New alternator wierd idiot light behavior - PeachParts Mercedes-Benz Forum

amptique · #1 07-03-2002, 12:18 PM

The alternator on my 85 300SD finally died so I replaced it with a rebuilt one. I also replaced the alternator wiring harness. Since replacement I am getting some strange behavior with the "idiot lights" and with charging voltages. Upon first turning the ignition switch the antilock light and glow plug lights come on but the battery and brake idiot lights do not. After the engine starts the antilock light stays on. At this point the voltage at the battery measures 12.3 volts at idle speed. If I rev the engine to about 2300 rpm (either in park or by driving) the antilock light goes off. The voltage at the battery now measures 13.8 volts at idle speed. The antilock light will stay off until the engine is shut down and restarted. I had a spare voltage regulator so I tried replacing it -- no change in behavior.

Any help would be greatly appreciated.

BIll

JimSmith · #2 07-03-2002, 03:17 PM

amptique,

There are a number of potential problems but to sort through them may be tedious.

First, the new alternator may be defective or undersized, and unable to put out the necessary voltage to trigger the dash lamp response. In this case the battery is still providing juice and you may wear down your battery.

Next, the connections may be less than ideal leading to Voltage drops from the alternator, which will also give you the response you see. Was anything covered in a preservative on the new wiring harness? If so it may need to be removed to make a good contact.

I can't remember, but how is your battery? If starting is really stressing out your battery, it may be drawing so much current to recharge the alternator just can't keep up. You did not specify where you got the rebuilt alternator. I would suspect it may be under rated for this service.

What is your idle speed? It may be that the car has a really low idle speed and the alternator just does not have the speed to make the power at idle when it is also recharging the battery.

By the way, once you get to 13.8 volts by revving the engine, you say it stays there at idle. Do you mean fast idle or normal idle?

Good luck, Jim

amptique · #3 07-03-2002, 03:53 PM

Jim,

The alternator came fro O'Reilly Auto Parts and was rated at 65 amps. The battery is about 3 years old and is of the traditional style (not maintenance free). I have not had very good luck with maintenance free batteries.

The voltages I mentioned in my original post were at normal idle speeds. My tach reads about 750rpm at normal idle.

I don't recall if there was any preservative on the wiring harness connection to the alternator but I will check it out.

Thanks for the help. I'll post my progress.

Bill

JimSmith · #4 07-03-2002, 04:28 PM

amptique,

What is the Voltage of the battery without the engine running? It should be over 12.5 Volts.

Just to make sure I have this correct, once you run the engine to around 2300 rpm and then let it idle, the idle speed voltage across the battery is 13.8 Volts, right? This is a good idle speed value, so you have an issue reaching some threshold value without revving the engine it seems. The battery may be weak and initially drawing the Voltage down.

If the car is warmed up after a significant drive, say half an hour or more with an average speed that keeps the rpm indicator over 2300 rpm, and you shut it down, then immediately restart, do you get the exact same results? Or is this only after it sits for a while?

Well, good luck. Jim

amptique · #5 07-03-2002, 05:04 PM

Jim,

Your restatement of the problem is correct. I just checked the battery voltage without the engine running and it is 12.56volts.

I will try the extended drive at higher RPM test you mention and see if the problem is reproducible. Most of my driving is stop and go in town and under these conditions the symptoms are the same no matter how short the "engine off" time is.

Thanks,

Bill

joe p · #6 07-03-2002, 07:46 PM

Just went through that on a 89 260e and a 88 300te, both have the same problem, a bad diode in the recitifier. In other words, replacement alternator time. The bulb prove out for the IC in the 124 cars is done through the diode trio in the alternator, the ABS and SRS are powered differently. This is why you keep the ABS and SRS prove out and loose pretty much everything on the l/h side of the cluster. However, I've seen them still light the oil level sensor bulb and I had one that the oil level light stayed on that was eventually traced to the alternator.

Joe

BTW, if you can get a retrofit for a 90 amp, do it.

amptique · #7 07-03-2002, 11:28 PM

Joe P,

The alternator is still under warranty. If it is a bad diode they should replace it. Would a test of the charging system detect a bad diode in the alternator?

Bill

amptique · #8 07-06-2002, 05:58 PM

I have replaced the alternator again (warranty replacement from O'Riellys) and the symptoms are exactly the same. I also drove the car on the highway for about 1/2 hour to see if the symptoms went away. Still the same.

From the wiring diagram it sure looks like the "D+" output from the alternator is not sufficient to "out" the antilock light. But shouldn't I still get the "charge indicator" and "brake wear indicator" lights to come on with the ingnition switch on if there is a complete connection to ground through the alternator?

Could both alternators be bad?

I'm confused,

Bill

joe p · #9 07-07-2002, 11:26 AM

The 123 body still runs buld check through the alt. I'd attach a jumper cable from the alt body to the ground post of the battery, start it and see what its up to. You also might have a bad charging harness, find the block for it on the r/h fenderwell area and do a resistence chack on it end to end.

Joe

amptique · #10 07-07-2002, 02:28 PM

Bingo!!! Problem fixed.

When I replaced the original alternator (alternator #1) I also replaced the wiring harness. Apparently I stripped the threads on the screw that connects the "D+" alternator output to the wires going to the antilock system and the idiot lights (at the connector block) when I replaced the wiring harness. The screw was tight but the wires were not making a good connection. A trip to the hardware store and $0.38 later I'm back in business.

Too bad I did a warranty replacement on a perfectly good alternator (alternator #2). Hope they don't come looking for me.

Thanks for all your help.

Bill