Speedometer only works when I push the brake pedal

[DmaxRvRrat]

Speedometer only works when the brake pedal is pressed (brake lights come on). Cruise control doesn't work either. Tested the brake pedal switch, replaced it anyway, didn't fix it. checked all of the wires under the dash best i could.

Anyone know what would cause this?

 

[DmaxRvRrat]

Anyone have this problem before?

 

[duratothemax]

Any codes? Sounds like a short in the VSS wire from the ECM to the cluster.

Pull the cluster out, and look for the dark-green/white-stripe wire. I forget what pin position off the top of my head.

Use a multimeter and check for voltage between that wire and ground (key on engine off, and then check with engine idling). Then press the brake pedal and see if the voltage (if any) changes. It sounds like the TCC/brake circuit (which is +12v with brake released and 0v with brake applied), its the purple wire going to the brake pedal switch, is somehow feeding into the 4k ppm VSS circuit.

Ben

 

[DmaxRvRrat]

No codes....I've got the cluster out, there are two green and white striped wires.


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[DmaxRvRrat]

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[DmaxRvRrat]

I tested the green/white striped wire on the left side in the picture. I believe this is the VSS wire in A3 position on the connector.

This wire has 12V with the key on engine off, and once the brake pedal is pushed it has 5.5 V.

With the engine running this wire has 14V, and once the brake pedal is pushed it has 6.5 V

Any clue as to what could be causing this?

Thanks for your help!

 

[DmaxRvRrat]

Also tested the purple wire at the brake pedal switch and it has the same voltage that the VSS wire at the cluster has, and acts the same when when brake is applied.

 

[trashygasman]

Check to make sure all your fuses are getting good contact. My truck was doing some wierd stuff a while back and one if the ECM fuses under the hood wasn't getting good contact.

 

[DmaxRvRrat]

Check to make sure all your fuses are getting good contact. My truck was doing some wierd stuff a while back and one if the ECM fuses under the hood wasn't getting good contact.

Ok, I will do some more digging today.

Might have stumbled upon another clue last night. With the key off, the cruise and high beam indicators on the cluster light up dimly 10 to 20 seconds after everything is left idle. (turn lights on or key on and they go off)

Could the multi-function switch at the column cause the issue I'm having i wonder? :screwy:

 

[DmaxRvRrat]

bump for some educated guesses? lol

 

[jc1843]

Poor ground can cause strange problems. Might want to add another one for a test.

 

[DmaxRvRrat]

I'm not sure I know what you mean by adding another ground


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[DmaxRvRrat]

Still trying to diagnose this....pulled the switch off the column and found this




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[DmaxRvRrat]

Well, the speedometer doesn't work with the connector unplugged at the multi-switch, must not have anything to do with this problem. I followed the wires all the way to the fuse box, couldn't find where they would be physically shorted.

Ben, is there a module or something that uses both of these wires? I unplugged the ABS module on the frame just in case, but that didn't solve it either.

 

[DmaxRvRrat]

Bump


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[DmaxRvRrat]

Found the cause of the issue finally!! Mouse got underneath of the fuse box and did some damage.




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[LBZ_Duramax]

Glad you found it, was chasing a problem with a speed sensor on one of our work vans and skinned wire was to blame also.

 

[Sierra2000]

2 1/2 years later and thanks for this tip. I have the same chewed up wires under the fuse box under the hood. Once I got you in and had a look I could see GM designed nice little entrance doorways for the mice to come into from the cold and all those wiring harnesses... Nice when these trucks just had a simple speedometer cable from the trans to the dashboard. Never had a mouse chew into one of those. Thanks again - Happy Holidays.:hello:


Reply to #16
DmaxRvRrat

Gearhead
Join Date: Mar 2014
Posts: 82
Feedback Score: 1 reviews, 100%


Found the cause of the issue finally!! Mouse got underneath of the fuse box and did some damage.




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'01 Sierra
30 overs, EFI, Built Trans
Grizzly locker, Sleeved and Braced
Lift Pump, AFE Intake, Horn, Straight Pipe
4" lift on 35's.