Facts:
1. Early TPI units and all Throttle Body 305 engines that came in Firebirds/camaros require the 2 pulse vss signal (Can't recall whether it's square or sine wave).

2. Late TPI Units require a 4 pulse vss signal that is the opposite (square or sine wave) of the Early vss.

This vss signal is then interpreted by the computer. The computer relays a signal to the speedo.

I need to know if the signal the computer relays to the electric speedo is the same for all electric speedometers. (ie 86-92 firebirds with both throttle bodies and tpi engines).

 

I wouldn't think it is for 2 reasons.

1) The cars came with different speedos and to calibrate them right they would need to send a different signal to each to get the right speed. (ie 60 mph is not in the same spot on a 85 mph speedo as a 115 mph speedo)

2) In tunnercat there is a constant variable for instument pannel pulse divisor, this appears that it goes along with the VSS, but I could be wrong. IMO this would be in there for different cars and speedos that needed different calibrations.

 

I believe that while the SD cars allow calibration of the signal sent to the speedo from the ECM, the MAF systems take the dash-displayed speedo info directly from the VSS. Could be wrong, but it seems that when I was combing through the harness that this is the case.

If anyone knows different, please speak up...

 

Maybe calling a speedometer shop would work. They would know more than anyone I would think.

 

There are the 2 pulse, 4 pulse, and reluctor VSSs.

You need to figure out what you have, and what you want to go to. There are numberous wrong combos.

Might try www.jagsthatrun.com or Riverside Speedometer in Riverside CA, for the info you need, but a wiring diagram should answer you guestions.

 

Doesn't a 140mph 'bird elec. speedo swap right in for a 85mph 'bird elec. speedo? I know cable driven speedos are set to a std. (1001rpm = 60mph) so I think your statement is false. X # of pulses from ECM putting 60mph on the 85mph speedo will also put 60mph on the 140mph speedo.

 

The basis for that statement was the thinking that the ECM knows what speedeo you have and it sends the proper signal to that speedo. I was thinking that because of the Instument panel pulse divisor. I would think that is used to divide the pulse given by the VSS so that it can be sent to different speedos. I have never gotten a clear answer to what this does so if anyone has an idea, please let me know. If it is the speedo cluster that interperates a generic signal from the ECM then I would think to make things simple the ECM would send out the same signal to all the different speedos.

On another note, not all TBI and TPI cars had an electronic speedo. I know my '88 is cable driven. I think they started the electronic driven speedo later, 89 or 90. The VSS in those years was only for the computers benifit.

 

Just for some FYI and FWIW: My 87 has a VSS. Never looked at which one or anything though....

 

In 87 the 'birds switched to a electronic speedo. In 90 the Camaro followed.