W124 m103 alternator whistle

[snow200]

Hi. I have many times combed through the forums for help but this is the first time I've asked a question.

I have a 300TE which I have been slowly restoring, it having sat unused for 10 years. New fuel tank, pumps, injectors and distribution head, pumps, SLS struts, pipes, spheres... and a new headgasket and head rebuilt. Etc. It is running pretty well now but it has an annoying whine / whistle coming from the alternator. This is from idle and low revs. It is louder (not higher pitch) with lights etc on and when taking my foot off of the accelerator. I have used a hose to isolate the noise - it is definitely the alternator. The sound goes away if the alternator cable is unplugged.

The alternator, which is a 70A Bosch re-manufactured unit with about 10K miles on it is charging fine and has been checked by an alternator specialist. It's correct for the car.
Car has a new correct MB battery with good ground.
Car has a new correct MB ribbed belt tensioned properly.
Voltage Regulator brushes are like new.
Swapped out OVP and Fuel pump relay (not because I thought it would make a difference but because I have spares!)
Battery is 12.5V standing. 13.5V running. 13.2V running with fan and lights on.

I have tested a used genuine Bosch alternator and it makes the same noise. I feel fairly certain that it is not the alternator, though accept it could still be.

The car has an after market Cobra alarm, which works, but also the wiring remnants of an earlier alarm and a towbar. I have no electrical problems with the car - everything works. It is just the noise.

Does anyone have an idea of the next thing to look at? A dud MB battery? The alternator cable? The ECU? An earth point somewhere? Perhaps it is normal on the m103 (previous 200TE, 200E and 220E didn't make this noise)?

Hoping someone can help!

Thanks

Chris

 

[LostKiwi]

13.5v running is on the low side. I'd expect 13.8v absolute minimum through to 14.5 max. Ideally 14.2v is the optimum.

 

[mioba]

Greetings to the first post.

The post is a bit consfusing as you say its the alternator and then its not the alternator.

Since using an alternative alty the noise is still there, I think you can rule that out. But as the noise is more under load then I get the feeling this is another pulley that the belt runs over. Take the belt off and inspect all pulleys for free rotation, a car sitting for 10 years is also not good for the tensioner. they like to move.

 

[snow200]

Thanks Lostkiwi. I will look to try a different battery. Mioba, the tensioner and other pulleys were smooth (water pump had a little play though). The whistle definitely comes from the alternator but I am suspecting it is because it is having to work too hard rather than the alternator having a fault, since two have made the same noise. A low battery might do this I guess. Is it worth trying a 2nd hand ECU?

 

[DREAMER NO2]

Remove the alternator drive belt , and spin the pulleys if you can hear them they will need replacing .

 

[LostKiwi]

Thanks Lostkiwi. I will look to try a different battery. Mioba, the tensioner and other pulleys were smooth (water pump had a little play though). The whistle definitely comes from the alternator but I am suspecting it is because it is having to work too hard rather than the alternator having a fault, since two have made the same noise. A low battery might do this I guess. Is it worth trying a 2nd hand ECU?

Battery will not affect running voltage from the alternator.
ECU will not affect the alternator on that car.

If charging voltage is low it suggests an issue with the alternator or its regulator (if separate). Or earth's between engine and chassis and chassis and battery.

 

[snow200]

Battery will not affect running voltage from the alternator.
ECU will not affect the alternator on that car.

If charging voltage is low it suggests an issue with the alternator or its regulator (if separate). Or earth's between engine and chassis and chassis and battery.

Battery earth is good. But I will look at engine to chassis next… thanks