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Brake light comes on at exactly 39mph

1992 Chevy C2500 - 150k.



This truck has been sitting unused in Michigan for 8 out of every 12 months for the last 3 years. I recently drove it down to North Carolina and noticed that every time the vehicle is started and is accelerated to 39mph, the brake warning light comes on. The light turns off only when the key is turned to the off position.



It’s always exactly 39mph, and it happens every time.



It doesn’t appear to be tied to engine RPMs because it won’t produce the problem from revving the engine in Park, nor will it prevent the light from coming on if I reach 39mph while coasting down a hill.



The vehicle otherwise brakes normally, with no indications that anything is wrong except the warning light.



What gives?



I suspect rodent damage to the wiring. It is hard to say exactly which wires get shorted togeter without a schematic and some time, but you have an odd occurance and a vehicle that sits a lot,so I go with rodent damage to the wiring.

How’s the brake fluid level?

My first thought was the Rear Wheel Antilock Brake (RWAL) controller. I’ve had problems with those in the past and simply disconnected them. That system is speed related as to when it does it’s system checks. For grins, I did a google search and found this thread- http://www.truckforum.org/forums/gmc-truck-forum/12653-brake-warning-light-goes-on-at-35-mph.html

Read it carefully. On this truck it turned out to be the brake switch! I learned something I did not know about how the system works on this vintage. Hope it helps you fix your problem…

First thing to find out is what is the brake light wired to.
Is it simply connected through a float sensor in the brake fluid reservoir?
Or is there more to it?
If there’s an engine computer it might control the light.

I assume it doesn’t have ABS.

Edit: I wrote this before I saw TwinTurbo’s interesting link.