Speedometer stopped working

ROSS71

MEMBER
1990 CHEVROLET SUBURBAN

The other morning my speedometer, and cruise control stopped working.
The heater blower stopped too, but at the time I didn't think that was related because I've had problems with it before, now I'm not so sure.

I changed out the speed sensor after doing some internet searches, but that didn't seem to do anything.

The "Check engine" light comes on now every morning after driving it about a mile.

I checked the fuses under the dash (drivers side), but didn't see any bad ones.

3 Replies

MERLIN2021

MECHANIC
17,250 POSTS

Read the codes! Service engine soon is telling you where to start. Use a piece of wire and cross the two top right terminals, then turn the key to "ON" count the flashes to get the codes, then post the codes for us. A 22 will flash as 2 flashes then a short pause, then 2 flashes, if more than one code is stored, the pause will be longer. The codes will display three times and end with 12. Write them down and post here. This connector is right under the middle of the dash...

Ground A and B terminals together.

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Wednesday, May 26th, 2021 AT 11:24 AM

ROSS71

MEMBER
2 POSTS

Thanks for the tip on checking the codes. The code is 24. My manual says that's the speed sensor (which I replaced). Is there a way to test the wiring or something?

Thanks.

Ross.

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Wednesday, May 26th, 2021 AT 11:25 AM

MERLIN2021

MECHANIC
17,250 POSTS

CODE 24: VEHICLE SPEED SENSOR (EXCEPT 3.1L) ECM applies and monitors a 12-volt signal on circuit No. 437 to Vehicle Speed Sensor (VSS). VSS alternately grounds and opens circuit No. 437 when wheels are turning. This pulsing action takes place about 2000 times per mile. The voltage level and pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH. The ECM uses VSS information in calculations to determine vehicle adjustments. A Code 24 sets when MPH is less than 2 MPH, transmission is not in Park or Neutral, engine speed is greater than 1400 RPM, circuit No. 437 voltage is constant and all of these conditions are met for 10 seconds. These conditions are met during a road load deceleration. The 2.8L may set this code while accelerating or at highway speeds. NOTE: Test numbers refer to test numbers on diagnostic chart. Test Number 1) Monitors ECM voltage on VSS circuit. With wheels turning, pulsing action will result in a varying voltage. Variation will be greater at low speeds to an average of 4-6 volts are about 20 MPH. Test Number 2) A voltage of less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect vehicle speed sensor connector. If voltage is now greater than 10 volts, vehicle speed sensor is faulty. If voltage remains less than 10 volts, circuit is grounded. If circuit is not grounded, check for faulty ECM connector or ECM. Before replacing the ECM, PROM should be checked for correct application. Test Number 3) A steady 8-12 volts at ECM connector indicates VSS circuit is open or speed sensor is faulty. Test Number 4) This is normal voltage which indicates a possible intermittent condition. Diagnostic Aids A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter. "Scan" tester MPH reading should closely match speedometer when vehicle is moving.