I recently noticed the Brake light going onfor a few seconds during either normaldriving or idleing then it goes back out. Checked the Brake fluid and it is Fine. Then I noticed that When the Brake light comes on, both the needles for the speedometer & the RPM drop to zero.When the brake light goes back Out, then the needles go back to indicating the speed and RPM. What could be causing this problem and how can I fix it?

 

I'm working off an older schematic, but here it goes:

In order for the brake dash light to turn on, the circuit with the dash bulb must be completed (in this case, to ground). This occurs by design under the following circumstances:

1) Low Brake fluid switch closes (due to brake fluid actually being low)

2) Parking brake switch closes (when parking brake handle is pulled, engaging/closing the switch)

3) During the brake "bulb check" sequence for the dash indicators when you initially turn the key to "on".

If the brake light comes on at any other time, it would seemingly indicate that the circuit is being completed (to ground) when it shouldn't be.

The fact that this coincides with the odo and speedo dropping to 0 is actually the key to diagnosis -- they share the same power feed thru the IGN 1 circuit.

I believe there is a dedicated cirucit which is responsible for the brake bulb check which occurs when you initially turn the key to "ON"; I'm thinking it is flaking out and grounding internally at "random" times (other than at startup). Like it is doing a bulb check while you're driving down the road. Or it is malfunctioning in a less friendly way, since it draws enough current to hose the speedo and odo operation.

Unfortunately, I don't think this circuit can be replaced separately -- it is most likely integrated into the instrument panel electronics.....

Derf



Edited by: derf

 

Thanks for the info! Good, comprehensive answer. (Now if only the crazies on Craigslist had been so helpful).[img]smileys/smiley1.gif[/img]