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#2
03-08-2014, 06:12 PM
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Bad voltage regulator or bad diodes...both in the alternator. It happened to us a couple times, so we changed the voltage regulator which is attached to the back of the alternator and can be r&r'd with 2 screws that hold it in the alternator. Some you can do without even removing the alternator, others might be tough to do without removal. It fixed the dash light issue on this one car.
It is easy to see if the v.regulator is bad, with a multimeter on each the positive and negative battery cables.... test values on correct multimeter setting to avoid damage to the meter. There are voltage values that determine it's condition...and the engine needs to be running, with the meter probes hooked to the cables. It worked to change the voltage regulator on one vehicle, but we did the same with another vehicle and the battery light kept flickering on and off....I found out from someone on here, that the diodes can cause this. We never messed with the diodes, cause I don't know where they are, but rather just ended up swapping a whole different used alternator in with the new voltage regulator. That fixed the second issue for the other car. Voltage regulator was $30 bucks. There could be other causes, as well, but these worked for us. I think there is also a small blue wire that connects to the alternator and feeds back to the dash cluster from the alternator or from the alternator to a junction box on the passenger fender, then to the guage cluster. It has something to do with energizing field coils in the alternator itself and if the wiring is compromised in it's travel, may cause the dash light to come on. Have you had dead battery issues? 
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#4
03-08-2014, 06:38 PM
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Yes, something around 14v would be good, but...at a certain point you don't want it overcharging as you don't want to boil your battery out...especially a new one. Conversly, too low and it won't recharge....
On the second car...with the apparent buggered up diodes, I checked with the meter from time to time .....to make sure, I was not damaging anything by overcharging, or undercharging and we always got around 14.21. I kept an eye on it, but drove it that way for almost a year before finally switching the alternator itself...and everything appears fine...although I am not sure if continuing to drive a car this way could damage stuff. I guess a person would have to know what purpose the diodes have to be certain. In my case, the dash light bugged me and it does not give one piece of mind with the dash light on all the time, especially since the second car was a gasser. Don't forget, the diesels don't rely on the battery to stay running, and may run down the battery during the course of the day...then you find out when it gets dark that the headlights are getting dim....and you get stuck in the dark....or you turn it off somewhere and it does'nt have the juice for a restart....could catch a person by surprise, but if the battery light is on...it will come back, and not seem as much of a surprise. With the cost of a battery and being that a voltage regulator is between 20-30 bucks .....it is cheap enough to keep a spare in the glovebox...We do.
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#5
03-08-2014, 06:40 PM
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I guess the other thing to do while testing, would be to raise the engine rpm...to make sure, that as the rpm rises.... that the voltage regulator continues to regulate, even with the engine rpm increase. Of course be carefull working around a running engine with mechanical accesories spinning (fan and belts), that a multimeter test lead does'nt get sucked in and spit back out at you...they are sharp. Bunny hug strings are bad too...might give a person a close shave.
Last edited by macdoe; 03-08-2014 at 06:52 PM.
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#7
03-08-2014, 07:28 PM
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I had this problem on an 81 300SD. Changed the voltage regulator since it was easy and cheap. No change. Replaced the alternator. Bingo. It was just not putting out enough juice.
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