hillpc

??

1997 Canadian A8Q.

I did jumper a couple of pins in the DRL (daytime running lights) change-over relay socket (relay broken and removed) to shut off the headlights except when turned on by the headlight switch (parking lights still come on at all times with key on). I don't think this should affect the brake lights.

hillpc

So now my lighting system is back to original, but the warning light's still on.

I also replaced the brake light bulbs themselves with the original spec P21/5W bulbs (from Philips, not the dealer). The car had had 1157 bulbs before.

Now looking at the wiring diagram for clues. I did see a thread somewhere saying that the system measures a voltage drop across a resistor in a relay somewhere. Maybe the relay needs replacing?

jfrahm

Do you have a trouble code or just a bulb message? If you have a trouble code check both circuits of the brake pedal switch, they can get so that one circuit is normal and the other not, goofs up the cruise and the trans lockup and throws a code sometimes.

hillpc

Just a bulb message. My Bentley wiring diagram seems to show only 1 circuit for the brake pedal switch controlling the 3 brake lights (left, right, and high center), though there's something across the contacts; I don't know what it is. See the picture below. Maybe the other circuit you're talking about is for something other than the brake lights.

Also (I embarrass myself more than I'd like), the high mount brake light is dead, so no, the brake lights don't work fine. All 4 bulbs come on when the light is removed from the car and hooked up to a battery. My LED voltage tester shows that both terminals coming to the light are at ground, all the time, brake pedal pressed or not. But the other brake lights work, so it's not the switch. I figure it has to be the J123 "Lamp Control Module", this is just a large mechanical relay, no?

audinutt

check your 3rd (middle brake light) mine had led lights and caused the same error message.

hillpc

I could see how that would cause it, because LEDs pull less current. But that's not the case here. They are 4 incandescent bulbs, 5 watts each.

jfrahm

I had a problem like this with one of my 928s and had to replace the bulb module. Maybe 4B0919471 , $20 used, not sure if it's the same part worldwide.

hillpc

I'll bet that's it. Probably check the wire coming out of it to the middle brake light for juice this weekend. I did see one on ebay this morning for $15. Even though it's used, I was surprised it's so cheap.
Thanks.

hillpc

Last night I tried to jumper wires in the lamp control relay socket (relay removed) to turn on the middle brake light, but it wouldn't turn on, and heard the gear selector solenoid moving, which it should when in Park and the brake pedal is actuated. Otherwise the selector can't be moved out of Park. I messed around with it some more and came to the conclusion that I'd need the schematic or logic diagram of the light control relay to make sense of what's supposed to happen in there. So for $20 I just ordered a used relay.

This morning I couldn't get the car out of Park, and saw that I had blown the brake light fuse. Replaced and on my way. Some fun, eh? I still might try to check continuity between the relay socket wires and the middle brake light socket.

hillpc

You were right, jfrahm. The old light control relay had a bad section inside, not sending juice to the middle brake light. However, the new (used, actually) relay blows the brake light fuse immediately when the brake is stepped on. When I disconnected the red/blue wire going to the middle brake light from the relay socket (well, I accidentally tore the wire out of the terminal trying to get the connector to release the terminal with the wire attached), the fuse no longer blows. And when I jumper the relay socket terminal directly to the middle brake light lying on the passenger side floor, it lights up just fine, with the other brake lights. So there's a partial short (measured at about a half ohm, so it should pull 24 amps on just the middle brake light part of this 10 amp circuit) to ground somewhere from the relay socket to the wire at the middle brake light.

Interestingly, I had already bypassed the brake light fuse position in the fuse box because the plastic fuse holder position for the brake light was melted, indicating some past abuse. Maybe someone installed a big *** fuse in that position at one time since it kept blowing. Probably fried that one circuit in the relay as well. I cut the 2 wires out of the brake light fuse position and installed a separate fuse holder with pigtails right above the fuse box.

How do I find the short? Is there some way to follow the wire through the hidden catacombs of the car? At the relay, a giant bundle of wires disappears upwards towards the top of the passenger footwell, and at the middle brake light, the wires appear out of an area above the trunk ceiling somewhere. I imagine that pulling a new wire somehow could be as cumbersome as tracing the existing one (how much stuff would I have to take apart to hide the new wires or trace the old ones?). I'd have to repair the terminal at the relay socket as well. Maybe I just forego the middle brake light and learn to live with the constant brake light warning. Or maybe install the right value resistor at the relay to fool the instrument cluster into thinking that the middle brake light is operating. It would have to be 20 watts though so it wouldn't overheat and burn out, since each of the 4 middle brake light bulbs is 5 watts. Or do multiple resistors in parallel.