pulses per mile or 4000 pulses per mile...  


let us know if it works..

i could not read the part number off the 4 output speed buffer on the JTR page..

[enjenjo]

I cannot say if it will work. as i have NOT TRIED it.. and don't know what the actual signals look like.. but it is the closest i can get without experimentation..

I have done it. it will work.