Alternator help - PeachParts Mercedes-Benz Forum

Rmcfarland · #1 11-02-2006, 01:50 AM

I really could use some advise. My '79 240D won't hold a charge! I highly doubt it's the battery (New in May 06) and the starter is ok, I believe, since with a jump start it cranked with a vengence I haven't heard it do for a while. But the battery won't stay charged! The amp light does not come on during driving. Anyway, my questions are:

I installed a new head unit a few weeks ago, and a small 300w cheapo amp with a sub. I run it at a very moderate volume, could this just be draining my battery? I know the alternator's not very big, but still..... I would hope it could handle this.

Does the alternator have an internal or external regulator?

What voltage should the alternator produce (to check if it's OK)?

And, if alternator is dead, will Autozone or Advance etc. carry them? I need the car to get to school every day! Thanks for the help.

maddogg20 · #2 11-02-2006, 01:58 AM

Hey Bro,
I had the same problem, it would randomly not start and I'd need a jump, and the car would overheat, and I didnt put 2 and 2 together until my charging light came on. I have an electric fan instead of the stock one, and what happened was the alt wasnt charging the battery and the fan wouldnt go fast enough and the car would overheat. I just got a new battery tonight because I think the old one was tired, because I needed to be jumped once again, and I replaced the alt the other day. Just to let you know, out alternators, (I have an 83 300td by the way) are very small. Like 55 amps, and at idle ud be lucky to make like 20 amps, I would say that you should look into the alternator upgrade that a few other members have done on here. They got an alt from an older saab, like an 89, and those alts push out around 115 amps. Do a search for alternator upgrade and see if it looks like ud wanna do that because our cars were totally not made for any high demand electrical devices. I hope this helped u, from one newbie to another :-)

Andy

carnut · #3 11-02-2006, 01:48 PM

The regulator is internal. Two screws and it slides out. Very likely the problem. You need to verify amp output, not voltage. With a load tester, it should put out approx 45 amps. Make very sure the belt is not loose. You should not be able to rotate the pulley on the alternator by hand against normal rotation. (WITH THE ENGINE OFF!) Try www.dieselgiant.com for regulator replacement. The alternator itself very seldom fails.

vstech · #4 11-02-2006, 01:57 PM

I have an 80 amp in my 79TD, and the original 65 amp in my SD, but both had the same problem when I got them. the belts need to be NEW on the alternator. if not new, no matter how tight you make the belt, it will slip under load. and of course if you are under load, the battery discharges... I put on new belts, and have not had a problem starting since.
John

Rmcfarland · #5 11-02-2006, 06:07 PM

Well, I know voltage isn't the end all for diagnosing alternators, but I did hook a voltmeter to the battery leads and got the following:

~11.8 volts off (needed a jump)
~13-14 volts running
~All lights, radio, brights, flashers, blower dropped it to 10...then 9....then 8.
~Let it off and it never raised back past 10.5 with no load. I think the jump gave me the temporary voltage boost.

I did have the car on high idle the whole time, and revved it lots. Voltage actually dropped slightly when revving. After reading lots of forum posts, I decided it was probably the regulator. I took it off, and the brushes had maybe a couple millimeters left. I think they are shot, especially since I have had slowly growing power problems, not sudden failure. I did not hear/see belt slip, but i could be mistaken. So, I ordered in a new one, will replace tomorrow and report back. Figured worse case, I have a spare. Thanks all for the help.

Jeremy5848 · #6 11-02-2006, 06:16 PM

Hopefully the new regulator will solve your problem. However, there is another, less likely possibility.

Part of all alternators is a set of solid-state rectifier diodes that turn the alternating current (AC) generated by the alternator (alternator = alternating current generator) to direct current (DC). If one or more diodes fail, the alternator may seem to work until you load down the circuitry with lights, glow plugs, ACC, etc. Unfortunately, the diodes are part of the alternator rather than part of the regulator. The only fix for a fried diode is to replace the alternator.

#7 11-02-2006, 06:23 PM

You don't have to pay Bosch to rebuild your alternator. There are independant shops that usually specialize in alternators and starters. They can rebuild your alternator to generate more current while they are there too.

Rmcfarland · #8 11-02-2006, 09:25 PM

Well, hopefully the repair does work. I need my car. But, if I do have to get the alternator rebuilt, what kind of price would it be? I can get a crappy aftermarket one at the auto store (forgot which one) for 88 bucks. Probably not the best, but it has a lifetime warranty, so if it blows up, just get another. A new Bosch was ~170 bucks from the little European shop in town.

Also- is there no way to replace those diodes, if they are the problem, myself? I am no expert at car repair, but I am ok when it comes to electrical (going to school for EE, in fact.) I don't want to rewind an alternator or anything, but has anyone messed with these diodes before? Thanks again.

FamilyGuy · #9 11-02-2006, 10:15 PM

Go to an "auto supplies" store instead of a "auto supermarket", I recently got a rebuilt Bosch 65 amp alternator for $119 + core.

blueranger · #10 11-02-2006, 10:35 PM

you have an electrical leak and its usually caused by a wire with broken insulation touching the car body.... I just had this problem and it took forever to figure it out... it was the positive battery cable arching up against the body... I slid some heater hose down it to insulate it and it solved the problem.

vstech · #11 11-02-2006, 11:25 PM

stupid upgrades... stock belt is supposed to be 15385, but it is too small, so I decided to play around with belt sizes until I find one that fits well... it looks like a 17408 or 17410 is going to work best for me... too bad nobody stocks that size. rassin frassin smagickal ..... three hours to change a stupid belt! innermost belt, loosen a/c. remove, loosen ps pump remove.... try belt, return try next belt return, repeat, aaaaaahhhhhhhaaaahhhhhh.
I guess the 80 amp alt is a little more outboard than the original 55... I guess, I'll try autozoo or NAPA tomorrow, driving the gas guzzler tomorrow... almost out of gas, so I'll have to fill it up... 80.00 down the drain for gas in it.... arrgh. discover ps pump belt is cracked up too, price it, not in stock. special order it.... Ya gotta love driving 27year old cars...
John

patbob · #12 11-03-2006, 02:45 PM

Quote:

Originally Posted by blueranger

you have an electrical leak and its usually caused by a wire with broken insulation touching the car body.... I just had this problem and it took forever to figure it out... it was the positive battery cable arching up against the body... I slid some heater hose down it to insulate it and it solved the problem.

I had a similar problem on an old Horizon many years ago.. battery wouldn't charge, ampmeter indicated fine, no alternator idiot light. Worse, car was only running off battery with no indication of the fact. Turned out to be a bad alternator ground.

Made for an interesting story though. Wedding day

, -15F, 60mi from home, power fade at 60MPH, coasted to a stop on an offramp, had to be rescued by father-in-law... Would have made it too if I hadn't been listening to some religious motavational tape while I was driving that my brother strongly insisted I needed to hear. If it had only been a diesel..

FineOlBenz · #13 11-04-2006, 11:35 PM

I just went throught the same low voltage problem recently. Car could sit for weeks and be ok. But if I drive it the battary will be dead in a 150 miles. I found that the GLO PLUG TIMER WAS STUCK ON!!! Nothing like leaving 5 toaster ovens plugged in and left running

After you get it fired up again unplug the wirng harness from the glo plug timer and check your volatage again.

Rmcfarland · #14 11-08-2006, 06:50 PM

Well, here's what I did.

Friday:
1. Replaced regulator in alternator.
2. Drove home (~40min on highway)
Car started once, but then later that night, had to glow cause it had been sitting a bit, and it wouldnt go.
3. Got a jump, drove it back home.
Saturday:
4. Got to looking around, belt looked a little loose on alt. hmmmmm.
5. Investigated closer. Belt is a piece of junk and loose. Tightened old belt and liberally applied belt dressing.
6. Put battery on battery charger overnight.
Sunday:
6. Drove car home in the dark, used headlights, blower, radio, etc. Seemed fine. Been driving every day since, and it has been ok. Seemed a regulator, and/or old crappy belt are the problem. I am going to get a new belt before it breaks. Naturally, it has to be the innermost belt....

I have noticed there is a few different belt/pulley configurations that could be used. I was wondering what is proper. Mine is as follows:
Innermost-alternator and fan
Middle-Air conditioner compressor
Outermost-Power Steering Pump

Is this the correct setup, so if I get an alternator belt, it'll fit right? Thanks for the help, and sorry for the slow reply. At least I seem to be back on the road.

carnut · #15 11-08-2006, 08:17 PM

All the belts should have numbers on them. Go by the numbers to get the proper length. The belt routing you mention appears to be correct. If it were my car, I would change ALL the belts because all will be removed to get to the alt.