Inexplicable (?) alternator light behavior

[awolff280sl]

Driving in the rain today, my alternator light came on. This hasn't happened in years and I will go through the usual drill to try to troubleshoot it. BUT, what is different this time is that the light burns brighter red with higher revs, and dims to almost out at idle. This is just the opposite of what I have experienced in the past, either due to a loose belt or idle too low, where the light goes out with higher revs. Please let me know where to start looking. (I've got a 55amp alternator with an external voltage regulator, all relatively new). Thanks.

[ejboyd5]

Moisture, corrosion, loose connection on the alternator wiring harness.

[Allenh]

Loose wet belt!  It will grab better at low RPMs And slip wildly at high RPMs. The alternator hasn't quite stalled at slow speeds, the regulator is just turning the lamp on at high speed it stalls and the light is full bright.

Check it out.

Allen

[awolff280sl]

Thanks Allen and ejboyd5, I wish it was that simple. Belt is dry and tight, connections are clean, dry and snug. I let the car dry thoroughly with all connectors removed, then replaced, and same behaviour.
I will trial a new voltage regulator next.

[menesesjesse]

Check the battery voltage while it is running and you should get a good indication of if the regulator is doing its job.  When the car is running you should get 13.7 - 14.7 volts for full charging but anything around 12 is not good.  Turn your lights on and other accessories on to see a drop.  If the drop is not too much and the alternator keeps up the regulator is  probably doing its job.  The red light is a no charge indication so if the belt is good I would suspect the connections first and then the altenator. Good luck
Jesse 

[awolff280sl]

Thanks. Had the alternator checked and the diode is gone. So, I'll need a new alternator. Trying to decide whether to stay externally regulated or go with internal.
I also have to see if I'll be able to get it out from under the car since my rotary compressor is mounted up above and I'm not sure that I have enough A/C hose slack to rotate the compressor out of the way to be able to remove the alternator from above. Otherwise, I'll need to evac the system and remove the compressor. Maybe someone else has been there, done that?
I also spoke with Gernold since I was disappointed in the lifespan of this rebuilt alternator I had bought online, maybe 18 months. He told me that he only buys rebuilt alternators from MB because it's been his experience that they last longer, maybe better quality control.

[graphic66]

They come out the bottom,you need to take off the horn. It is a knuckle buster, but possible. The hardest part is the belt tension adjusting part. The original alternator should work fine if you have not added any more electrical demands than AC. You can also just open up your rebuilt alternator and solder in a new diode and not buy another, very easy to do. I have done this in the field on my Unimog.

[awolff280sl]

Thanks graphic66, it's reassuring to know it comes out the bottom, so that's how I will procede. I know that belt tensioning issue is problematic. I have 3 different belt lengths that I plan to try in an effort to find the one that gives me the most room in regard to the angle of the tensioning rod.

[abe280SL]

These days you can get a new alternator for under $80!  Don't need to buy a rebuilt.
abe

[Allenh]

If the belt is good, I believe the brushes will be worn out.  One of the two reasons for that alternator behavior.  I just had one of these apart last year, the brushes are not that difficult to get to but I have CRS tonight and I don't remember how to gt into them.....  If it was hard, I would remember.

Allen

[waqas]

These days you can get a new alternator for under $80!  Don't need to buy a rebuilt.

As far as I know, the units running for that price are all remanufactured (e.g. by Bosch). For example, see this one. The advantage of buying the Bosch remanufactured unit is the warranty that comes with it.

[ja17]

Hello Andy,

I sorta doubt that MB rebuilds their own alternators.  However I am sure whoever does it for Mercedes Benz (most likely Bosch) does it to MB specs.  So if you cannot find a good rebuilder you have to rely on the higher priced MB rebuilds.  I would suspect that a authentic rebuild by  Bosch would last.