Instrument Cluster missing light bulb cause havoc - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 05-24-2010, 07:07 PM Thread Starter
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Instrument Cluster missing light bulb cause havoc

I took out my instrument cluster (97 SL500) due to dim lighting, and found almost 10 bulbs out. I'm going to order all new bulbs, but in the mean time, I've found that driving around with missing light bulbs seems to cause havoc on the instruments: i.e., speedometer & tach sticking at various speeds; whole cluster blinking off and on with seal belt light flashing; outside temperature readout showing about 30 degrees more than actual; fuel level at random levels, etc....

The thing is, I don't really know if the problems I've been having are due to the bulbs or not, because it began infrequently and randomly, but it has now become a schizophrenic acid trip. I can't go a few minutes without the cluster wigging out. But, the bulbs have been blown for quite some time, so I don't know why the problems have accelerated all of a sudden.

Does anyone have any insights? The problem is acute when the accelerator is depressed, and goes ok for a while. The harder it's pressed, the more crazy the behavior. Thought is was a low voltage issue with the battery, but after getting new battery, the problem persists. Alternator seems OK at 14.4v.

I do have a few questions:

1. I noticed that the bulbs have a thin wire attached to the filament. Does this serve to provide current continuity so that even if the light bulb blows, there is still current flowing through to charge the rest of the circuits?

2. Are bulbs necessary to feed power to the entire circuits, or can I just pop the bulbs out?

3. What ill effect is there if I test the car with the instrument panel disconnected? Also, will those test miles not be reported on the odometer?

Thank you in advance!

Last edited by jsap; 05-24-2010 at 07:52 PM.
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post #2 of 3 (permalink) Old 05-25-2010, 04:26 AM
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cluster f-cked

Sounds like a grounding problem. Ten bulbs out at once is unlikely as is intermittent instrumentation. Try running a clip lead hard wire ground from the cluster baseplate to a good place on the body/chassis. If that fixes your problem, wire a permanent ground point. Let us know how this works out.
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post #3 of 3 (permalink) Old 05-25-2010, 12:32 PM Thread Starter
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I try grounding, but after what happened today, I think it might be something else. Today, the car idled poorly, had misfires, ran on 7 cylinders, and the check engine light came on (I don't have a scanner yet--just ordered Actron CP9180). The car runs poorly, and this is rather sudden after getting a new battery (Napa Legend Premium 84 8449).

Things to check:
- Trickle charge the new battery 2amps
- Check ground (are there some obvious locations, other than at the dash?)
- Voltage Regulator?

I've already checked the coils and applied die-electric grease to the coil connectors....
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