What is killing my alternators? - PeachParts Mercedes-Benz Forum

terminator · #1 08-19-2009, 10:35 PM

Tell me what you think. I have a 1990 300D 2.5 with 435k on it. This has been a great car except that it loves alternators. The first time I ran into this problem was February of 2006, I am sitting at a red light and the idle starts to drop and all accessories die then the car. I have it toed to the dealership (I use to work for Mercedes) and it’s diagnosed as an alternator. No big deal, I get them to install a new (reman) one and all is well till June of 2008. I am on a road trip in Florida and get in the car and with no type of warning my battery is completely dead, no big deal I buy a new battery (Shirley it‘s not the alternator right?). No problems till I get back to NC. A week after my trip with no warning the new battery is dead. I take the car to a friend of mine who owns his own Mercedes repair shop and once again it needs an alternator. Now… August of 2009 I am on a road trip and stop for a break, try to start my car, with no warning the damn battery is dead. Mercedes road side comes out, jumps it off and I drive it home. With a full charge on the battery I have the car checked again and it needs another alternator!!! What gives 3 alternators in less than four years? In each case it’s the diodes in the alternator. All 3 where Bosch. Everything works perfect on this car. If anything ever broke I fixed it. There are no drains on the system when the car is shut off. That’s all I can think of...

mramay · #2 08-19-2009, 11:03 PM

Two suggestions.
First, my W126 ate alternators until I found a loose connection in the main alternator feed wire up to the secondary firewall. The nut was loose on the stud for the big wire and it let the current spike as it made and lost connection.

The second one is that I found a place here in Vegas that made me a custom alternator from new Delco parts for only $195. It almost bolted right in, I needed to whittle on the mounting bracket with the adjustment bolt just a little. I now have a 140-amp alternator that is easily rebuilt, except that it puts out so much power that I haven't needed to fix it yet!

terminator · #3 08-19-2009, 11:12 PM

Did the loose connection fix your problem?

mramay · #4 08-19-2009, 11:56 PM

Tightening the connection fixed it on the W126. I had put a set of dual electric fans on the radiator instead of the viscous clutch fan and just didn't tighten that nut sufficiently.

terminator · #5 08-20-2009, 12:02 AM

Thanks for your input. I will check for loose connections in the morning.

pawoSD · #6 08-20-2009, 12:29 AM

That seems really unusual....something is wrong to cause those alternators to fail like that....I too vote for a loose connection

wbain5280 · #7 08-20-2009, 01:42 AM

Is the alternator idiot light burned out in the instrument cluster?

#8 08-20-2009, 06:58 AM

If you're not at least a little bit savvy about electricity, take it to a local auto electric shop and they can quickly diagnose the problem. If you are electrical savvy, use a DC Voltmeter across the battery with the engine running looking for 13.4 to about 14.0 Volts. If it's not that, do a Voltage drop test along the entire charging circuit. THEN, use a DC ammeter in series with the battery while everything is turned off to make sure there is no maverick current draw.

pmckechnie · #9 08-20-2009, 07:51 AM

Do like Larry says. Also find someone with an inductive amp meter that can check the charging amps. With the engine running at about 1500 rpm and all other electrical devices (a/c, lights, etc) off, check the charging amps and watch it to make sure it drops as the battery charges. It should drop to 10 amps or so depending on the car. Remember, if cooling fans come on this number will go up as the alternator is supplying all power needed for the car and charge the battery. I know you have a new battery, but I have seen many battery's that will charge up but only to a "not quite charged" level and will continue taking the charge from the alternator. This puts an undue amount of stress on the alternator and usually over heats it to the failure point. Also, if possible, never jump off a car with a dead battery and then drive the car to recharge it. I KNOW, this is done all the time but it puts an undue stress on the alternator that is trying to charge a dead battery, run the A/C on high at night with the lights on and the radio on full blast with and the kids in the back seat running the windows up and down. Well I think you get the point. If you have to be jumped, and then drive, at least keep everything off that you can until the battery can be charged with a charger or the alternator can do its job of recharging with out the extra loads.
If any of this makes sense, I hope someone will learn something. If I confused you, let me know and I will try to explain what I meant.
Paul

mbdoc · #10 08-20-2009, 08:06 AM

Bad glow relay can cause that type of problem & is somewhat common!

I'd try getting a used one & replacing it.

terminator · #11 08-20-2009, 11:29 AM

Quote:

Originally Posted by M.B.DOC

Bad glow relay can cause that type of problem & is somewhat common!

I'd try getting a used one & replacing it.

I will check this. My glow plug light stays on longer than any other diesel that I have had. I have also seen the glow plug light flicker a few time after starting it.

The battery light is not burned out but like most Benz' dosen't always warn you when your alternator is not charging.

I did check for loose connections and did not find any. Thanks for everyone's imput.

Oldwolf · #12 08-20-2009, 11:34 AM

435k miles is an impressive number Mike.

Did you pull the regulator and brushes each time to see if they were worn?

Mercedes Guy · #13 08-20-2009, 10:58 PM

Quote:

Originally Posted by terminator

(Shirley it‘s not the alternator right?)

Don't call me Shirley(sic).

compress ignite · #14 08-21-2009, 12:03 AM

mike,

This is gsxr's alternator upgrade tutorial:
http://www.peachparts.com/shopforum/showthread.php?t=53238&highlight=143+amp+alternator

'have done the same...acquired used 143 Amp...New Bearings...New Regulator
less than $100.00 invested.
Now, the Rectifier Bridge for this Alt is about $143.00...But I don't forsee
needing one for many miles.

kerry · #15 08-21-2009, 12:35 AM

Quote:

Originally Posted by terminator

The battery light is not burned out but like most Benz' dosen't always warn you when your alternator is not charging.

Can you elaborate on this? Why wouldn't it? If something is wrong with the charging system badly enough for the battery to die, the idiot light should give some indication of the problem.