Just an update. Instruments worked flawlessly ( for almost two weeks), until yesterday. Again, depressing brake pedal shuts all gauges down. Also, no brake lights and a very faint (almost inaudible) buzzer sound. Never affects engine performance and all instrument cluster lamps work. Anyone care to comment. It appears as though this is a thread very few are interested in or know anything about. All suggestions are greatly appreciated

This is just a guess, but an educated one.

To me, this set of symptoms sounds like you have a short in the brake light circuit that causes havoc when it gets energized and causes a current drop.

I would check for:

1) The wrong bulb in a brake light socket
2) A short in the brake lights wiring harness
3) Problems inside the "bulb monitor computer" or whatever you call that little box that contains all the circuitry that monitor for bad bulbs. These things have metal strips used as resistors. I have seen them burn up and then you have metal pieces kicking around inside the box with all the circuitry.
4) A problem with the brake light switch
5) A wiring problem behind the cluster such as a bad ground or cracked trace on the circuit board.


Good luck.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

Hey tinypanzer, Thanx for the info. Could you, or anyone for that matter, tell me where the bulb monitor computer is on a '88 300 SE?

Hello,
#'s on Brake light bulbs-7506, 12vP21W. Are these the CORRECT bulbs? Removed those bulbs, still have problem. Could the brake light switch be causing all this? Also no one has told me where to look for bulb monitor computer. Anybody know?

Sorry, I've been distracted lately.

The bulb monitor box is in the fusebox. It's the large black rectangular box. It plugs in like a relay. If you get it out and it rattles, there is probably a burned out reference resistor in there. The reference resistors are just little strips of metal, and they can short things out when they are loose in there. To open it, you use a few pieces of thin, hard material like a driver's license. You slide them in each side so that the tabs no longer catch the bottom, and then you slide the guts out.

If the problem still happens with the bulbs out, then it's not the bulbs. Do they light up properly and with even brightness?

Again, I'm shooting in the dark here. I hope I'm not wasting your time.

Have you pulled the cluster out yet? It's really not hard at all. Just telescope the wheel all the way out, and pull it out with the hooks you make from a coathanger.

__________________
-tp


1990 300SE "Corinne"- 145k daily driver - street modified differential - PARTING OUT OR SELLING SOON - PORTLAND OR. AREA - PM ME FOR DETAILS
1988 560SEL "Gunther"- 190K passes anything except a gas station
1997 S420 - 265k just bought it with a rebuilt trans. Lovely condition

Thanx, tinypanzer for the quick reply, and no you are not wasting my time. I cannot tell if the bulbs shine w/ same brightness unless I put them in a car that will energize them, or rig them up to my battery. They don't come on at all with the problem I am experiencing. Can the brake light switch be the problem. I can't really check the bulb monitor computer till tomorrow. Can the reference resistors be changed or replaced?

I just had the same problem with my 1986 300E. It was just a fuse, No. 5 I think. In your case, it could be a fuse with a hairline crack. I'd find it and replace it--or at least clean up/tighten up the fuse holder.

Brent Smith
Atlanta, GA

Hi Brent,
Thanx for the feedback. I will look again at the fuses. Do you know what fuse #5 is related to or would that same fuse be protecting same circuit as in my car? I have no English owner's manual, only French.