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Old 11-15-2005, 09:36 PM
speedy's Avatar
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Join Date: Mar 2005
Location: Texas
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W124 Electrical - could use some advice please!

This is a 1991 300D 2.5 Turbodiesel. Monday morning I started it up in the garage and most of the "idiot lights" in the dash stayed on. Some glow brightly (Battery, ABS), some moderately bright (parking brake) and some dim (oil, coolant). Here is what it looks like with the engine running at idle:

The ABS light goes off temporarily when the engine revs above 1000rpm. I searched the archives last night and what I found indicated that the regulator/brushes might be bad. I bought a new regulator and removed the old one about an hour ago. The brushes were very short so I was sure I had found my problem:

BUT I was wrong! After installing the new regulator, there was no change in the dash lights. Some additional info:

I put a voltmeter on the battery. After driving two solid days like this, about 70 miles with half of that before dawn with the headlights on, the battery reads 12.80V at rest. That is fully charged. When I start the car and let it idle, the voltage actually drops down to about 12.60, but goes up to 12.65-12.70 when I rev up the engine. Turning on the headlights takes it down to about 12.45 but again the voltage increases slightly when I rev up the engine.

So, what could it be? The alternator must be doing something or the battery would not be maintaining such a high voltage, but it's definately not working to it's full potential. Should I bite the bullet and buy a rebuilt alternator? Could it be some other electrical gremlin?

Thanks in advance for any replies,

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Old 11-16-2005, 03:05 AM
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Location: Surrey, Beautiful British Columbia, Canada
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What would cause some lights to be dim? My OVR is new.

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Old 11-16-2005, 09:35 PM
speedy's Avatar
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Location: Texas
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How can one be sure that the OVR is the problem? It seems most people with a bad OVR are not able to start their cars - mine starts fine.

I think I am going to remove my alternator and have it tested before I spend any more money on other parts.
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Old 11-16-2005, 09:44 PM
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Location: Florida / N.H.
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Give the OVP [ OVR to you] a good wiggle and watch the light .. the problem with OVP when there are lamp and ABS gremlins is the connections in the base ... give it a try , as it is in line with power feed .. also, pull the fuse on top and clean it and re-install.
These are just diyer things to try when one has no electronic diagnostic tools/meters/schematics..
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Old 11-16-2005, 10:38 PM
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Already removed the fuse, verified that it was not blown, and reinstalled. It is not plugged into a base but is hanging loose near the two computer modules. I will take another look at it tomorrow when it warms up a little.

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Old 11-17-2005, 08:26 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
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The overvoltage relay has NOTHING to do with the BATTERY light staying on!

IF the battery light remains on after starting then the alternator is at fault!

The other lamps are designed to be dim with key on only as a BULB check, making sure that they are ABLE to come on bright if there is a malfunction.

THE ABS lamp stays on bright until the charging system operates!
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Old 11-17-2005, 11:12 AM
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Location: Los Angeles
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Some of the warning lights on my wifes 2000 C280 stayed lit during her morning commute.

While doing a little research I came to this site and tested my battery.

In testing the charge of the battery, you first have to eliminate the "surface charge", or the charge done by driving the car and then measure the charge:

If you have just recharged you battery or driven your car, eliminate any surface charge by one of the following methods; otherwise, go to the next step:

Allow the battery to sit for two to three hours or
Turn the headlights on high beam for five minutes and wait five minutes, or
With a battery load tester, apply a load at one half the battery's CCA rating for 15 seconds and wait five minutes.


Using the following table, determine the battery's state-of-charge:

Open Circuit Approximate Average Cell
Battery Voltage State-of-charge Specific Gravity
12.65+ 100% 1.265+
12.45 75% 1.225
12.24 50% 1.190
12.06 25% 1.155
11.89 0% 1.120

The most common charging system failure is a loose, worn or broken alternator belt, so check it first. If OK, then with a known good battery and the engine running at 2000 RPM or more for two minutes, depending on the load and ambient temperature, the voltage should increase to between 13.0 and 15.1 volts. Most cars will measure between 14.0 and 14.5 volts on a warm day.

Looking at these numbers, I'd check the battery again after it has rested 3 hours and check the alternator as it seems your charging voltage is low.

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Old 11-17-2005, 02:17 PM
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when my car displayed the same symptoms, it turned out to be the voltage regulator. my mechanic tested the alternator and that was charging fine.

mines a 1990 300E 2.6
bmw 2002.
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Old 11-17-2005, 03:57 PM
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Location: Sherman Oaks, CA
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when it happened to me the alternator was bad and the regulator was fine.

I replaced it with a 143a unit and all is well.

Not sure ont he diesel if you can go to that extreme, but I would suspect the charging system.

First telltale sign is the voltage. at idle it should read around 13 volts to 13.5 volts. under load it should read about the same, maybe a little higher.

That is typically how most cars I have seen are at with voltage.


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