Brake Light Just Came On.

Had my 95 RMW in the driveway which is a moderate incline. I put the car in reverse, and felt a slight knock as I let off the brake. The slight jolt putting the car in gear on an incline is normal, just felt something slightly extra. The light came on then and stayed on. The brakes and rotors are fairly new. Could a caliber or pad be stuck? Drove the car a bit. The brakes stop the car just fine. Any ideas?

Try engaging the parking brake on flat ground and then release.If it still stays on try pulling up on the parking brake while holding the release handle.

Flasheroo wrote:

Try engaging the parking brake on flat ground and then release.If it still stays on try pulling up on the parking brake while holding the release handle.

Thanks Jim. I will try that, but I did not have the parking brake on. Just had the car in the driveway in park. It happened for sure when I put the car in reverse and felt an extra little clunk, other than the transmission engaging. I came to work this morning and still have full brake power but the light is still on.

There's a couple of things that can trigger the brake light. First, as mentioned, is the parking brake pedal switch. The brake doesn't have to be applied, just the pedal depressed a tiny bit or the switch out of adjustment/stuck. Or a short from the wire to ground, for that matter.

Second is the brake system pressure differential switch. Since the days of Ralph Nader, cars have had dual brake circuits -- in case one circuit fails, there is still some brake capability -- and the pressure differential valve actuates the brake light in case one circuit fails. If pressure from one circuit exceeds the pressure in the other, a spool valve like mechanism is forced towards the low pressure side, theoretically sealing it to prevent further fluid loss, and at the same time, grounds the brake light circuit.

So a tiny leak in one circuit or a short from the wire to ground or a failed switch could light the light too.

Update......... Brake fluid fine. I am starting to thing there is some other issue. A week ago the airbag light came on and it flashed after a few minutes and went off. Now in addition to the brake light on, the change oil light is now on. The oil was changed 2000 ago. This starting to sound like something else???????

Let's assume you reset the oil change light at the time (no disrespect intended, I've forgotten things like that at times) 2000mi could be reasonable. The oil change algorithm is flexible and based on usage (cold starts, short trips, heavy foot etc. all reduce the interval). So I wouldn't read too much into that right now. And it's controlled by the PCM.

The SIR light is controlled by the airbag system, separate from the PCM. I'd do a scan of the history codes and see why it came on -- probably a corroded sensor wire or similar. But not likely directly linked to the brake light issue, as the brake light is an independent, non-computerized circuit. It's like the interior light -- switch grounds wire, light comes on. No computer required. (The ABS light of course is another matter).

There is one other possibility -- but only if you have the Daytime Running Lights option (mandatory in Canada, but I think I read it wasn't even available as an option in the US?) The brake light circuit is tied into the DRL module so the headlights won't come on when you start the car until you release the parking brake. If the DRL module was to short that line, the brake warning light would come on.

buickwagon wrote:

Let's assume you reset the oil change light at the time (no disrespect intended, I've forgotten things like that at times) 2000mi could be reasonable. The oil change algorithm is flexible and based on usage (cold starts, short trips, heavy foot etc. all reduce the interval). So I wouldn't read too much into that right now. And it's controlled by the PCM.

The SIR light is controlled by the airbag system, separate from the PCM. I'd do a scan of the history codes and see why it came on -- probably a corroded sensor wire or similar. But not likely directly linked to the brake light issue, as the brake light is an independent, non-computerized circuit. It's like the interior light -- switch grounds wire, light comes on. No computer required. (The ABS light of course is another matter).

There is one other possibility -- but only if you have the Daytime Running Lights option (mandatory in Canada, but I think I read it wasn't even available as an option in the US?) The brake light circuit is tied into the DRL module so the headlights won't come on when you start the car until you release the parking brake. If the DRL module was to short that line, the brake warning light would come on.

Thanks for the ideas. I wasn't aware the change oil was dependent on anything else but miles. One more thing to add. The starter was replaced a couple weeks ago. The battery and alternator were replace last week. But when I got the car back, no lights were on. Could this have anything to do with this? BTW, I don't have daytime running lights.

From the FSM:

Quote :

The PCM monitors the distance the vehicle has traveled, Engine Coolant Temperature (ECT) and engine revolutions to determine when the next service interval is required. The PCM also estimates engine oil temperature based on various inputs. Once these calibrated values are attained, the PCM will ground CKT 1439 and the change oil lamp will illuminate.
The maximum mileage that can be accumulated before the PCM illuminates the change oil lamp is 7500 miles. However, the lamp may com "ON" sooner depending on engine load and temperature (trailer towing, high ambient temperatures, etc.).

So basically, instead of an arbitrary "Severe Service" schedule, the PCM tries to estimate how severe the service really was. At the upper end, the light will come on at 7500 miles even if the car was babied along on the freeway criss-crossing the nation for the entire time. But at the opposite extreme, the light could come on at 0 miles if the car was left idling in the driveway for a few weeks, just based on engine revolutions.

Anyway, while it is always possible that the mechanic disturbed the wiring when working on the other electrical repairs, the starter motor is on the other side of the car from the brake pressure differential switch and the alternator is at the front of the car, so it's not likely that the brake circuit wires were damaged at that time.

As you don't have DRL, the brake light circuit is a very simple affair and one of the few things that doesn't connect in any way with a computerized system. Power goes to the bulb, from there a single wire connects to each of the brake pressure differential and parking brake switches, both of which are "Normally Open" (meaning there is no contact through the switch in it's normal state). If either switch is actuated (difference in pressure between the two brake systems or the parking brake pedal is depressed), the respective switch then closes and provides a path to ground. There is also a "bulb test" in the ignition switch. When you turn the key to "start" the circuit is grounded to check the bulb.

If any wire (after the bulb) to any switch is shorted to ground, the bulb will light up.

You sure do know a lot about these cars! I think I am going to have to agree with you since your knowledge is great and mine is not. I think you are right that the change oil and brake light are unrelated. As for my driving habits, I drive the car gently at startup, and the car is garaged. I think it is likely that a reset wasn't done at last oil change. That would make the computer think there are about 5k on the oil. The brake issue is too serious for me to address and am taking it in for service tomorrow. I do appreciate your thoughts on this.

cdcdguy wrote:

You sure do know a lot about these cars!

My first Buick wagon was an 81 Electra. I bought it used in 83 or 84 and replaced it with a 92 RMW. Drove that for 17 years, tried a pick-up truck for a year or two and came back to the fold with a 95 RMW two years ago. I do my own wrenching, so call it 27 years of maintaining these cars? Some things were bound to stick; for the rest I have the FSM

Something you could try: "fan" the brake pedal a half dozen times. By "fan" I mean depress lightly but rapidly. If there is an incipient leak in one brake circuit causing a pressure differential, this may equalize the pressure long enough for the light to go out.

If it works, it confirms a brake fluid leak caused the light. If it doesn't work, it means nothing -- could still be a leak or a wiring issue or a bad switch.

Unplugging the wires from the respective switches one at a time would be another way to help isolate the issue. These are simple things that you can do with very little experience or cost.

The oil change light was not reset, so no problems. The left rear wheel cylinder was leaking, so it was replaced and all is fine. Have full braking power again. Thanks, all.

So the brake system warning light did it's job: it alerted the owner to a potentially dangerous leak in the system, who then investigated and had the system repaired.

Perfect.

buickwagon wrote:

So the brake system warning light did it's job: it alerted the owner to a potentially dangerous leak in the system, who then investigated and had the system repaired.

Perfect.

True. While sometimes I complain about these idiot lights, turned out I was the idiot! Glad the car did it's job. The airbag stayed on, them flashed a few times too. They ran all the codes for it and said they could find nothing wrong with it. Another member had a possible explanation for it, as it occurred during the same drive when my starter would fail. Anyway, I am all good to go, and getting back to enjoying this big, beautiful car.

If the left rear wheel cylinder failed,its time to repair the opposite side as well or at least check it closely.

You are right Jim. And I had them both done.