well for the longest time now i have been driving without speedometer, and not logged close to 5k miles i'd guess. so its time to fix this thing.

with a multi meter, what kind of signal/voltage should i look for at say 30mph? i've heard i should put my multimeter on AC voltage, is this true?

Also, i've been reading that the signal from the VSS to the speedometer is AC. didn't think this was true as for the reason for the yellow buffer box.

How can i test the buffer box, a/c voltage going in, and what coming out? and out of which ports?

where is "the trickster" when i need him??

i searched, and searched, and have been searching since it quit over a year ago... its time to get it fixed...

 

Yet another of the often ignored differences between Camaros and Firebirds is the differences in the VSS systems. Camaros of that vintage had the VSS incorporated onto the rear of the speedometer, while Firebirds had them on the transmission since 1985. Camaros caught up in about 1989 and moved them to the transmission as well, eliminating the cable drive to the speedometer.

The signal from the speedometer-mounted optical VSS on a Camaro should be a square wave DC output varying in frequency. At 30 MPH the output should be in the range of 16-17 Hz. Camaros used an optical trigger array which produced a pulsed (square wave) DC signal which was split at the VSS buffer module for use by the ECM and cruise control module.

Firebirds use a PM tacho-generator that produces a sinusoidal AC wave, varying in frequency with sensor (drive line) speed, including your 1986 model. At 30 MPH, the frequency of the AC wave produced by the VSS should be closer to 33 Hz, and can vary in P-P voltage, but should be at least 3 VAC. The VSS signal buffer converts that lower voltage sinusoidal AC wave output to a square wave DC signal usable by the devices, and on the Firebirds it also performs some division so that the signal rate is appropriate for the ECM and cruise control module. The signal output to the ECM and cruise module should be between 16-17 Hz at 30 MPH. The output of the VSS buffer to the electronic speedometer should be a 12VDC square wave which varies in frequency at the same rate as the input signal, or about 33 Hz at 30 MPH.

A plain analog or digital multimeter will not be very useful in diagnosing the system, unless your meter has a frequency scale (most don't unless they are higher-end units). About the best you can do is test the AC output voltage of the VSS to make sure it produces 3V P-P at 30 MPH. You could theoretically test teh VSS Buffer DC outputs by connecting a DIGITAL voltmeter to the output and ground, then turning the VSS sensor very slowly (one pulse at a time) to cause the VSS Buffer output state to pulse off/on. If you detect the state changes at the VSS outputs, you might presume the buffer module is doing its job correctly.

 

vader i am using this gauge. its a kal equip 3001, i'm told she's pretty expensive, Kal-Equip®

anyways, at an estimated 30mph i was getting the 33hz you said.

next step is what? which of the pins on the buffer do what?

 

well i am pretty sure your going to laugh....

i fixed it..

after taking a drive testing the dc frequency, i found out it just wasn't happening.... well low and behold, the ground to my buffer was not connected. dont ask me, i have zero clue what happened to it, but when i was testing the buffer for the obvious, the ground was absent...

a whole year of no speedometer because of a stupid ground... wow.... stunning...

 

Thanks for the info Vader. I have been looking for this for a long time. Is there a thread archive on TGO?