Alternator/Voltage Regulator Problem??? - PeachParts Mercedes-Benz Forum

HaagIII · #1 12-06-2006, 08:02 AM

I had my bonehead move of the day on Mon. My glow plug relay has been out since I aqcuired the car, so there is a lead coming from the battery to bypass the relay to turn the glow plugs on with a momentary switch. My problem started when the switch broke and I used a jumper wire to turn the glow plugs on. I forgot to unplug the jumper wire and drove home most of the way (approx 25 miles). Once I noticed the lights dimming, I realized what I did. I unplugged the wire and the lights were now brighter again. I restarted the car later that night with no issues and then the next morning. But the next day at work I could barely turn it over. I jumped the car and got it to start. On the safe side I turned on all the lights and let her warm up. Slowly the lights dimmed. All signs point to alternator. My question is, is it just the regulator that I fried, or is there something else in there that I took out? Is there a way of checking this or do I just replace the alternator. Do they make rebuild kits that would fix this. The alternator was replaced within the last year by the previous owner.

SD Blue · #2 12-06-2006, 10:03 AM

1) A meter across the battery terminals, while running, should tell you. In Volts DC, you should be getting close to +14 volts.

2)If not, switch to Volts AC and you should have less than 1 volt.

If #1 and #2 are not satisfactory........... change the alternator. The rectifying diodes are fried (and probably the regulator as well).

If only #1 is not satisfactory..........change just the regulator.

HaagIII · #3 12-07-2006, 05:58 PM

I'm getting 12.5V DC with lights and heater on with an elevated idle. But I am getting ~20V AC under the same condition. Based off what you said, I'm not sure what to do with what I have. If #2 is out of range and #1 is close to the 14 V, what do I do if #2 is not satifactory. Under an elevated idle, the voltage across the terminals was not changing with lights and heater on.

JimSmith · #4 12-07-2006, 06:25 PM

You need to run the test as described. No loads like lights, A/C, fans, heated seats, etc.. etc. Then, with the engine running at normal idle speed you should have 13.6 V minimum, DC and very little AC voltage, definitely less than 1 Volt. If you don't get both of these, you have a problem with the diodes, which are devices that either work or they don't, and when they fail you have no choice but to replace them. A good rebuild shop can handle this for you, but from the sounds of what you did I would expect the insulation is damaged. I would just replace the alternator. Jim

HaagIII · #5 12-07-2006, 07:09 PM

That's what I planned, but I just wanted to check to make sure I wasn't missing anything. Thanks for all the info!

networkboy · #6 12-07-2006, 08:25 PM

Under NO condition should you ever see 20VAC. Your battery will not survive even an hour of that. replace the alternator.
-nB

SD Blue · #7 12-07-2006, 11:21 PM

what Jim and NetworkBoy said. You have a diode rectifier, in the alternator, that has failed.

Usually you can find auto electric rebuild businesses that can repair an alternator but you might have to wait.

If you get a rebuild from a parts exchange, it would be a good idea to put your dead battery on an external charger before using the new alternator. A near fully discharged battery will put a lot of strain on that rebuilt unit. It might save you having to replace it twice.

networkboy · #8 12-10-2006, 12:10 AM

Quote:

Originally Posted by SD Blue

A near fully discharged battery will put a lot of strain on that rebuilt unit. It might save you having to replace it twice.

Totally been there done that, to the tune of about $800 for the camry. Battery shorted, killed the alternator (didn't know it though). Bad alternator killed new battery. vicious cycle. Mech replaced alternator and new battery at the same time. No issue since.
-nB