Hi,

My alt has died and at the local part shop they show two possible alternators.
A 110amp or a 120amp.

How do I tell which one is in there now? Or does it matter?

Thanks in advance.

Ian

It would be printed on the alternator itself. If the price difference is slight, and the connections are the same, my as well upgrade

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

Pelican Parts on this page lists and early and late alternator for this car, but as a US Import for 94 and 95, both as 115AMP. You want to know about parts, check the catalog, though it is true that the catalog does not cover your car exactly.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Thanks guys,

It's a $100 more but I'll grab the 120 amp version. Not so much cause I think I need the power but because it's in stock I'm hoping to get it installed on Sat.

The procedure in the FSM looks pretty easy. Any surprise "gotcha's" I should know about?

So... I've got a bit of a headscratcher.

My old belt tensioner was giving out and I had lots of slack in the pulley, battery wasn't charging.

I replaced the belt tensioner. Now the belt was tight but still no charge.

I pulled the alt, had it bench tested (it was not charging) and replaced it with a rebuilt 120 amp alternator.

Now, the new alt is in, belt is reasonably tight, but I'm not getting a charge.
Battery reads 12.4 with the car turned off.
At 3000 rpm and lights on I get 11.65

It seems unlikely the rebuilt Alt is dead right out of the box.
Is there something I'm missing?
I've replaced plenty of alternators in other vehicles and never run into this.

I have not yet pulled the battery clamps and cleaned the connections, but given that the battery takes a charge from a plugin charger, I can't see that the connections could be so bad that the alt wouldn't charge.

Help a guy out, cause if I bench test the alt and it's fine then there's gonna be some cuss words flying.

Many thanks,

assuming its the same 2 wire bosch alternator most benzes of that era used - do you have the battery indication on the cluster when you switch on the key?

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Very astute question. No, the battery indication light doesn't come on. In any position, running or not.
And yes, two wire alternator.

the battery lamp on the dashboard is mandatory to excite the alternator and hence start charging.

It is crucial that lamp be 3W only.

To test the alternator - get a 3W bulb and holder to test the alternator. The wiring on MB is usually junctioned in a heavy duty block near the firewall. the small dia alternator exciter wire is also there - undo it and use the bulb there. Key it on and you should see the bulb glow bright, start it and it should go out and the battery voltage should reach 14V.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Thank you very much!
I'll test that tonight after work.

Well a big thanks for pointing me in the right direction.

I didn't have time to pull the instrument panel out or figure out which was the correct wire at the junction so I just put clipped a test light to the 2nd terminal on the alt.

Probably not the best way but I got positive results. 14.5 volts at idle. and 14.13 with the lights and heater on. My stress levels have dropped a ton.

None of the warning lights in the area of the charge indicator (left side of the panel) are working. (other than the parking brake and high beam indicator)

Seems unlikely that all three bulbs would be burned out but I'll check that.
Is there a known fault in the instrument panel circuit board that could be responsible for this?

You are most welcome - this is what this place is for, sharing information.

The charge warning lamp is a direct path to the alternator, I dont recall how a 129/140 is done up between the cluster and key but in all MB fashion it would be quite similar to other models. e.g. on my car which is a 95 w124 the charge lamp positive (15 in bosch talk) is supplied by a fuse in the fuse box which only gets hot when the key is on.

This charge lamp ground (from the alternator when engine is off and key is on) is also responsible for the warning lamp check, that is why when it illuminates all the warning lamps light up and go out as soon as the lamp goes out. It prevents backwash from other lamps through diodes in the wiring. I think your car would be very similar.

btw the indications that do not get a lamp check test are the high beam and parking brake.

For starters you can check for all fuses to be intact.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

I'm happy to report a successful fix.

With the advice of Zulfiqar, I was able to test my alt to make sure it was good, then do some better research which led me to this thread: 89 300e charging sytem and dash lights - Benzworld.org - Mercedes-Benz Discussion Forum

Turns out the wire from the 2nd terminal of my alt to the x4/10 junction had failed. A quick replacement and everything is working as it should.

The hardest part of the fix was printing the wiring diagram and then figuring out where the junction box was.

Cheers,