Diagnosis Needed - Idiot lights, alternator?, seat belt warning relay? - PeachParts Mercedes-Benz Forum

Bob Albrecht · #1 07-17-2013, 01:52 AM

1991 350 SD

About 2 months ago the idiot lights flickered on, I pulled over, stopped the engine, started, and the problem was gone.

Today the same thing happened again, and this time it is not going away.

The battery, ABS, and fuel low (on fuel gauge) lights are on bright.
The break wear, oil, low washer fluid, and light bulb out lights are on, but dim, and they flicker.

I tested the voltage at the battery:
12.56v not running
12.30v running hmm, not charging

The voltage on the alternator housing is 0.0v.

My fuses all look good.

Some threads here mention the Seat Belt Warning Relay as a source of the identical idiot light problems. Could that cause the symptom at the battery? Should I get one?

Can just a bad alternator cause all of those lights to go on?

The alternator was replaced with a rebuilt Bosh on 6-24-2011 about 26K miles ago. Do they go bad that fast?

jay_bob · #2 07-17-2013, 10:37 AM

It's possible the rebuilt unit has gone bad. Did the place you got it from give you a warranty, and if so is it still effective?

Absolutely a bad alternator causes the warning light symptoms. The bulb check feature relies on the voltage difference when the key is on but the alternator is not producing output.

First I would pull out the regulator and see if the brushes are good. If they did not replace the regulator when they rebuilt the alternator chances are the brushes are toast.

If the brushes look good, I would pull the alternator and take it to your favorite auto store and have them give it a spin.

whunter · #3 07-17-2013, 11:46 AM

Quote:

Originally Posted by Bob Albrecht

1991 350 SD

About 2 months ago the idiot lights flickered on, I pulled over, stopped the engine, started, and the problem was gone.

Today the same thing happened again, and this time it is not going away.

The battery, ABS, and fuel low (on fuel gauge) lights are on bright.
The break wear, oil, low washer fluid, and light bulb out lights are on, but dim, and they flicker.

I tested the voltage at the battery:
12.56v not running
12.30v running hmm, not charging

The voltage on the alternator housing is 0.0v.

My fuses all look good.

Some threads here mention the Seat Belt Warning Relay as a source of the identical idiot light problems. Could that cause the symptom at the battery? Should I get one?

Can just a bad alternator cause all of those lights to go on?

The alternator was replaced with a rebuilt Bosh on 6-24-2011 about 26K miles ago. Do they go bad that fast?

My best guess:
The regulator is toasted, a LONG brush unit MUST be used = one week to six month total life for the short brush in a 300SD alternator..

.

Bob Albrecht · #4 07-17-2013, 11:54 AM

I don't think the warranty is still effective.

I'm going to pull the alternator tonight and have a look.

Isn't the regulator part of the alernator?

Is it easy to replace the regulator if the brushes look good?

DrLou · #5 07-17-2013, 12:38 PM

Bob; Yes, regulator is part of the alternator...

But is easily replaced in situ - no need to remove the entire alternator. Regulator is easily had for $25-sh... It's two philips screws on my -ator, also a Bosch rebuild.

Also: (with several grains of salt here) Sounds like that filckering you are seeing might well be consistent with 'un-regulated' alternating current; ie, the raw output of an otherwise working alternator...

That is to say: If you connect a Voltmeter to an alternator putting out raw current (alternating), you'll see a voltage - depending on the latency of the meter you're using, as the voltage flips positive/negative. However, the battery will never charge without Direct Current - which the regulator gives you. It's also consistent that your running voltage might be lower than your static voltage - also related to the meter's latency.

A smattering of conjecture here (but reliant on a few solid principles of physics) - I'd replace the regulator first. Or, if the alternator's out already, most AutoPartsStores will test it for you...

funola · #6 07-17-2013, 05:05 PM

" I tested the voltage at the battery:
12.56v not running
12.30v running hmm, not charging""

12.30 running is too low.

But
"The battery, ABS, and fuel low (on fuel gauge) lights are on bright.
The break wear, oil, low washer fluid, and light bulb out lights are on, but dim, and they flicker."

I don't think are related to the charging issues and may be a different issue altogether. Grounding problem in the cluster?

Bob Albrecht · #7 07-17-2013, 06:11 PM

I am going to have a look at the brushes on the voltage regulator when I get home.

The symptoms from from the idiot light matches what others have seen related to the Seat Belt Safety Relay.

I hope it isn't two things, but it could be.

Bob Albrecht · #8 07-19-2013, 09:21 AM

I put a new voltage regulator in this morning.
The symptoms are the same.

I disconnected the alternator and started the car to see what would happen. No idiot lights come on, including the the battery light, which is supposed to be on.

Any ideas?

Is the other thing attached to the back of the alternator the diode? Could it be that?

Or, what about the seatbelt warning relay?

funola · #9 07-19-2013, 09:43 AM

Probably interference suppressor. Diodes, 9 of them are inside the alternator.

No battery light due to alternator missing. As for the other lights IDK w/o more info.

Can you post a link of the seatbelt relay?

Bob Albrecht · #10 07-19-2013, 10:51 AM

Here is a link to the seatbelt warning relay.

Electrical Issues........1986 300SDL

Does this still look like the alternator? I can take it out and have it tested.

jay_bob · #11 07-19-2013, 11:11 AM

Yes I think your next move is to have the alternator tested. If you end up having to get another alternator, hang on to that new regulator as a spare.

You might find out that the seatbelt warning issue goes away when you have a working alternator. It too uses the difference in voltage between the alternator output and the battery to figure out it needs to buzz and light up.

Bob Albrecht · #12 07-19-2013, 11:20 AM

Can it be tested by running the engine without the alternator connected and test the voltage between the big alternator connector and the chassis.

Are the two connectors on the alternator both (+) one leading to a sensor?

If I don't see a constant 14V then I would think the alternator is dead.

Is testing at an auto parts store more comprehensive? I don't know what they do.

DrLou · #13 07-19-2013, 11:54 AM

Quote:

Originally Posted by Bob Albrecht

If I don't see a constant 14V then I would think the alternator is dead.

Not exactly. This could well be consistent with a dead voltage regulator.

Quote:

Originally Posted by Bob Albrecht

I put a new voltage regulator in this morning.
The symptoms are the same.
...
Is the other thing attached to the back of the alternator the diode? Could it be that?

Not sure exactly what you replaced. 'Voltage Regulator' I'm referring to is definitely 'a thing' attached to the back of the alternator: Here's what one looks like for your car: (competition LINKS removed).

Again, this installs with two small Phillps screws; pretty easy to do with the alternator still in the car.

Quote:

Originally Posted by Bob Albrecht

Is testing at an auto parts store more comprehensive?

If done properly, yes. normally, these automated tests will look for correct amperage output all along the (shaft) rmp curve. The test should also then load the circuit, and test that output voltage is properly regulated. So you'd get a full test - ostensibly - of both power generation and voltage regulation, all along the output curve.

funola · #14 07-19-2013, 12:12 PM

With the multiple idiot lights lighting up when they shouldn't, it seems there is a sneak path due to bad grounds. I'd suggest removing the battery terminals and clean them, also clean the starter terminals, alt terminals, ground strap at the bell housing.

Charge up the battery with a home charger, start engine and see if the idiot lights are fixed.

Bob Albrecht · #15 07-19-2013, 12:30 PM

I did replace the voltage regulator attached to the alternator with a new Bosch one (long brush). The alternator is a rebuilt Bosch.

The short to ground is something I will look for, but the lights don't come on at all, when the alternator is disconnected and the engine is running. The glow plug light and other lights come on normally before the engine is started.

Because I see the flashing idiot lights with the alternator connected, but do not when it is connected I suspect the alternator.

I think I will see if there is any voltage at the alternator before I remove it and take it in for testing. Are both of the terminals on the alternator (+)?