240D alternator problems - PeachParts Mercedes-Benz Forum

nategodin · #1 05-13-2004, 02:23 PM

Hello,
I bought a 1982 240D a couple months ago, and up until recently it's been great... a real step up from my previous car, a 1992 Dodge Spirit which was a hand-me-down from my uncle.

The problem I've been having is that the alternator isn't charging the battery. Both of them are new. I can easily jump start it, so I know that the starter works fine, and the battery will hold a charge if I use a charger on it. I had it checked out by a local mechanic, who told me that after an hour of tracing electrical connections, he bonked the alternator with a hammer (!!!) and it started working again. He also told me that my voltage regulator was to blame, and that as a result, I would need a new alternator. Apparently he was not aware that on these Bosch alternators, the voltage regulator is a separate unit. He also made some ignorant racist comments which aren't worth repeating... so I gave him his 40 bucks for running his diagnostics and charging my battery, and went off on my merry way. Later that day, the battery was dead again. Needless to say, I won't be going back to that guy! I did remove and inspect the voltage regulator, and aside from a little rust, it looks OK to me... anyone see anything wrong with that? (see attached pics below)

The guy that I bought the car from is a DIY mechanic, and had mentioned that he'd replaced both the battery and the alt before selling it to me, so I got in touch with him to find out what the problem had been with them. He described the same symptoms that I've been seeing... dim headlights, slow wipers, problems starting. He told me to tighten the tensioning bolt on the front of the alternator, and that would take care of it. I have a copy of the technical manual CD, and I attempted the procedure since it looked fairly simple. Tightening the bolt didn't have any effect, although without the tension measuring tool, I didn't dare tighten it too far. I also noticed that the bolt and bracket don't look much like the ones in the manual. (I'll post pics) The bolt is something that the previous owner engineered himself, not sure about the bracket, but it looks bent at the point where it meets the bolt, so I'm considering replacing those with OE parts. I am aware that there are two different designs for the alt. bolt/bracket assembly, the one with the L-shaped adjustment bolt, and the one with the starwheel bolt. If I were replacing both at the same time, could I use either combination? If there's any advantage to the starwheel design, and it will fit on my engine, might as well go with that, right? Any advice would be GREATLY appreciated.

Thanks,
Nate

mbdoc · #2 05-13-2004, 04:53 PM

Try un-plugging the pre-glow relay unit with the engine running & then see if the alternator starts charging.

MrCjames · #3 05-13-2004, 05:04 PM

I'm with DOC

nategodin · #4 05-14-2004, 09:36 AM

Thanks for the replies. I'll try to give a complete description of what I did, I'm new to this so I tend to not take much for granted.

Drove to work this morning, parked the car (engine still running) and disconnected the wires going into the glow plug relay... this was a black box above the front driver's side wheel well, with 2 clusters of wires, each with square plastic connectors that attached to a small grid of pins. There was also a thicker wire held down by a nut, but I assumed that was a grounding wire and left it alone. There was no increase in voltage when I did this. I never get anything above 13V. Any more ideas? The V-belt is definitely spinning the alternator, which should result in SOME electricity generated, right? Maybe the wiring from the alternator is faulty... how can I measure voltage directly from the alternator?

#5 05-14-2004, 10:24 AM

Don't overlook the connector at the alternator. I have seen the connector crimps get loose and cause this problem. Simply disconnect the battery and then pull the connector from the alternator. Pry the plastic cover off the top of the connector and examine.

Good luck,

MrCjames · #6 05-14-2004, 12:17 PM

Check your "Output Voltage" with: Headlights on high, flashers on, Blower motor on High and the fog lamps on. Should stay above 12.9-V, over 13-V would be best.

Try disconnecting and reconnecting the glow relay to see if there is a significant change in output voltage. 13-V does not seem low enough to raise my eyebrow, assuming the alternator is under an adequate load.

jrmd01 · #7 05-14-2004, 01:34 PM

I agree w/Larry. Your battery and alternator is new so that can be eliminated as your source of problems. I would go as far as to replace the wires, at the very least check the connectors.

That mechanic you used was an idiot. Any mechanic who works on Mercedes should know that the alternator regulator is a separate part located in the alternator. Don't go back to him. He didn't tell you anything you didn't already know. Your money would have been better spent going to the dealers to diagnose the exact problem...it would have costed a little more but you would have gotten your money's worth.

Good luck.

MrCjames · #8 05-14-2004, 02:22 PM

Take a DMM (Digital Multi-Meter)or a Digital Voltmeter and try this.

Test Conditions: Engine Running (head lights, blower, hazards ON)

Checking Ground circuit integrity
Connect the meters POS lead to battery NEG post, connect the meters NEG lead to the Alternator bracket. Set meter to DC Volts, >.500mV you have ground circuit issue somewhere.

Checking Positive circuit integrity
Connect the meters POS lead to battery POS post, connect the meters NEG lead to the POS lead at the terminal block located at/near the Auxiliary water pump. I believe your alternator harness should make a connection at this terminal block. Set meter to DC Volts, >.500mV you have a positive circuit issue somewhere.

The results of these tests should give us a better idea of how we can help pin point the cause.

nategodin · #9 05-16-2004, 09:53 PM

Hello,
The connector to the alternator looks OK... I did the ground integrity check... that looked fine. Where is the aux. water pump?

BobK · #10 05-16-2004, 10:42 PM

Lemme add one more idea... up on the right side fender, near the battery tray is a black housing with three screw terminals and wires inside it. Should be two screws with fairly heavy wire and one with a smaller gauge wire. With the neg. terminal disconnected from battery, take the screws out one at a time and take a stiff wire brush to the rign lug connectors and to the screw terminals and the screws. Put it all back together and start the car. now check the voltage at the battery. I have seen oxide buildup on those connections keep a battery from charging.

MrCjames · #11 05-21-2004, 10:19 AM

Have you repaired the problem?

nategodin · #12 05-25-2004, 10:28 AM

I took it back to the guy who sold it to me... he checked it out, and it looks like the cable that runs to the alternator must be bad. I brought the car to Autozone because they do free alternator testing, and their tech agreed. From at the wiring diagrams, it looks like it's a pair of red wires that run from the alternator to the starter assembly. How can I test/replace the connection?

MrCjames · #13 05-25-2004, 11:05 AM

The voltage drop test I outlined earlier in this thread should help you pinpoint your fault. Follow the wires from the alternator, where do they go? If they run to the starter then you will need to test for voltage drop between battery positive and the alternator connection. Should you have a high voltage reading then suspect the connection at the starter motor.

If the wires from your alternator run to a "Terminal Block" then this will be your test point. Test between battery positive and the terminal block, if you have a high voltage reading then suspect dirty or loose connections at this block.

In my many years working on 123 cars I have yet to replace an alternator harness due to excess line resistance! Loose and or poor conections repaired, yes. Harness replacement, no, then again there is always a first.

nategodin · #14 05-25-2004, 05:04 PM

Hello,
The ground circuit looks fine, as I mentioned in an earlier post. I found what I guess is the aux. water pump connection you were talking about... the wires from the alternator are connected to a bundle of wires that go back up to the starter, which also connects to the positive lead of the battery. Another wire from this bundle goes down past the alternator to what I suspect is the aux. water pump. Voltage drop between that and the positive terminal of the battery is ~13V. Obviously the two are not connected, though the they should be, right? The car starts fine if I charge the battery, so the connection between the battery and starter must be OK... leading me to believe that the problem is with one of the wires further down the line.

MrCjames · #15 05-25-2004, 05:40 PM

Try attaching the red lead from the meter the battery +
black lead from the meter to the connection(s) at the alternator. (one at a time) Take meter reading. You should have near 0 volts as a rule of thumb, if not then the connection at the starter motor might be suspect. At which point you simply move the black meter lead backwards through the circuit, first to the starter, then to the next connection and so on working toward the positive terminal on the battery. When you get the near 0V reading than you have pinpointed the electrical fault, it is somewhere between the test point that reads 0 and the one that reads >.500mV.

One last question, was/is either one of your battery leads heavily corroded?