Its not the first time this has happened. Specially the lifetime alternators! Some people win, other people lose. It goes with the starters too...so far I have had luck with both.

Well at least you learned a few things

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Ok, alternator #3 is in place. It's giving 13.8vdc after the GP relays click off. Alt #2 was giving 14.4 until it just quit. Not sure what to make of that.

I had AutoZone test alt #3 before I put it on the car and it tested good, three times in a row in fact. So that's great. Then I put it in and got 13.8. Seems ok but a bit lower than last time, so I drove the car up to AutoZone and had them run their on-car alt test. Came back with failed voltage regulator. Alt still giving 13.8. I'm guessing their machine is wrong.

Thing about their test is that they start with the car off, hook to the battery, push a bunch of buttons then have you start it. Now I have my aux fan set to run whenever the key is turned, plus the GP relay does it's thing and continues to run the glowplugs for a while after start, so voltage after a start is gonna read low for a bit. I'm guessing that's why it said failed voltage regulator, but I dunno. I think I'm going to have them test again with the GP relay unplugged and the aux fan unwired. As of this morning I'm still reading 13.8VDC.

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Own:
1970 Mercedes 250c, 1976 International Scout II, 1982 Mercedes 240d, 1997 Honda Accord Wagon, 1998 Acura Integra Type-R, 2009 Mazda5.
Owned:
1974 International 100 pickup, 1975 International Scout II, 1981 Mercedes 240d, 1985 Mercedes 300SD, 1988 Honda Accord, 1988 Ford Thunderbird, 1991 Honda Accord, 1996 Geo Metro, 1996 Ford F150, 1996 Dodge Grand Caravan, 1997 Honda Accord, 2003 Acura TL-S, 2003 Yamaha V-star.

Hi Jeffroooo,

13.8v is a good enough voltage to charge a battery. It's at the lower end of the scale, but anywhere from 13.8-14.4 is correct. Lower voltages of 13-13.7 are also acceptable but not correct for a Bosch system. It's a bit of an old debate on 'what should the high voltage charge be'. Bosch have always said 14.4/5v but other car manufacturers say 13.8v because they think anything higher might fry sensitive electronics.

Are these new alternators you are getting or used?

There are plenty of people selling 'rebuilt' alternators around that replace the voltage regulator and that is about it. Give it a lick of fresh silver paint and that's 'fully rebuilt'. Hope you get better luck with this one.

Remember if your battery is fully charged, your alternator output will drop off. Drain your battery down a bit (engine off, lights on) and then see what your alternator puts out. If you changed your battery between tests, then that will also give you a different result.

A tired battery will place continuous strain on your alternator. Best to replace your battery when you feel it's on its way out.
Trying to get the last out of a battery is taxing on the alternator (and electric motors).

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1995 E300 Diesel

I have/had the same issue with my SD.

Reman unit 120 days old, no battery light, but jumping a ground the the blue lead=battery light. Not charging, killed a new battery, starter is fine.

I installed a "new" Iskra, that's what the fella at Motor City Reman siad. No where on the packaging or paperwork is the word Iskra. My mechanic says it's a reman. It has a two year warranty tho.

Problem solved.

It's charging at 14.33 right now.

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83 SD

84 CD

I think I'm going to have to entertain the thought that perhaps my car is killing these alternators. Alt #3 just crapped out this morning. Got about 100miles out of it.

Maybe GP relay? I don't know. It seems like it's shutting off properly. I can put a multi-meter on the battery and see when it clicks on and off by the voltage change. Once the glow plug relay has clicked off, unplugging it does not elicit a change in reading. I've also tried pulling the fuses one by one to see if any voltage changes occur, they don't. I don't regularly run a lot of electrics either, hell it's been cool out so I haven't even been running the A/C. Since I do most my driving in the day it's been the radio and that's about it except the brake lights and blinkers.

Battery tested good and comes back with 12.7vdc when fully charged.

I read back through this thread and noticed there hasn't been any mention of checking all of the grounds for this problem. In particular, the engine-to-chassis ground strap. If this strap is corroded/loose, it can cause very intermittent problems. This could also be the reason the alternator keeps failing. Also, check the grounding of the voltage regulator to the alternator casing.

I've encountered both of these problems and scratched my head for a bit, trying to resolve a no-charge problem.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Jeffroooo,

How are you with the multmeter? You're really going to have to start measuring currents now.

Fastest way to measure consumption (or leakage) is with a clamp on meter. Once you have found there is a large draw on the battery, you can then start checking the sub-circuits one by one, or consumer circuits as they are often referred to in the land of auto.

Alternators are for keeping your battery charged and shouldn't be used to charge a flat battery. If it is your only option, say if you were away from home, then sure you have to. Alternators only like a 75% duty on them, and not continuously either.

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1995 E300 Diesel

I've checked them in that I've physically inspected them for corrosion, tightness, and measured the ohm load between the major components, such as the engine block to chassis and batter negative terminal, aren't too high. I reliably see .01 OHMs in all cases. I've not checked the OHM load from the alternator positive leads to the starter lug, and from the starter lug to the battery. This is on my checklist for tonight. I did check the grounding of the voltage regulator to the alternator housing with alternator #1, but not #2 or #3. I just assumed they'd be screwed together decently well enough and they came with new voltage regulators.

Well like I said in the video, I'm no electrical genius. I don't own a clamp-on meter. I'm guessing they measure amperage as well as voltage? I'd like to own and play with one, maybe this will be the impetuous to finally purchase one. My battery is typically fully charged or nearly so when I swap on an alternator, I've always charged it with my external charger prior to swapping on a new alt. I've tried pulling the fuses one by one while watching battery voltage with no change. Speaking of continuous draw though, I have had my aux fan wired to run continuously on key-on. I did this because the switch on the dryer has been flaky. Do you think that would do-in the alt? I know plenty of other cars with electric fans that run all the time without that problem.

The clamp on meters are good because you can go beyond the usual 10amp maximum.

Auxiliary fans pull a lot of juice. Mercedes engineers would have performed a maximum demand calculation when selecting the alternator. When we do maximum demand calculations, some items count for 100% of its load but others might only be 50%/75% etc. This is presuming not all consumers will be running all of the time or at the same time. eg electric windows- a relatively high consumer, four of them, but would all four be operating at the same time, not likely.

If you are running an older amperage alternator (when I say older, I mean as in terms of what was standard on earlier Benz diesels) such as 50/60/70 amps, then you will definately be running into problems with permantly 'on' auxiliary fans, especially during start ups with the glow circuit add to that cabin fans and headlights etc.

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1995 E300 Diesel

Well Autozone messed up and gave me alt #4 without taking alt #3 back. So now I've got two again. What a deal.

First thing I did was remove the intake and exhaust and intake manifolds again. (I did this earlier this summer to install that shiny new block heater there.)



Not a fun job but I've had much worse. This gave me good easy access to the wiring. I found a reading of 0.4 ohm on the two wires from the alt to the lug on the starter, and 0.4 from the two wires on the alt to the positive terminal on the battery. My multimeter was giving 0.1 connected to itself. Still, I thought 0.3 ohms of resistance was too much for a cable 14 inches long. After cutting off the old connectors at the alt, I found the bare wire was giving 0.1 ohm resistance in the same tests. So I crimped on the new connectors and assembled them in the new plug, then swapped in alt #4. Put everything back together and fired it up.

Since I've been driving the last couple of days with no alt, the battery when disconnected was reading 12.1vdc. Pretty low but then again there is something over a dozen starts in there, and I've been running the radio when driving as well as lights and wipers here and there. The aux fan however has remained disconnected. Once the engine was running at idle and the glow plug relay shut off, I got a reading of 12.8. Not very encouraging. It raises up when I rev the motor, maybe as high as 13.5. It was late though so I just said "f-it", put the tools away, shut off the car and placed the external charger on the battery. I'm hoping to see higher voltage on the battery tomorrow morning after it's charged back up.

Tommorow I will also have access to a clamp on meter. So that's good, but I was thinking. If I had some giant parasitic draw somewhere, wouldn't it have munched my battery over the last couple of days driving with a non-op alternator?

My alternator puts out 12.5v at the battery and so far no issues....and I am pretty positive it's the original alternator to the car..

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Don't think you have a parasitic drain-as you said, it would have come to surface when you disconnected your alternator and drained your battery.

13.5v will charge your battery. It's not ideal but it will do providing you don't keep those aux fans locked on.

A bit concerning that you only get 12.8v out of your alternator at idle. At an engine idle speed of 600rpm, your alternator should be spinning at roughly 1200rpm (alternator pulley is about half that of your main pulley) which is ample to get results. You won't reach spin-up speed until engine rpm is 750-1000rpm which is where your regulator should be working to cap your peak output.

It might just be your alternator, some models are better than others. I've owned cars over the years that have given low(ish) output like yours is doing.

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1995 E300 Diesel

I would move along to voltage drop testing. For example with car running any voltage present between the centre of negative battery post and block? Centre of positive post and output of alternator. Resistance checking is not dynamic testing where under load voltage drops are.

There are some indications as well you could just be getting a string of bad rebuilds. I would not take a Mexican rebuilt unit home. They may have improved over the last few years though to be fair.

The logic being your original failure was an internal alternator mechanical situation causing the original alternator in you event to fail is almost beyond question. It was not external to the alternator in my opinion. So if we assume that this is true there may be no other issue there.

Believe it or not if the voltage drop tests under load especially on the negative loop where basically satisfactory. I would grab a used alternator from a pick and pull. You have to establish if your problem is really external to the alternator. My feeling is the junkyard unit would survive if you voltage drop tests where within reason. You really want clear of this headache by now. Test the used alternator before installation at one of the free testing places. You do not want to increase the variables in the process.

Sometimes I cannot see the trees for the forest either. Also get an old harness that plugs into your cigarette lighter for your voltmeter to keep a watch on the system voltages for awhile. These are just my thoughts, suspicions and what I might do if it where me.

Thanks guys, your posts have truly been helpful. After charging the battery all night I came out to a battery that was reading 12.8 at rest and disconnected, then 14.39 at idle once the GP relay clicked off. After a 20 mile drive, it was still reading 14.39 at idle. I'm picking up the clamp meter at lunch today. I drove with all my electrics off except the radio. The aux fan remains disconnected for now. After my drive I reached down and felt the alternator, it was warm but not uncomfortably so. I also felt the GP relay and while it was slightly warm to touch, it felt like it was from radiant heat inside the engine bay and not from within the relay itself.

If I have an excessive draw that's killing these alternators, it's something I can apparently switch on and off. Which means lights, wipers, A/C clutch, aux fan, or cabin blower. Ya know, I just realized that my power antenna recently seized up. I recently put the switch in the down position, but it's not typically there. I wonder if there be a short inside it.

You do not really need a dc clamp ammeter. The voltage from your cigarette lighter would sag substantially if there where a transient system overloading. In fact you cannot monitor for transient situations in service with one.

Even glow plug relays coming back on intermittently will not usually kill the alternator but will lower both the stored charge in the battery and reduce the voltage coming from your cigarette lighter. This is about the only practical way to monitor your system for the next while.