'83 240D..... the alt light on the dash comes on at moderate engine speed. Belts are tight, regulator is good, recent battery and the alternator bench tests fine at the independent electrical/machine shop that rebuilds em. Was newly installed a few months ago.

I have checked the wiring harness from alt to starter also from battery to starter..... cut open the outer sleeve and wires have no insulation cracks, no noticable problems. There are no other electrical probs with the car, no fuses blowing no headlights dimming etc.

Right now I'm thinking of running a ground strap from the chassis to alternator mount. Also will probly replace the battery negative ground wire even though it looks fine. Does anybody have any other ideas?? Like maybe there's a problem with ignition key switch or something? All other electric appliances in the car are fully functional.

I don't know anything about electricity but I will pretend that I do for this thread...
I think that idiot light works on something like a diode ...or several... and when back loaded past a certain amount it allows electricity to that idiot light...
I suggest you hook a real ammeter into the proper place ( I do not know where that would be ) and not worry if it is indicating something different from the idiot light.. there is a reason they are named that....
Or check to see if the diode or resistors which affect that light can be renewed by normal people with soldering irons and good procedure... it is obviously a comparator circuit... or something like a zener diode if you are lucky...

If your idle is low then you will have the light slowly begin to glow and when the engine speeds up a bit, the light goes out. If you have no dead battery problems, then you could turn up the idle a little. If you've got AC this will help quite a bit!

Thats the weird part - at idle there is no alt light. It glows at moderate/high rpms as if the belt was slipping but it aint.

I will check the system with ammeter also the idiot light relays if i can find em. Meanwhile will be following up on my negative ground theories. Any other ideas?

It never ever hurts to have great grounds ... from battery to engine, battery to body, body to engine....

240Dog,

Hook up a voltmeter across battery. Wire it so U can read meter inside car.

Note if there is any change in voltage when the idiot comes on.

No voltage change, no problem.

I'm getting the like the voltmeters in the newer cars instead of idiot lights or ammeters. Unfortunately that leaves out all my MB.

P E H

How do you know the belt is NOT slipping? This sounds like an example of how a glazed belt reacts to higher engine speeds! Try some belt grip and see what happens! If the alternator cannot keep up with the cars electrical demands, then the current required will be drawn from the battery more and more. Belts don't necessarily squeal when they slip and if the friction surface has a glassy look to it then this could very well be the source of your problems. Do a voltage check before and after applying belt grip and see if there is any difference in your reading from both battery terminals and let us know what happens!

Assuming the belt is tight and not glazed, I would suspect a diode is faulty within the alternator. It wont show up on a conventional charging test but will show up on a scope. It will continue to charge but not to full capacity. A diode allows voltage to move in one direction, if one diode fails then electricity feeds back and re illuminates the dash idiot light dimly.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

I don't have access to the wiring diagrams at work.. does this alternator have two sets of diodes? One to rectify the current for charging, and a secondary set to rectify it for the idiot light? I was looking at a wiring diagram the other weekend that had that but can't remember whether it was for the MB or the Volvo.

Anyhoo, if so, one of the secondaries could be bad. Depending on how, it might only "suck" current at higher reverse voltages (i.e. higher RPMs). That would cause the light to glow (probably faintly) only at higher RPMs.

Ok, maybe unlikely, but you asked for ideas so I figure you'd like outside-the-box ones too.

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Thanks for the tips..... Have installed a couple of ground straps, no change. But discovered some hacked wiring under the dash for aftermarket radio. Dunno if the alt is dual or single diode. Whats a diode?? Tomorrow if time allows I'll get the volts and amps checked, hooking up a multimeter. Yeah a volt meter inside the car is probly a good idea.

Can you rotate the alternator pulley against the normal direction of rotation by hand? If you can, the belt is loose! Also if the voltage regulator brushes are extremely worn, they will lift off the commutator at higher engine speeds and might cause a dim light. The regulator can test fine at idle, but without removal and inspection of the brushes you cannt tell for sure. Simple job, just two screws retain it to the back of the alternator.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

The belt is not loose and not slipping, it's tight as a guitar string. Regulator and alt are recent, just a few months old and bench test fine. Have a fully operative parts car - might swap the alts just to eliminate possibility of alternator probs. I believe its somewhere in the wiring. Still considering the key switch. Am honestly beginning to believe Mercedes Benz has the crummiest electrics of any car built.

The typical alternator I think has 6 diodes either in it or somewhere close .... three biased one direction and three the other.. giving the same effect as a full bridge rectifier....
In other words.... the reason an old style generator was inefficient was that you could only draw off half the electricity it was making... because you wanted direct instead of alternating electricity..which a generator makes.... an alternator uses electromagnetic coils instead of permanent heavy magnets...and with the diodes is able to convert all the electricity into direct... so it is much more efficient.... so that is the purpose of the diodes...
But it would also not be unusual to have something called a zener diode hooked up to that idiot light for a comparator circuit... so once you check the actual alternator diodes also look for another diode(s) in the circuit...

For you old guys that might remember.... many motorcycles of the early 1950 's used a large Zener diode to use up the excess electricity made by the generator.... they would design the machine for the most load.. ligts on for instance.... but if the lights were not on then that would still be being made by the generator... so a Zener diode was placed in the electrical system to take that excess and turn it to heat and dissipate it into the air.. they looked like horns.... round and had radial fins... and were mounted in the air stream typically under the headlight.... a Zener properly placed in a ciruit acts like an overflow dam... only allowing flow ' over the top ' of the set amount.

you mention regulator and alt are recent. are they NEW? are they Bosch?
chopped up wiring near the radio should not cause charging problems. does the radio work? it should be fine.
I am sorry to mention this again, but Tight and slipping are two different things.
the way the alts are mounted and driven, a VERY tight belt can still slip. get a wrench and hook up to the bolt on the pully, and use a little pull and see if you can turn the motor with it. if you can, then gently turn in the opposite direction, only a little though, you don't wanna turn the motor the wrong direction. if the pully lets go of the belt and you can turn it, you could have a glazed belt that slips even though it is tight... trust me. it happens.
John

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