88 300 SE- Strange behavior w/ Instruments
 

Hope someone can point me in the right direction on how to troubleshoot instrument cluster. Today, while starting my drive home from work, I had no speedometer, temp gauge, oil pressure gauge, or tach for about half the trip (10 miles). Then, all of a sudden, all the needles of all gauges came to life and landed in their normal positions. What could cause this. Within the first 300-500 feet of my trip, a buzzer sounded momentarily and then the gauges all stopped as if the key had been turned off. Any Clues?

I am having the same problem

http://www.peachparts.com/shopforum/showthread.php?t=247712

I hope that you get some answers.

Texholdem,
Thanx for the reply to my instrument cluster question. My car did just fine this am. Again, however, when leaving work---Same ole stuff. Buzzer first within 300 feet of starting engine, cluster fine. Then within half mile cluster goes bonkers. This time the cluster didn't come back to life until I had traveled about 16 miles. Then went out again then revived again. Sure is weird.

Sounds like a loose common ground. Pull the cluster out (using homemade tools which you can search on here).

Look behind the left side of the cluster. There should be several brown ground wires bolted to the chassis. Make sure it's good and snug.

If I recall correctly, there is a single ground wire that goes to the back of the cluster, it plugs onto a little pole coming off the circuit board. Make sure that there are no cracked solder joints in that area.

If you poke around long enough, you'll find it...... That's the only thing I can think of that would show up on all your instruments at once.

I have some bits of that wiring harness if you find yourself needing a new plug head.....

Thanx Tinypanzer for the reply.

Hi everyone.
Here is another CLUE. The problem always happens now with the prssing of the brake pedal AND there are no brake lights and a very weak or faint buzzer sound. I can kill the engine and start it right up--everything works except brake lights. All gauges are fine, but as soon as I apply the brakes, gauges go bye bye. What could cause this?.....

oxidized contcts
 

when i replaced ALL the fuses with a fresh set of brass type fuses the instuments have become perfect, flicker free. seems the old fuses had hailine cracks and the tangs were oxidised.

mak
300se/89

Thanks for the reply mak. All my fuses look like new. They're all fine. The gauges work just great until I press the brake pedal. No blown fuses.

Is anyone else out there that has a clue about my problem. Surely this has happened to someone else before. I feel the problem is something other than fuses or a loose connection on rear of cluster. I can start the engine 100 times, and 100 times EVERYTHING works great until I depress brake pedal. Then instruments go haywire. Any takers out there?????

I'm not sure if its the same, but once I took my cluster out to change the bulbs and after that it went crazy. If I turned the brights on, the gauges would die, and if they werent dead, they were maxed out. I was told here that it was probably a ground. I used little alligator clips to ground everywhere I could to fix it. After a while, I didnt need them anymore for some reason. However, My cluster dummy lights still come on and off for some reason. Especially the low oil light when I know its full, and the bulb out light.

So, your brake lights don't work at all, and when you hit the brake pedal the dash goes out?

That sounds like a short circuit, causing a nasty voltage drop when you hit the brakes. Why that wouldn't blow a fuse is beyond me.

The other symptoms sound like the ignition key switch is flakey, acting as if you are taking the key out with respect to the instruments, but somehow not the ignition system. That the buzzer comes on when the instruments come back to life suggests this possibility.

I dunno. I'm a but stumped, but I have the same car and I will poke around and see if I can come up with a reasonable suggestion.

I still think that a bad solder on the back of the cluster circuit board might be to blame. Have you pulled it out yet? It's actually VERY easy to pull it out once you make a set of hooks from coat hangers.

Thanks for the replies. This problem is the weirdest thing. I've not been able to take the cluster out yet because I've been replacing Wheel bearings and exhaust hangers and brakets and general cleanup underneath. Last night when I finished the wheel bearing job---Took it for a spin, and low and behold--FULL working instrumentation, EVEN after applying brakes and I have break lights. I did not change the bulbs or do ANYTHING electrical. All I did was replace wheel bearings replaced front right wheel bearings and clean and detail under fender and shock area. Today everything still works --no problem with instruments whatsoever.

Do you think it could STILL be fuses or a loose connection or short somewhere? Surely there is an explanation for this.

Sunday nite, Monday, Tue,--Instruments all working perfectly continuously-- because I replaced a wheel bearing????? C'mon, someone out there knows the answer to this.
What kind of VOODOO is this?

2 more days --Instruments working just fine---any comments? What is difference in fuse replacements? What fuse do you think is the best? Thanx.....

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.

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.

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.