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Intermittent VSS signal. Replaced the speed sensor and it…

Customer Question
Intermittent VSS signal....

Intermittent VSS signal. Replaced the speed sensor and it improved somewhat but I am suspecting a ground issue and trying to figure out where to look for associated ground connections and wire harnesses.

Mechanic's Assistant: Okay, I'll connect you to the mechanic to help find that for your Chevy. Before I do, is there anything else you want him to know?

The vehicle is actually a 2004 GMC Sierra 4.8L, ext cab wt

Submitted: 8 months ago.Category: Chevy
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Answered in 6 minutes by:
11/14/2017
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago
Ivan Alvarez
Category: Chevy
Satisfied Customers: 188
Experience: lead technician at Performance Tire and Automotive
Verified

C​an you send me the trucks vin number so I can pull up accurate wiring diagrams.

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Customer reply replied 8 months ago
2GTEK19V141411132
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago

There is no hard ground on the vss circuit. It gets a ground directly from the pcm. This would be on the light green/black wire. The purple/white wire will be reference voltage also provided by the pcm. Is the speed not accurate?

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Customer reply replied 8 months ago
Occasionally the speedometer needle will get just a little "jittery" and the ABS warning light will come on for "erratic vss signal" if I recall correctly (C0237) and the general break system warning light will go on and ding. Before I replaced the VSS the speedometer needle would be very erratic and sometimes drop to zero and transmission shifting would be impacted as well.
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago

Do you have access to a scanner that lets you view abs data?

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Customer reply replied 8 months ago
I have an OBDII scanner. I don't have my own ABS code scanner currently but I borrowed a friends to check the code initially. If I needed to I could probably borrow it again or buy one myself.
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago

It would be helpful to see what the abs data is. There is a wire that relays vehicle speed from the pcm to the abs module. .he abs module then calculates rear wheel speed based on front wheel speed sensors and that input from pcm. Its possible one of the front sensors are giving an erratic signal also. Depending on the scanner front wheel speed can be viewed

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Customer reply replied 8 months ago
Ah, ok. It would probably take me at least a few days to get ahold of a scanner and check that. Assuming the vehicle would need to be moving at the time and I'd keep my eyes on the live data from all sensors looking for stability in all three readings?
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago

No problem just let me know once you have access to that data. This post will stay open for 7 days.

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Customer reply replied 8 months ago
Will do, thanks Ivan!
Chevy Mechanic: Ivan Alvarez,
 replied 8 months ago

Your very welcome.

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Chevy Mechanic: Ivan Alvarez,
 replied 7 months ago

H​ello, today is the last day for your post just wondering if you had any new info?

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Customer reply replied 7 months ago
I was able to check the codes and data again with my buddies scanner yesterday evening and did not observe the erratic signal during monitoring however the scanner only displayed a "veh speed mph" and did not show individual wheel sensor speeds for comparison. The codes on the ABS module history this time included C0268 (pump motor circuit shorted), C0236 (rear wheel speed signal missing), and C0237 (rear wheel speed signal erratic).
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