1987 300 TD - Warning Lights - HELP! - PeachParts Mercedes-Benz Forum

Gauge · #1 02-02-2009, 05:57 PM

I have a 1987 300TD. The car is in immaculate condition with less than 90K miles. The other day I was driving, when I looked down and noticed that the warning lights were on - all of them. I stopped the car and checked the fluids, etc.., but everything seemed fine. I drove home with no ill effects.

The problem it seems is that the gauge cluster warning lights don't go off after the car is started. The battery light is on, the ABS light, the washer fluid light is on - all the lights you would normally see go off after the car is started. But when I test the car, the ABS is working and the battery is charging, the antifreeze is not low, nor is the windshield washer fluid.

I pulled the gauge cluster out and tried to find a loose connection, but everything seemed good and tight.

Has anyone experienced anything like this? How do I fix it?

I appreciate the help. Thanks!

sixto · #2 02-02-2009, 06:08 PM

The blind fix is to replace the voltage regulator in the alternator. Or at least pull it and check the brushes. Chances are one or both are worn to the point that there isn't good contact with the armature. Both should be nice and shiny. As they wear thin and the spring can't push enough, carbon will form on the contact surface.

Of course it could be a bad ground or bad alternator or bad battery or...

Sixto
87 300D

Jeremy5848 · #3 02-02-2009, 06:08 PM

There is a test circuit that lights all of the lights when you first turn the key to position #2 and wait for the glow plug light to go out (does it?). I'll have to look through my schematics; in the meantime, maybe someone else has already had and solved this problem.

Jeremy

Jeremy5848 · #4 02-02-2009, 06:14 PM

OK, the test circuit uses the charging system (Sixto is on the right track). You say "the battery is charging" but how do you know? What are you measuring? I agree with Sixto that the regulator (part of the alternator, has the brushes and a transistor, feeds current to the rotor, not expensive) would be the first thing to check. You can pull it and check the length of the brushes. If they appear mostly worn down, I'd replace the regulator.

Gauge · #5 02-02-2009, 10:08 PM

Yes the glow plug light does go out as well as the SRS light. All others stay on.

Well shame on me. I inspected all the fluids and tested the ABS and it worked. I haven't driven the car much, but since the battery hasn't gone dead, I ASSUMED it was charging, but I did not confirm that. I will start the car, turn on the headlights and disconnect the battery. If the lights go out, then I'll know that the alternator is not working.

Assuming it is the alternator, should I replace the entire alternator or just the regulator?

BTW - You guys are AMAZING!!!

Jeremy5848 · #6 02-02-2009, 10:12 PM

CAREFUL! Disconnecting the battery with the engine running may not be healthy for the electrical system. There are better ways to check for an operating alternator, such as with a voltmeter, watching for an increase in voltage as the alternator charges the battery.

Jeremy5848 · #7 02-02-2009, 10:25 PM

Quote:

Originally Posted by Gauge

Yes the glow plug light does go out as well as the SRS light. All others stay on. . .

Good information. The glow plug and SRS lights are not part of the test program. If they go out then their circuits are OK and this also confirms the suspicion that the problem is in the charging circuit. The warning lights that stay on are supposed to ground through the alternator (for the test mode before the engine starts). When the alternator starts making electricity, the lights lose their ground and go out. If the alternator does not produce any electricity, the lights continue to ground through the alternator and so do not go out.

Although the regulator is the most probable cause of your problem, a bad diode in the alternator itself could also be the cause. You might be able to remove the entire alternator and take it to an auto parts place for testing. Whether they will be able to tell you if it is the regulator or the complete alternator I do not know. They may want to sell you an entire alternator and that could prejudice their opinion.

If you replace the regulator and the problem continues, remove the new regulator and put the old one back in (no core return for regulators). Then buy a new (rebuilt) alternator and turn in the old one as the core. That way, you keep the new regulator as a spare.

Jeremy

babymog · #8 02-02-2009, 10:33 PM

I had this when the brushes wore and one wasn't touching the alternator commutator ring. I was far from home on Christmas Eve in a blizzard, with the wife and little critters in the 4matic, so, ....

The State Police car PUSHED my 3year old Mercedes into the fire station, I pulled the regulator assembly with my pocket knife for a screwdriver, stretched the braided brush wires a little and re-installed (while the Firemen ate some of the food we were taking to the in-laws and the young-uns toured the fire trucks).

The alternator charged and all lights went off and it worked for a week until the new regulator assembly arrived.

Wish I had photos.

Gauge · #9 02-03-2009, 09:10 AM

You guys are AMAZING!!

This has been driving me nuts for weeks and no mechanic I talked to could figure it out.

I ordered the regulator 5 minutes ago. I'll be popping it in this weekend. I'll let you know if it works out.

I have another electrical problem on my 1999 E300 TD, but I will start a new post so future users can find it with the "serach" function.

I LOVE this website!

Gauge · #10 02-09-2009, 12:20 PM

Well - it looks like you guys were right once again. I received the voltage regulator on Friday and popped it in this weekend. Started the car and....darn, the lights were still on. Talk about disappointing!

I climbed back under the car and figured, well, I'm this far into it, so I pulled the alternator and ran it over to Autozone for them to bench test it. They hooked it up to their machine and it failed! So it looks like the regulator was fine, it was the alternator that was no good.

I ordered a new one and will be putting it in this weekend. I'm quite certain this will solve my problem. I will keep you posted.

Thanks again for all the help.

TX76513 · #11 02-09-2009, 02:11 PM

You have one or two backout diodes bad in the alternator. $3 at Radio Shack

Gauge · #12 02-16-2009, 09:08 AM

Quote:

Originally Posted by TX76513

You have one or two backout diodes bad in the alternator. $3 at Radio Shack

Wish I would have known that earlier.....but even if I bought the diodes, I wouldn't know what to do with them. This actually makes sense, because I figured it had to be something simple. The car only has 85K original miles - this is hardly the life expectancy of a Mercedes Alternator. In fact, the "core" I returned looked as good as the new alternator I bought.

I installed the new alternator this weekend. Works perfectly. Problem solved. Thanks again for all your help.

pholway · #13 02-16-2009, 01:07 PM

where did you order your alternator from?

Jeremy5848 · #14 02-16-2009, 01:18 PM

Quote:

Originally Posted by Gauge

. . . So it looks like the regulator was fine, it was the alternator that was no good. . .

Remember to keep the new regulator as a spare. The brushes do wear over the miles and it's nice to have a good one in the trunk of the car with your tools, just in case. Put the old regulator in the old alternator as your core and return it. If you have already tossed the old regulator and can't get it back, return the core alternator without the regulator, they will replace it anyway as part of the rebuilding process.

Jeremy