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.

no the bulb doesn't light, but I also haven't had any time to tear into the dash yet, is there another way to get at this little bugger?
also pulled the voltage regulator and it looks fine.

Unscrew the knee panel and push the cluster out from behind with your hand.

You might be able to get to the light with out pushing out the cluster, remove the panel and take a look.

ok so I checked the bulb all seems well there, so I guess its pull it and take it to my local rebuild shop so they can have a look at the alt.

timely post. I've been dealing with my screwy alternator all this weekend, capping off the fun last night with a tow due to no headlights. I replaced the voltage regulator after having a situation where I was getting amperage from the alternator but only 10.5 V. After replacing the voltage regulator I'm getting nothing. I'm going to take it back out tonight, but I can't see how I could have mis-installed it.

Should I just buy a new alternator perhaps? And can anyone out there tell me the part number for the alternator bracket spacer? Its the one on top that the alternator adjustment bolt goes through. Car is an 83 240D. Thanks all.


Per the problem the originator of the thread is having. The last time I replaced an alternator it would do nothing until I replaced the battery with another one that had some charge left in it. Even though the original battery was connected via jumper cables to a good battery, somehow the dead battery was causing the voltage regulator to see a full charge scenario. Putting the second battery in was one of those last ditch, I don't know what else to try, type deals.

I am continuing to have a very similar problem to the original poster. I had a screw that had fallen out on the VR, which I replaced, but I am still getting nothing from the alt as far as I can tell.

I wanted to check the alt directly, to make sure it isn't a bad connection between the alt and the battery (I have been measuring from the battery and getting nothing).

Is it possible to just check the alt directly with a volt meter? Last night I took out the plug from the alt and started the car. I tried the volt meter in a couple of configurations on the alt, but got nothing off of it. I didn't know, however, which terminals on the alt to put the VM pins to.

Does anyone know how to test an alt straight from it's terminals?

U needn't test it at the alternator. It sounds as though your alternator is not primed/excited. Try swapping your battery out. And then test again, at the battery.

I have learned your not going to get anything without first exciting the field, so your alt needs to be plugged in to start producing power. if you would like to test the posts on your alternator you need to pop the cap off the alt plug so the terminals are exposed, then you can test across the back of the plug.

I've done this, both by charging the battery with a charger and by swapping out the battery. No rise in voltage with rise in RPM. So the alt will not produce voltage simply by turning with the engine? I guess I don't understand how an alternator works. I just thought it was like a generator which produces voltage via rotation. I'm pretty ignorant on the subject, obviously.

I've been driving my car daily by charging the battery at night every few days while I try to figure out what's up with my alt. This car gets me to work every day!

I will try on the back of the plug with the plug in the alt, as Masterful suggests.

from my feeble understanding of how the alternator works I gather that it is not a permanent magnet generator, meaning it requires some electric current first to prime/excite the electric field meaning that it contains electromagnets. once the field is excited and it is rotating it then generates its own field using some of the electricity it is generating. the circuit that primes/excites the field of our alternators is the battery light in the dash when you turn the key to let the glow plugs cycle. if your battery light isn't coming on then you are going to have problems. you need to check the bulb and the circuit that goes to the alt, should be the smallest of the wires in the alt plug.

I used a proper length of appropriate diameter pipe.

If you are going to replace the alt...
 

I would recommend going with a Bosch AL129X. I did it to my 300SD and the difference is immense! I got a rebuilt one for $80 on ebay. It bolts right in and really gives a lot of power even at idle. check out this thread for more information...
http://www.peachparts.com/shopforum/diesel-discussion/93569-alternator-115-amp-bosch-al129x-works-123s-9.html

There aren't any permanent magnets in the alternator, they are in the starter. The field on the alternator is how it controls the charging internally. It is not necessary to take anything apart to do the measuring as there is a fantastic terminal strip available on the fender for this (the alt. plug ties to it directly).

To check the alternator, connect your meter (+) to one of the large red wires and (-) to ground and 20VDC range. Start the car and you should have at least 13 or more VDC and watch as it slowly drops off, indicating charging of the battery. If less than this, switch your meter to 20VAC and the reading should be much less than 1VAC. This checks the rectifying diodes of the alternator.

If the AC voltage is too high, then you have an internal failed diode and you will need to replace/rebuild the alternator. If the AC voltage is OK but the DC voltage is too low, then the regulator is not working. There may be other factors causing this. Check for corrosion at the terminal strip, battery cables, grounding (both batt-chassis and chassis-engine), and something I found recently, voltage regulator to alternator housing.

When you disconnect the alternator plug and remove the voltage regulator, check the brushes and also check the comutator rings. If the brushes are severely worn, chances are the rings are heavily grooved. Then you are looking at strictly replacement of the alternator. If there isn't severe wear, then very likely just replacing the voltage regulator takes care of it.

The alternator doesn't really have that much to go wrong with it. Voltage Regulator, rectifying diodes, brushes and comutator rings, windings and bearings. The only time I have seen a problem with the windings was with a major bearing failure causing physical damage.

I have a hypothetical for you. IF someone were to forget to disconnect the main battery leads before installing the new voltage regulator causing sparking at the alternator housing during said installation, AND IF the new regulator didn't want to go in at one point during install and after taking it back out the brushes were pushed out of their respective places and kind of hanging by the little springs, HYPOTHETICALLY is all I'm sayin'. Could something have gotten fubar'd? Before I was getting 42 amps and 1.5 volts and after this little debacle, no amps or volts. I mean, not me personally, but I mean, damn never mind,:rolleyes: it was me.

Hmmm... so are you saying a faulty battery light can cause the alternator to not do it's job?

I notice that when I glow the car before starting, my battery light and other lights on the dash (brake and brake wear indicator) sometimes come on, and other times only the glow light comes on - most of the time, only the glow light comes on.

Also, when I shut the car off, the battery light usually doesn't come on, though I think I remember in the past, that it did come on. I have seen it come on once in the past week when shutting down, but I think only that one time (though I don't watch it religiously, because I didn't know it really mattered - but I know it doesn't always come on).

did you get the bulb to light up? try it in another socket. I still think that that circuit is the problem...

Try tracing it back from the bulb to the thin wire on the alt plug.

YES YES YES!!

That is the circuit that excites the alternator. if the light is burned out or if it is not working you will not get anything out of the alt.

To check voltages, you can get the car started, then disconnect the negative and positive leads from the battery. If you get 13+V from the leads, the alternator is working. If the car is a gasser and it is still running the alt is working too :rolleyes:

You can also check the voltage at the black junction box (on the right fender on W126 cars) I would almost guess that either the thin wire is bad at the box or you have corrosion in the box...

Oh dumb question, but you did clean up all the corrosion on the battery terminals and everywhere else you found some, right?

My junction box has 25 years of dirt and dead spiders on it. I can try unscrewing the terminals and cleaning them off. There is supposed to be four connections at the terminals, right?

My battery is new and the connectors themselves are good and clean.

The alt appears not to be getting voltage to the battery, as there is no rise in voltage when the car is running from when it is off (actually, a slight dip in voltage when it is running, and even more when you turn on the lights, etc)

It is hard to say what kind of damage may have been done but you may have only damaged the voltage regulator....requiring a new one. It's kind of a gamble as to whether this took out the rectifying diodes also. The gamble is...$20 for a regulator, which may not fix it or an additional $120 for a replacement alternator. Personally, I would take it to an alternator shop and let them deal with it after this type of scenario.

yes I am saying that a burnt out bulb could possibly cause the alternator not to work

Check the Battery Light
 

I fought this problem this summer, ended up replacing my alternator twice (the first replacement alternator was bad). Anyway, one thing I found out is that if the check battery bulb is burnt out, then the alternator will not charge. The bulb is in the circuit that supplies the field voltage to the alternator, no field voltage = no charge. I also read a post somewhere that the brake pad light is in the same circuit, but I can't verify that.

To check to see if the bulb works, follow the alternator plug back to the terminal strip. The field wire will be the blue one (at least on my 82SD). Run a jumper wire from the field wire to the (+) terminal on the battery. Check to see if the battery light on the dash lights up. If it does, then most likely there is something wrong with your alternator.

Good luck!

So I can't visually see if damage occured? By looking into the alternator through the VR hole? Also where does one find a voltage regulator for $20, I was out of town when this occured and got hit for $73 by the mercedes dealership in Lancaster, PA. Should've just bought the $100 remanned beck-arnley alternator from the local import parts place.

Just happened to me this morning!
 

I was running up the highway when my RADAR detector started blinking "Low Voltage" At the same time the headlights started dimming. I got off the road and checked things out. It turned out that the wire from the alternator to the battery wire had come loose:eek:

Not only did this prevent the alternator from operating, it meant that the dashboard light did not come on!

I popped the wire back on but had drained the battery enough to require a jump start. Once the car was running I had to goose it up to 1200 RPM one time before the alternator would excite and start charging.

This proves that that circuit is a major potential failure point in the charging system. It also proves that these cars really NEED a volt meter...

Yikes! The local car electronics rebuild shop quoted me $13!

Try here for about $30: http://catalog.peachparts.com/;)

I pay $7 for a voltage regulator...NAPA

wow how about that canadian dollar! I just looked online and NAPA wants $84.89 for the bosch part and $54.89 for the Beru. This is voltage regulator bosch part # 1197311021

ok to sum this thread up and maybe give someone else an immediate answer to their charging problems, here is my trouble shoot on the charging system now that I know it like the back of my hand.

scenario 1:
Dead battery, no prior warning via dash lights, and all the dash lights come on when you switch the key on (once you recharge/jump your battery of course).

1. test battery, easily done at local parts stores for free.
2. test alternator, can be done several ways, easiest IMHO is with voltmeter across battery terminals (reading 13+ at idle means your charging)
3. check the terminal box on the right fender where the alternator leads meet battery leads for corrosion, and general dirt.

this one is most likely the battery.

scenario 2:
Dead battery, warning light has been glowing at idle but goes out at higher RPM, dash lights all come on when you switch the key on.

check the things in scenario 1, but odds are its your voltage regulator going bad.

scenario 3:
Dead battery, no warning lights, only glow plug light comes on when you switch key to start.

check things in scenario 1
also pull your cluster out and check the bulb for the battery light, brake indicator light. plug them in to a light you know you can get to light IE high beam indicator light and test them.
also check the small wire on the alternator plug for continuity to these bulbs.
if all that checks out you will probably need to replace your alternator.



mine was scenario 3 and I checked everything there was to check without actually taking the alternator out and looking at it. I took it to a local alternator rebuilder, good guy have taken a few things to him... he took it apart and gave me the lowdown of what was going on with it. he told me he could rebuild it but it would cost as much to do as just getting a new one, and he said the new one would be more reliable, due to the amount of wear on the housing. I got a new one and put it in, no more charging troubles, yet.

also as a side note if you are ever traveling through mid-Missouri on I-44 and have alt/starter problems I highly recommend Harold in Cuba, he's honest and fast, has been doing it for almost 30 years now. I also like that he will bring the part out to you and point out what exactly has went south in it before he replaces it. I once took a starter to him which he rebuilt in about 30 mins, and he only charged me $20. starter still works just fine over 6 years later.

Thanks Masterful, I am personally trying to figure out scenario #3 right now, and it is good to see this all in one place.