Real Fiero Tech

Does the stock Speedo need the stock computer to work?

Speedo hasnt worked since I installed the microsquirt. I verified with the c203 that the purple and yellow wire goes straight to the sensor on the trans.

I was told the M location on the c203 is the ground for the Speedo. I have that grounded on the engine head with the microsquirt.

Speedo still doesnt work. Are there more grounds to the system? Maybe more at the c203 or c500?

Ideas?

No, it doesn't. It's been years since I've looked but the PCM never conditioned the signal on any of the factory cars. The V6 cars speedometer conditioned the signal (halved it right?) for the cruise control module, but the signal was tee'd between the speedometer and the computer. If you take the computer out of the circuit it doesn't know the car is moving for idle and fuel control, but the speedometer should still work.

Both the yellow and purple feeds come into the bottom of the speedometer on the third pigtail (C1). Are you sure it's plugged in? If it were unplugged everything else would work except the speedometer. Take the kick cover off and you should be able to see it. M is the correct ground for the cluster.

Which transmission?

Emc209i wrote: ↑Fri Aug 27, 2021 4:44 pmIf you take the computer out of the circuit it doesn't know the car is moving for idle and fuel control, but the speedometer should still work.

I can confirm, I drove around for about 2 years with exactly that setup, and a MegaSquirt computer that didn't have a VSS input. The speedo still worked.

Shaun, does your odometer work? If the odometer works, that means the VSS signal is reaching to the cluster.

Maybe you dropped your speedo gear into the transmission as you installed the VSS?

Odometer doesnt work either. Trip odometer doesnt move either

I checked the fuse. Pretty sure the radio fuse controls the Speedo. Looked ok.

I never removed anything from the trans. Its a getrag 5 spd.

I did modify the purple and yellow wire for the micro. I rerouted it. Factory it runs through the harness over near the c500 by the battery and on the trunk side of the motor. I shortened the wires so its run through the firewall now and over the top of the trans.

I checked continuity on both sides of my splice on both wires.

I never removed the kickplate under the steering wheel for my rewire.

Stock the VSS produces a sine wave that goes to the speedometer. If you have continuity from the VSS to the instrument panel AND neither wire is connected to ground or +12V, then the speedo should work, if the gauge itself is not broken. As noted above, the speedo halves the frequency and sends that signal to the cruise control and ECM.

In most engine swaps, the ECM reads the VSS and outputs a signal for the speedometer. That signal is usually a 0-5V square wave instead of a sine wave, which drives the need for the speedo signal conditioning circuit also commonly used in engine swaps.

So anyway, check continuity of the purple and yellow wires from the VSS all the way to the instrument panel, then check isolation of each wire from ground and +12V.