speed sensor


guidogto
06-03-2016, 06:19 PM
Help! my 1994 century won't shift, the speedometer shows 0, my 97 service manual shows the transmission won't shift without speed input, trouble code 502, low speed sensor input, changed the speed sensor but the problem persists. I could only find 2 fuses in the front passenger side under the hood with relays in front of them, the fuses are good, all the fuses in the glove box are good, I still believe it's a fuse problem. is there a fuse on the speedometer? the speedometer is analog, not fluorescent. thanks for any help.

gmtech1
06-03-2016, 08:17 PM
Help! my 1994 century won't shift, the speedometer shows 0, my 97 service manual shows the transmission won't shift without speed input, trouble code 502, low speed sensor input, changed the speed sensor but the problem persists. I could only find 2 fuses in the front passenger side under the hood with relays in front of them, the fuses are good, all the fuses in the glove box are good, I still believe it's a fuse problem. is there a fuse on the speedometer? the speedometer is analog, not fluorescent. thanks for any help.

Since you have a service manual, you should have access to a schematic. It will show you any fuses in the system. You need to verify that the wiring is ok between the the speed sensor and wherever it runs to. No shorts or opens. I'm no trans guy, but there is also the possibility that the tone ring that the speed sensor uses for the speed signal is loose/not spinning. You can check this by removing the sensor and use your finger or a small tool to try and spin the signal ring. It should not spin freely.

guidogto
06-03-2016, 08:27 PM
I don't have the 94 service manual, I get an ac signal out of the sensor, the wires are clear, but the 97 manual is not much help with schematics on a 94.

gmtech1
06-03-2016, 08:56 PM
Ok, If you're getting the signal, then that's good. Really need to know where the wires run to. I imagine to the ECM, then to the IPC. I won't have access to a diagram till Monday. Maybe someone else will have a little more info before then!

Blue Bowtie
06-04-2016, 03:32 PM
The VSS signal connects to the PCM via purple and yellow wires at PCM connections D13/D14. The AC signal from the VSS also needs to be present at those connections.

The PCM places a low DC bias voltage on the circuit and monitors for the pulses as raw input. Verify that all the connections are clean and intact. PCM connectors can oxidize, and at these very low signal levels the meaningful signal can be lost. It might be helpful to disconnect the battery, remove the PCM header connectors one at a time and inspect the contact condition. Many times the act of simply disconnecting and reconnecting the header connectors will wipe the contacts clean and restore connections.

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