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Brake warning light

Tiny
RIESLING
  • MEMBER
  • 1992 GMC SIERRA
  • 5.7L
  • V8
  • 2WD
  • AUTOMATIC
  • 140,000 MILES
I had the brakes serviced and soon after the brake warning light started coming on intermittently. Recently the light has been coming on regularly. As a test the sensor wires from the parking brake and proportioning valves were disconnected but the light still came on. Furthermore, I've notice the light comes on only once the speed reaches 35 mph. Any ideas how to correct this problem would be appreciated.
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Thursday, August 27th, 2020 AT 9:12 AM

5 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Good afternoon,

Is this the yellow brake light or the red light?

The red light comes on with the proportioning valve, e brake switch and the float in the master cylinder if it has one.

Can you verify?

Roy
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Thursday, August 27th, 2020 AT 11:05 AM
Tiny
RIESLING
  • MEMBER
  • 3 POSTS
Thanks for the quick response. This is a red brake warning light. It covers the items you mentioned except there isn't a level sensor on the master cylinder. As an added note, if the e brake is pressed when the warning light is on the light becomes brighter.
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Thursday, August 27th, 2020 AT 12:25 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Okay, I attached a wiring diagram for you to view. In the diagram there is a diode in between the cluster and the switch in the e brake. See if you can find this diode. It is most likely wrapped around the harness under the dash.

Roy
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Thursday, August 27th, 2020 AT 1:08 PM
Tiny
RIESLING
  • MEMBER
  • 3 POSTS
Thanks. I'll check the wiring. Still baffles me why light will go on at a certain speed.
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Thursday, August 27th, 2020 AT 4:40 PM
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Most likely due to the charging system voltage.

Roy
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Thursday, August 27th, 2020 AT 4:41 PM

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