hey guys, I have an electric speedo in my 89 formula and its been going crazy latley, it jumps up and down and sometimes if i hit the gas it will go to zero. Now what could this be? its a manual transa if it matters and a 120 speedo, thanks

 

The metal drive ring on the transmission output shaft may be slipping, or may have been mounted too far off center of the driven gear on the VSS. You shoudl be able to remove the VSS and inspect the condition and location of the metal ring gear on the output shaft. You may also have a problem with the driven (VSS) gear. If the plastic driven gear on the VSS is intact, you may have a mechanical problem with the speedo itself.

 

Could it be a loose ground or connection possibly near the VSS buffer?

 

I could be mistaken (prob'ly am) but I thought the ECM got the buffered VSS signal and the speedo head got the raw signal. From that "logic" I thought that any VSS signal at all should operate the speedo, regardles of the amplitude.

 

I am not totally sure how it's wired up in a stock car.
When I converted to TPI, I had the buffer box (little yellow box that had 4 OUT Buffer on it) wired directly to my speedo head, since that is how it was when the car was an LG4.
The ground for the box was bad on my setup and the speedo would sometimes jump around and/or drop to zero.

 

Good info. I guess I should start digging through schematics before I offer up some advice. I had a similar problem on an '84 Fiero and found the VSS was wired straight to the speedo head. I guess I shouldn't assume that the 'F' cars are the same. OOPS! I have the little yellow quad-in buffer, too.

 

I know that the 90-92 cars had the buffer built into the ECM, at least that is what I was told.

 

Vader, here's some more info that Dale and I dug up at the v6 forum a while ago: Turns out that '86 Firebirds (engine size not mattering, haha) had both types of VSS setups.

'86 Firebirds with an idiot light panel received a cable-driven speedo, VSS on the back of the speedo. (My style of '86.)

'86 Firebird with a full gauge panel got a VSS in the tailshaft, a buffer box by the computer, and an electric speedometer. (I pulled one of these setups from an '86, it's in my basement.)

I couldn't tell you about pre-86 or post-86 with any certainty... I have my guesses though.