300E - flickering lights
 

When my '89 300E is at idle, all of the vehicle's lights (interior, headlights, etc.) flicker. The idle is good - not perfectly smooth, but as good as one can expect from a car of this age I suppose. The car runs perfectly in all other aspects. Is this an alternator problem? Voltage regulator?

When the car is moving, the problem disappears - no more dimming and flickering lights. I'm assuming that this is due to the higher engine RPMs and perhaps a more constant/higher output from the alternator? Any info appreciated, thx!

had the same problem on my 88 300e; mine was the alternator; had it rebuilt for about $300, installed. if you have a voltometer, try testing it as it runs. it should be reading consistently at 14.4 volts approx, with little variation.

I've got a voltmeter, how can I test the alternator when running - at battery terminal or do I have to connect voltmeter directly to alternator? thx.

i did it by attaching to the battery terminal; there may be a more direct way of doing it, but methinks the battery is the easiest, safest way.

You might get a stronger indication if you have the headlights on. This will pull down the battery more if the alternator is malfunctioning, making the indication easier to see because of the lower voltage.

If you still have the described voltage (14.4 ?) with the headlights on at idle, it may not be the alternator.

Typical failure mode is a diode going bad inside the unit, resulting in a lower charging voltage because less AC is getting rectified into DC. Could be the regulator too. Since these components are probably all inside your alternator, the typical problem resolution is to replace the entire unit.

However, even with a bad alternator I would think the lights might dim (and flicker some) but should not go completely out.

Another potential fix is to clean the alternator electrical connections - not too likely the problem but a cheap potential fix. :)

Ken300D

Replace the regulator/brushes. Be sure to lightly clean the slip-rings with a little emery cloth.

The BOSCH regulator/brushes gave me only 13.2VDC at load. I used a TRANSPO adjustable regulator and set it for 14.2VDC.

Works wonderful.

:-) neil
1988 360TE AMG
1993 500E

Thanks for all the info everyone. I'll dust off the voltmeter tonight and check it out.

Problem has been fixed! My brushes were completely worn out. Replaced the brushes/voltage reg. unit with a new one from Bosch ($50 CDN) and everything is working perfectly. Steady 13.70 V at idle.

Thanks for the help!

Zeus- that's good news. BTW- what battery are you using?

If you're using anything like a maintainence free battery or OPTIMA be sure to read, "Car Batteries Are Not 12 Volts " at: http://www.landiss.com/battery.htm

In short, it explains why 13.2 or 13.7 VDC may not be enough for certain types of batteries.

:-) neil
1988 360TE AMG
1993 500E

Interesting article, thanks. Brings up the usual cluster of manufacturers' contradictions. The information seems sound though - I have a new (1 year old) Interstate battery.

On your previous post, you said you used a TRANSPO regulator instead of the Bosch? Is this regulator compatible with a Mercedes alternator? Any mods needed?

The Transpo is a direct screw-in replacement.
A good automotive alternator/starter rebuilder should be able to get it (about $28).

It has an adjustable screw potentiometer to adjust voltage. I adjust for 14.2 VDC with A/C and lights on at 2500 RPM.

:-) neil
1988 360TE AMG
1993 500E

I have learned much in the last 30 minutes or so...I checked out Transpo's online catalogue. It's incredibly efficient and complete, especially with the references.

The regulator/brush assembly I received from my mechanic was in fact not a Bosch but a Monark regulator - part number 082966090. It puts out a set voltage of 14.2V. The equivalent Transpo regulator is the IB350.

The correct voltage seems to be a subject of quite a bit of debate. Browsing the Transpo online catalogue returns two possible regulators for my car - 1989 300E - a 14.1V and a 14.5V. It also states that I have an 80 A alternator. Is this correct?

My battery is an Interstate MTP-91 with 700 CCA and 875 CA.

So...after all of this, I should be fine with the Monark set at 14.2V...;)

Thanks again for the helpful advice...I continue to learn.
Transpo Online Catalog

The only way to make sure, is to turn on your A/C at HIGH, and HIGH-BEAMS, then rev the engine to 3000 RPM, hold it steady, and measure the voltage.

If less than 14.2 VDC, I would be suspect.

Transpo also makes "fixed" regulators- I think that's what you looked up.

FWIW, the BOSCH regulator (fixed) says 14.2 VDC (printed on regulator), but I never got that high even at 3500 RPM and no A/C or lights on.

:-) neil