I recently bought an 87 4 cylinder 5-speed Fiero. The speedometer was working but eventually stopped, pegging at 60 mph. I bought a new speed sensor and installed it on the tranny. For a brief time the speedometer was reading correctly, but then stopped and pegged at 60 again. The "Service Engine Light" comes on and stays on. When I checked the codes, the only one it is throwing is code 24 - Vehicle Speed Sensor. What do I need to do in order to get the speedo working again?
Code 24 means the ECM isn't seeing any VSS pulses for a period of time while it still sees the RPM up and some engine load. The ECM figures something is amiss and sets the code. The VSS generator goes to the speedo first. The speedo sends the signal on to the ECM and cruise. Therefore if your speedo is working and your getting a code 24, there is something wrong at the speedo, the wire from the speedo to the ECM, or the ECM.
My guesses are:
1. The connector on the speedo is loose or corroded (the output to the cruise and ECM is pin C1-U), 2. The ECM connector (pin C2-2 Brown) is corroded or broken, 3. The speedo output circuit is broken, 4. The wire from the speedo to the ECM (which goes through C203 under the console is broken). 5. C203 is pin is corroded or brokon.
If your cruise (if you have it) also doesn't work then that points to the speedo circuit since the signal for both come before C203 or the speedo connector C1-U).