Altenator plug
 

I have been having some alternator issues as of late... I wound up with a dead battery without any prior warnings, just 1 slow start and that was that. have recently had the voltage regulator replaced and that fixed it it was charging again. but now with taking it off to have it rebuilt and putting it back on I think I might have pulled a wire off without noticing it. I have read in related threads that the alternator needs to be primed to start charging with a little bit of voltage when the key gets rolled on, and this priming is indicated via the battery warning light which is to flick on when the engine is not running but the key is on. I know that the light used to come on when you rolled the key to glow the plugs, but noticed today after reading some threads that it no longer does...

what I am looking for is a picture or an explanation of what wires are supposed to be coming out of the alt, what prong they are supposed to connect to and where they go.

thanks.

Please give us a hint to what Model/Year MB you are asking about.

- alternators do not need "primed"

sorry forgot that in my first post, she's a '80 300sd, and if I could find the post that explained that the alternator needed priming from the battery I would, but the basics of it came down to this, the alternator isn't a permanent magnet generator, so it needs an electrical charge to produce a magnetic field first, then starts generating its own power and magnetic field after wards.

The 116 alternator has a 3 spade connector - are all three wires separated from the connector?

I think there are only 2 hooked up to the plug, I will have to crawl under there and have another look today.

There is no priming, I think what you are looking for is exciting the field. You just have to run the engine for a few minutes and turn on some electrical loads like the blower or headlights.

yeah that sounds logical, just got done driving it to work (25 mile drive with lights on the whole way) and checked it with the voltmeter when I got here, still no alt action:confused:. I will see if I have time today to crawl under there and inspect the plug again, also might pull the voltage regulator again and see if its fried, if it is fried (this would make the second time inside 3 months) would you all recommend getting a new one again, or just getting a different alternator? I am still concerned about my battery indicator light, it would have/ should have warned me of the impending doom of dead alt, but instead it just sits there completely silent. how much of a pain is it to get to the bulb on it? do I have to remove the whole gauge cluster?

poked around yesterday with my voltmeter and got these readings

resistance from block to ground practically nothing
battery volts sitting all night idle 12.53
battery volts post start up 12.42
battery volts with lights on for 20 mins with engine running 12.09
picked up some voltage on the alt plug from one spade to the other with the key off, switch the key on and it goes away.
checked my fuse box for any blown fuses, nothing new there
checked there for any power leaking that may indicate a short some where nothing.

You are not showing enought volts at idle.

yes I know this, it should be putting out at least 13.5 at idle and 14+ at 1500 rpm, but engine speed has no effect on voltage.
it is always around 12.4.

Check for corrosion at the voltage regulator. Recently, I had the same symptoms of flakey alt. light and low charge voltage. When I removed the voltage regulator, I noticed a green corrosion on the metal tab that grounded to the case of the alternator. I cleaned both and applied a very slight dab of conductive heat sink grease before remounting. After reassembly, headlamps are bright and 13+ charging voltage.

Do you have to remove the whole alternator to get at the voltage regulator? Is it the plastic thing with two bolts on the back of the alternator? Kind of diamond shaped with a raised circle in the middle?

I am having similar problems to what you describe, and have been told to change the voltage regulator and see what happens.

I think you know already, but the unchanging voltage with raise in rpm means the alternator is doing nothing.

You don't have to remove the alternator. However, be sure that the plug is disconnected to remove power from the alternator. The voltage regulator is just as you mentioned, the plastic diamond-shaped thing held with two screws on the back. (usually black)

The voltage regulator is the thing on the back held on with two screws. The plug should have three wires in it. The middle wire is much thinner than the other two and is the connection to the battery light and serves to excite the coil. the other two are both output lines that run in parallel to the junction box. If you turn the key to run and the battery light does not come on you have a problem in that circuit. I believe (but could be wrong) that if the bulb burns out, the alternator will not excite. I would try replacing the bulb and seeing if that helps.

ok I was checking on the plug and I got this kind of configuration, thick wire left thick wire middle and thin wire right. now I can take my voltmeter and get battery voltage from the thick wire to the thin wire with the key off, as soon as I turn the key on it drops to 0. and I get no voltage anywhere else other than outside thick wire and grounding to the chassis.

Can you get the bulb to light? try replacing it as I think that could be the problem... It is connected to the thin wire and if it is burned out it could leave an open circuit.