The speedometer is erratic or at zero

JOHNEFLEMING

MEMBER
1994 CHEVROLET TRUCK
V8
2WD
AUTOMATIC
193,000 MILES

I can drive the truck for 15 miles and if I stop for any reason the truck then begins shifting erratically, the speedometer is erratic or at zero, and the truck sometimes appears to remain in limp mode, stuck in 2nd Gear. Almost always the brake warning light comes on. I have had the transmission speed sensor replaced and the ignition switch replaced. The transmission guy said that he checked the 'dark', which I believe is the speed sensor buffer and it was okay.

3 Replies

MASTERTECHTIM

MECHANIC
4,749 POSTS

You need to check for ac voltage at the drac (digital ratio adapter controller) and make sure your getting a signal there from the vehicle speed sensor. If you get ac voltage there when its inop then replace the drac

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Wednesday, April 21st, 2021 AT 12:30 PM

JOHNEFLEMING

MEMBER
2 POSTS

I tested the voltage from the vehicle speed sensor at the drac, it rose steadily up to over 12v when disconnected from the drac. When I connected it to the drac, the voltage was erratic and would not rise even millivolts.

Does this point to the drac?

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Wednesday, April 21st, 2021 AT 12:30 PM

MASTERTECHTIM

MECHANIC
4,749 POSTS

The voltage from the speed sensor is around 1.5 ac volts at slow speed. If you get a close reading and still no speedo at all then yes I would suspect drac. I worked at gm when these trucks were new and I remember replacing alot of them, so they do fail.

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Wednesday, April 21st, 2021 AT 12:30 PM