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1983 240d
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Discussion Starter #1
My alternator keeps frying. I've had it repaired by a reputable vendor at least 3 times now. It will last about a week and then short out again. I got tired of taking it off and putting it on every few weeks and have resorted to using a trickle charge on the battery once a week or so for the last 2 years. But that is also trying my patience. I'd like to find the culprit and fix it. Its left me stranded too many times now. I've taken it to my mechanic, who found nothing suspect, but was willing to keep looking, indefinitely, at the shop rate. He's not cheap. Today I pulled an alt and a starter (for good measure) at the pick 'n pull so at least I have a fresh start. It doesn't really seem to lose voltage except from normal use (starting, headlights). In other words, it doesn't seem to drain just from sitting, and there doesn't seem to be anything quirky happening just from running off the battery. It's just that the alt blows soon after repair/reinstall. How might I go about discovering the cause? Any thoughts are greatly appreciated.
 

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1983 240d
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Discussion Starter #2
Crickets... I could sure use some help, but perhaps I am not asking properly? Clearly electrical issues can be tricky to track down.
I would like to trace the non-fused circuits in the car, starting with the blue 'exciter' that emminates from the alternator. It seems to disappear at the starter. My glow plug indicator light does not glow, though the relay itself seems to be working fine. After exhaustively searching forums, I gather these are related or on the same circuit? Any suggestions are greatly appreciated!
 

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1984 300D
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If you have a typical Bosch Alternator the skinny wire goes to the Battery Charging light which should go on when you first turn the Key and then go off right after you start up. That bulb need to be a 3 watt bulb because as mentioned that excites the circuit. From there I think it goes to the Ignition Switch.
Bad grounds can cause issues. Remove the ground cable where it bots to the Chassis and clean it so you get bare metal contacts and do the same to the Engine to Chassis Ground Strap which on mine is under the car about where your feet are if you are sitting in the drivers seat.
Another ground problem area is where the alternator bracket bolts to the Engine Block. I have see pictures of some that are extremely rusty.
Some people have run a separate ground wire from the negative battery cable to the alternator casing to make sure it well grounded.
 

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1984 300D
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If you have a Bosch Alternator this is a test cord that I made. See the attached pictures. I used an alternator electrical connector got at a junk yard.
In the diagram the skinny wire on the electrical connector has a 3 watt light bulb in the circuit and all the wires eventually join together and the clip on the end goes on the Positive battery terminal.
Using the test cord bypasses all of the wiring going to the Alternator except the ground.
I have driven around only a few miles with the test cord and it charged fine. Of course it bypasses 3 wires.
Alternator Test Cord drawing FEB 18.JPG

Alternator Test Cord with bulb 2019.jpg
 

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1983 240d
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Discussion Starter #5
Thanks for your reply! I checked the grounds a while ago, they looked fine, but I will pull them off and clean today just to be sure. If I were to add a ground to the alternator, where do I attach the lead to the casing? The starter I pulled at the pick 'n pull had a ground to chassis. I don't think my car has a bolt in the chassis in the same location.

What is the best way to test the exciter wire? Between the starter and the dash bulb I can't physically see it (or at least don't know where to look). Your patch cable looks like a good way to bypass the blue exciter wire to at least rule out other causes. I'll try to put one together.
 

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Ground can be attached anywhere between the engine and chassis. There is already a ground strap by the driver floor, it could be corroded. Get a multimeter, even a cheap $1 from Harbor Freight, measure voltage from battery positive to battery negative, chassis and the engine. They should all be the same at 13.8-14.1v at 1000rpm.
 

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1984 300D
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You can loosen one of the alternator bolts that holds the alternator together and use a forked electrical connector so you don't have to pull the bolt all the way out.
connector.jpg


There is sometimes also some screw holes on the back of the alternator where an anti-static condenser is. You could use those holes or if you have one with the condenser again a forked connector could go under the condenser ground.

Note that the reason for adding the direct ground to the alternator housing is you can have a good battery to chassis ground and a good chassis to engine ground but your alternator bracket can be so rusty that in is not well grounded. Rust does not conduct electricity like bar metal does. Any way it is cheap and easy to do.
 

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1987 190D Turbo
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1,274 Posts
The first question is whether or not the alternator was ever repaired properly. If it was, then the question is, excactly what is failing, and how was it fixed?

First of all, that test lead is a very frightening piece of kit. Discard it before you hurt yourself. The battery connection can potentially carry the full output of the alternator, depending on the state of charge. If you pump 60 amps through that ridiculous pigtail, you will end up with a blob of red hot copper and flaming plastic.

Don't be adding a ground to the alternator, either. The alternator is firmly bolted to the engine, a break in ground there is all but impossible. But if you're concerned, just pick a mounting bolt and polish it up. A ground between the engine and chassis has to be a very heavy duty connection, because it will need to carry the full output of the alternator, so those little crimp connectors would be quite dangerous, even as a test. The chassis ground strap is a heavy braid for a reason.

As to how to test the D+ circuit, that's done for you. If the light goes on when you turn the key and goes out when you start the engine, D+ is working and the alternator is charging. Done and done. If you want to test it "cold", turn the key to the on position without starting, then measure voltage between the small connector and ground. It should measure somewhere between 9-11V, but as long as it's greater than zero, the circuit is working. If you don't have D+ voltage, the alternator will never start charging.

It's not uncommon to see an alternator fry if the charging circuit is intermittent, and then the problem is more likely B+ than ground. If the cabling is removed while the alternator is running, the stator will discharge it's remaining energy in a huge inductive surge. You may find a bad connector or loose cable. What you want to do is to go over the entire circuit, B+ and ground, between battery and alternator. Start by inspecting the condition of the connector at the alternator, then check the battery clamps. If there is a distribution block between the alternator and battery, check that. Check the body ground on the battery negative cable, and the strap between the engine and body. And when I say check, not a visual. Open each connection and buff it up.
 

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1983 240d
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48 Posts
Discussion Starter #10
Thanks so much for all the suggestions! Today I pulled the wiring out to inspect it and clean contacts. It looks fine. Nothing burnt, continuity along the red wire accessible from battery to starter to alternator. Contacts are tight and clean.

Following the wiring harness under the battery it disappears into the firewall, then snakes under the dash. What happens from there? Eventually it goes to the dash, but my prime culprit is a short in the blue wire, so I'd like to see the length of it or test it in some way.

By way of answering some of the folks on the other forum, it's a Bosch, original to the car. The shop where I take it is 2 guys, who pretty much only do alternators. They've bench tested it in front of me every time. I think he said it was a 'rectifier' last time. I don't really know what that does or means. The first time I took it he replaced a bunch of parts, but I didn't really pay much attention because I just assumed it was fixed and wouldn't need to worry further.

There isn't any rust on the mounting bracket. It's pretty clean actually, since I've been messing with it so much. The starter, however, is looking a bit rough. I took some photos which I will post later.

2613352
2613354
 

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1983 240d
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Discussion Starter #12
The first question is whether or not the alternator was ever repaired properly. If it was, then the question is, excactly what is failing, and how was it fixed?

First of all, that test lead is a very frightening piece of kit. Discard it before you hurt yourself. The battery connection can potentially carry the full output of the alternator, depending on the state of charge. If you pump 60 amps through that ridiculous pigtail, you will end up with a blob of red hot copper and flaming plastic.

Don't be adding a ground to the alternator, either. The alternator is firmly bolted to the engine, a break in ground there is all but impossible. But if you're concerned, just pick a mounting bolt and polish it up. A ground between the engine and chassis has to be a very heavy duty connection, because it will need to carry the full output of the alternator, so those little crimp connectors would be quite dangerous, even as a test. The chassis ground strap is a heavy braid for a reason.

As to how to test the D+ circuit, that's done for you. If the light goes on when you turn the key and goes out when you start the engine, D+ is working and the alternator is charging. Done and done. If you want to test it "cold", turn the key to the on position without starting, then measure voltage between the small connector and ground. It should measure somewhere between 9-11V, but as long as it's greater than zero, the circuit is working. If you don't have D+ voltage, the alternator will never start charging.

It's not uncommon to see an alternator fry if the charging circuit is intermittent, and then the problem is more likely B+ than ground. If the cabling is removed while the alternator is running, the stator will discharge it's remaining energy in a huge inductive surge. You may find a bad connector or loose cable. What you want to do is to go over the entire circuit, B+ and ground, between battery and alternator. Start by inspecting the condition of the connector at the alternator, then check the battery clamps. If there is a distribution block between the alternator and battery, check that. Check the body ground on the battery negative cable, and the strap between the engine and body. And when I say check, not a visual. Open each connection and buff it up.
Thank you for your thorough response! I will try these suggestions. Sorry for my ignorance, but what are the D+ and B+ terminals? I gather D+ is the blue wire?
 

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1984 300D
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After looking again at the test patch cable, I was wondering, how do you start the car? It seems to bypass the starter altogether.
No. With the Engine not running you unplug the Electrical Connector on the alternator and push it aside some place safe and you put the Electrical Connector that is on the test cord into the Alternator and connect the Clip to the positive terminal of the battery. And start the Engine and use your voltmeter to wee if it is charging properly.

I only drove with it on for 2 miles and that was just to see if it was possible. But, it is mainly for testing. If your alternator works OK with the Test Cord is an indication that there is an issue from the Alternator Electrical Connector or in your wiring, charging light setup or the Ignition Switch.
 

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1984 300D
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Rectifier-electronic device: an electronic device that converts alternating current to direct current, e.g. a set of semiconductor diodes connected in a bridge circuit.
From what I have seen on automotive alternators is the Rectifier has 6 individual diodes and separate from the Rectifier is another Diode that keeps your battery from draining through the alternator.

Diodes only allow the current flow in one direction.
 

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1983 240d
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Discussion Starter #17
It has been raining all day, and likely will tomorrow as well. I only have street parking. Will pick up where I left off in a day or 2.I n the meantime, a few questions...

Is it possible that the starter is somehow causing the alt to fail? It doesn't have any trouble starting, given a charged battery, but the casing on the starter is pretty corroded (see photos above). As stated earlier, I pulled a pretty clean looking starter from a 300sd, but I'm not 100% sure it will fit, since my 240d has an additional L bracket attached to the front (and to the engine). Any experience with the compatibility of these?

Also, and unrelated, I noticed after removing the air filter housing that the exhaust manifold has some gaps where it connects (with the intake) to the engine. Is this normal? I'd never really looked down there much before. Photo attached.
2613472
2613473
 

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Ok. Repeat defect and failures, repeat faults is what we have.
Ok . This is a diesel car . Important detail!
Alternators fry , fail , burn out when circuit is interrupted during operation. Rectifier pack overload.
Check that your ignition switch is not overloaded with heavy keys, and inadvertently switching off during use. Try wagging the key with ignition on -engine stopped. Wiring from battery usually goes direct to starter solenoid and then on to alternator, worth a check or connection , if any are in poor contact they will warm up so you can check for temperature rise after running . I presume the ignition light stays lit when engine running.
 

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1983 240d
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Discussion Starter #19
Brief update. Junkyard alt and starter are in good working order. Original starter, despite it's ugliness, is also in good nick. Original alternator's solder joints have melted away. So the symptom is overheating, which is melting the rectifier and solder joints. Alt shop guy suggested that bad wiring or bad battery are typically the cause (too much resistance). I was really kind of hoping my corroded-looking starter was the cause, so at least I just replace it and move on with life. I may replace the battery (approx. 2 yrs., still under warranty) just to see if that helps. Next step is to bolt everything back up and test a few more of the suggestions mentioned on this page. Could heat from the engine itself cause the alt to get too hot? I would think that unlikely?
 

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Heat from the engine won’t be the cause. The solder melting suggests a heavy charge current caused by overvoltage feeding the rotor. This is what happens when the natural stabilising effect of the battery is removed while the engine is running, voltage increases which feeds back into the rotor circuit further increasing the voltage output.This happens very quickly. I’m not an expert, but modern systems are regulated by the ECU Old generator systems would do this to the solder if the regulator relay contact stuck, solder would melt after your head lights briefly turned into plane spotters lamps and blowing fuses.
High resistance battery can be a fracture in the terminal inside the case. This would show as very warm to touch after charging for 10 mins. Simple job to change out the battery or get it professionally tested. Love is faith is life.
 
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