Alternator doesn't excite until I blip the throttle

[dude99]

Ever since I bought my w126 the idiot lights have all stayed on until I give the engine a quick rev. I have verified that it does not start charging until I do this with a volt meter. I was under the car today and was going to replace the voltage regulator to see if this cured the problem but some how managed to brake one of the brushes off the new one I put the old one back in for now, honestly the brushes on it didn't look that worn down to me. I've also cleaned every connection that I can find and they all seems good. Should I go buy another VR and see if that is it, or am I barking up the wrong tree?

[sleepstar]

every german car i've owned or driven did this! vw, bmw, MB! dunno why!

[dude99]

The reason I wonder is that my 79 300D didn't do it, nor has any of the ones I've test driven.....

[pawoSD]

My diesel does it if the engine starts at a low idle (generally when really cold out). Some alternators need slightly more rpm to excite, in my case I have an 80A unit off a saab.

Most gassers fire up by revving to 1500ish rpm at first from the cold start injection, so this is never an issue.

[compu_85]

My MB doesn't do this, my old VWs do.

-J

[pawoSD]

It all depends on the alternator and your idle speed....thats about it.

[dude99]

So I shouldn't be worried then. Excellent, one more thing I can check off my to do list.

[Stretch]

Originally Posted by dude99

So I shouldn't be worried then. Excellent, one more thing I can check off my to do list.

It might be worthwhile checking the belt tension though.

[dude99]

Belts are nice and tight

[JamesDean]

My 300SDs would do this. I adjusted the idle speed control bolt and set the idle to around 750. That seemed to take care of the problem.

You can also try the diode trick on the voltage regulator to help out with charging.

[t walgamuth]

The brushes may also be on the edge of worn out. None of mine do this, but I have had them do it when the brushes are failing.

[Brian Carlton]

Originally Posted by dude99

Ever since I bought my w126 the idiot lights have all stayed on until I give the engine a quick rev. I have verified that it does not start charging until I do this with a volt meter. I was under the car today and was going to replace the voltage regulator to see if this cured the problem but some how managed to brake one of the brushes off the new one I put the old one back in for now, honestly the brushes on it didn't look that worn down to me. I've also cleaned every connection that I can find and they all seems good. Should I go buy another VR and see if that is it, or am I barking up the wrong tree?

Your post intrigues me as the SD has suffered this malady for the past three years. I cannot pinpoint the exact time whereby this issue has surfaced.

However, what is quite interesting is the fact that the following components have been changed in the past three years:

1) Instrument cluster

2) Alternator

3) Battery


The alternator was a properly rebuilt unit from NAPA with an unlimited warranty and I'm confident that it is performing properly.

The speed required to get the alternator to charge is close to 1500 rpm.........and sometimes approaches 2000 rpm.


I'm convinced that this behavior is abnormal and need to make some additional tests in an effort to identify the issue.

The possibility does exist for belts that are not sufficiently tight...........as I tend to be toward the "loose" end of the range. I recently purchased a proper belt tension gauge and will utilize it to readjust tension of the V-belts.

[JamesDean]

My gold 300SD has a newer alternator on it. I'll have to look up where it came from. It outputs 13.9-14.1 volts on start up without blipping the throttle. Next time I'm up in the garage I'll double check this though.

[dude99]

I should mention that I also have a brand new battery. I think I'll replace the VR just to error on the side of caution since it's less than $15 (I'll try not to break this one). It just bothers me that the other old mb's I've driven haven't done this. I can't comment on the RPM for mine as my tach is not hooked up. The person that swapped in the diesel motor did install the correct tach amp for it. I don't suppose that the gasser tach can simply be plugged into the amp can it? Or do I have to find a diesel tach?