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From digest.v5.n153 Tue Aug 27 13:26:51 1996
From: Larry Schwarcz <>
Date: Tue, 27 Aug 96 12:19:12 -0700
Subject: Re: 325iC questions (John Metzger) writes: >I've got a 1991 325iC that I bought a couple of months ago. The car has one
>fault. The rear brake lights check function comes on intermittantly. by
>intermittantly I mean, It always comes on somewhere in my 9 mile commute to
>work. I start the car up, press the brake and the light goes out. Somewhere
>along the route when I apply the brakes the light comes on. If I turn the
>car off and back on, the light goes out only to come on again sometime
>further on down the road. I've changed all the bulbs to no avail. The brake
>lights work fine. (it's kind of ironic that the computer to indicate the
>problem failed before the actual lights themselves).

I had the exact same problem. Here's what I wrote up on things to check. NOTE: I've heard the two most common causes of this problem on the convertibles is the brake light switch and the center brake light assembly (in my case, the switch was not fully plugged in).

Brake Check Light:

The normal and correct behavior is for that light to come on when you start the car. Then, once you depress the brake pedel, it should go out.

When the car starts, it can't check the brakes, so it assumes they're bad. Then, once the brake pedel is pressed, the system then can see that they're OK and goes out.

Now, if the light stays or comes on later, then there's lots to go bad (I had this problem on my '90 325iC).

Possible causes:

  • Bad bulb. This is an obvious cause. First, check to see if you actually have a burnt out bulb. If you do, replace it. If they're all OK, replace all three of them (L, R & center) anyway. Even though they all light up, you could have a filament that's on the verge of going out. It's the cheapest and most likely solution. COST: About $7.00 for three bulbs (the center bulb is more expensive

    than the others).

  • Bad contacts. Try cleaning all the bulb contacts. Disassemble the tail light assembly and do a *very* thorough cleaning of all the connections. COST: About $7.00 for a can of switch cleaner.
  • Replace the brake light switch (by the brake pedal). It may even be a bad connection. On my car, the problem was that the plug was not quite plugged all the way into the switch. I just had to securely attach the plug.
  • Center stop light assembly (on convertibles only). On the iC's, this light is mounted on the trunk lid and is subject to damage and abuse each time the trunk is shut. I have an extra one of these in working order. Call me if you need one.
  • Brake Check Relay. About $100 for a new one. I've got an extra if anyone needs one. I thought mine was bad and bought a new one. I've still got the old one. (See below on location for Brake Check Relay)
  • Check control panel (the box with the check lights by the mirror). For my car it's $222 with my BMW CCA discount from The BMW Store. According to a local dealer, it's very rare for this part to fail. There's a step-by-step diagnositic process in the shop manual (BMW, not Bentley).
  • Bad ground. The tail and brake lights are all grounded at the ground point G300. On the E-30 convertibles, this is right by the Brake Check Relay. The Brake Check Relay is under the left (drivers) side side panel in the back seat.
  • Connector problem in circuit board on instrument cluster. See posting from Laurence G. Hoffman for more details on how to trace and fix these type of problems.

To get to the Brake Check Relay and G300 on convertibles:

  • Remove the back seat (bottom and back).
  • Remove the trim around the soft top release latch by sliding it towards the rear.
  • Pop out the panel that covers the drivers side (left) speaker (pull from the top side).
  • Undo the two larger black phillips screws that hold the panel.
  • Open the drivers door and pull away the black weather strip.
  • SLOWLY pull the side panel out. The speaker wire will be connected still. When you get enough clearance, disconnect the speaker.

The ground will be the bolt at the bottom center with a bunch of wire attached. The Brake Check Relay will be visible through the hole above the ground connection. It's a larger relay (about 4") with 2 large plugs. It's attached with two screws.

The Brake Check Relay is in the trunk on coupes near the antenna. I don't know where the ground point G300 is on the coupes.

>Also, what manuals should I pick up for this car? I don't do as much work on
>my cars as I used to but I still like to know what's going on so I can avoid
>getting snowed by the mechanics.

I'd start with the Bentley manual and the BMW Electronic Troubleshooting Manual (ETM). Both are available through quite a few places (including a few suppliers on this list). The Bentley covers something like '86 through '90 (Don't let this throw you. Your '91 iC is still the E-30 body style) and the BMW ETM is year specific.

       Good luck!
Lawrence R. Schwarcz, Software Design Engr/NCD      Internet:
Hewlett Packard Company                               Direct:  (408) 447-2543
19420 Homestead Road MS 43LN                            Main:  (408) 447-2000
Cupertino, CA 95014                                      Fax:  (408) 447-2264

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