Difficult Overvoltage problem on 300SD...please help!

[300sd2000]

Hello,

I have a 300SD 1984 (126) with a difficult voltage regulation problem.

Initial symptoms would show a low charging voltage, and the alternator was replaced a year ago. The problem seemed to subside. In the last several weeks, I began getting overvoltage problems, where the voltage will rise from 12-18 depending on how fast the engine is spinning.

So I replaced the voltage regulator, now x 2 and the problem is STILL OCCURING....

Defective alternator??? Or is there something else responsible for voltage regulation on a 300SD????

I pulling out my hair on this one...can't take the car anywhere! The voltimeter must be accurate, as I have blew my headlights out from the overvoltage and the radio will stop responding during overvoltage readings...

Is there some other controller of voltage, or is there some kind of diode that could be causing this??? Again, the alternator is a dealer-factory rebuilt about 1-1.5 years old...

[300sd2000]

This is the voltage regulator that I have replaced, in case anyone is wondering what it looks like...it installed without difficulity (don't think I could have really installed it wrong)

[mbshop]

twice seems odd. test the battery and check grounds.

[jbaj007]

You've tried three voltage regulators with an RPM dependent voltage up to 18 volts still present......needs new alternator.

[300sd2000]

ok, looks like I need a new alternator....will try that and see what happens.

Yes, the voltage has become very much RPM dependent.

Someone suggested the battery, but I don't see how the battery can cause overvoltage.

[300sd2000]

THe new (Fastlane-BOSCH) alternator FIXED the overvoltage problem

HOWEVER, i am still back to square one...that is that the car becomes discahrged after several weeks of running around town...it's as if the alternator isn't really getting any current over to the battery.

I've had essentially everything changed form teh battery to the alternator. (NEW DIEHARD). No shorts that I can find, ground posts to the body checked and in good shape. I did a test light between the ground terminal and the body when everything's off and I don't see a current leak.

Here's what I think is wrong. It seems the old alternator had a ??Capactior or diode on the back of the alternator. STRANGELY enough, the other 300SD (1984 CA) does NOT have this diode...which leads me to believe that it probably doesn't ?? belong there: (maybe it came from an aftermarket alternator?? as the new BOSCH rebuilt I just installed didn't have it.

Removing it seems to have given me MUCH better charging voltages (13 idle 13.5-13.8 at 2500 RPM).

DUring warmup, I get about 12Volts for the first two minutes (I think this is still the newer glow plug relay that i have glowing and then it cuts out) adn then I'm up to 13-13.5.

QUESTION: Is the part below SUPPOSED to be there on a 300SD alternator? (The other car has done well for years without it).

If so, could a DEFECTIVE one be the root cause of my poor charging voltage?? WIll removing it cause damage to the car, or does the alternator have the equivalent on the inside of it already?

Here is picture of it below: BERU 0310034062 2.3 uF 110V 100 C

[JimSmith]

300sd2000,

The black thing you photographed (by the way, those were all excellent photos) is a capacitor and from what you describe it has failed. The point of the capacitor is to filter alternating current content from the DC going to the battery. The alternating current content comes from the brush commutation and the diodes rectifying the original alternating current produced by the alternator.

Capacitors conduct alternating current, and when healthy, they do not conduct direct current. The filtering function is performed by conducting the alternating current component to ground before it gets into the system. When these are not installed you may, depending on how the radio is hooked up and whether or not it has a separate filter, hear a buzzing noise with a frequency that is speed dependent.

I am a little surprised this unit lasted as long as it has. Once a layer of insulation in a capacitor has a hole punched in it, they usually don't last very long thereafter. As with most small electrical components in a power circuit they initially fail shorted, and then they ultimately fail open, because they vanish when the smoke comes out.

I think the capacitor, or condensor, is supposed to be there. If it is not failed it can only improve the life expectancy of the small electrical devices and boards in the car's electrical system.

Good luck, Jim

[300sd2000]

Still having problems....starting to get desperate

Symptoms: Car won't start up (slow engine turn over) after several weeks of stop and go city driving. Starter will turn over very slowly, lights dim etc. and voltage will drop to 8-9 volts when this happens. Car driven primarily in the city (HOT 90-100F) & R-134 Retrofit AC on continuosly full blast (california weather)

So far tried:

1) Two alternators (one MBZ and bosch rebuilt from fastlane)
2) Two batteries (both Sears Diehard)
3) Tried voltage regulator and removal of the capacitor (see above).


Still no avail.

-Highest charging voltage at full throttle is 13.5
-Idle voltage is 12.5 usually dropping to 12.0 at idle with AC full blast.
-Startup voltage is 11 volts (rising to 12-12.5 volts at idle with AC. The startup voltage will rise to 12-12.5 after approx 3 minutes as I think the glow plug relay (newly replaced bosch from fastlane) stays on until that time and then cuts out--- a warmup feature I think)

Here's the strange part: I have another identical 300SD that does mainly highway comuting 30 miles each way and its voltage as this problem cars above (if not worse) and it runs fine.


Could this be:

A) Bad alt wiring harness? Haven't tried changing alternator harness (original)

B) COuld the alternator light in the dash have something to do with this. I swapped this with the other car in a desperate attempt to try something. The bulb IS working.

C) Could a bad starter motor do something like this....its a 3-4 year old bosch rebuilt and I don't really feel like replacing it. The starter normally works fine so I doubt it.

D) Any suggestions before I give in and take it to MBZ dealer?

[300sd2000]

Still having problems....starting to get desperate

Symptoms: Car won't start up (slow engine turn over) after several weeks of stop and go city driving. Starter will turn over very slowly, lights dim etc. and voltage will drop to 8-9 volts when this happens. Car driven primarily in the city (HOT 90-100F) & R-134 Retrofit AC on continuosly full blast (california weather).

When this happens I jump it with a portable battery pack and I'm in business. I then charge it overnight adn then ready to go the next morning.

So far tried:

1) Two alternators (one MBZ and bosch rebuilt from fastlane)
2) Two batteries (both Sears Diehard)
3) Tried voltage regulator and removal of the capacitor (see above).


Still no avail.

-Highest charging voltage at full throttle is 13.5
-Idle voltage is 12.5-13.0 usually dropping to 12.0 at idle with AC full blast.
-Startup voltage is 11 volts (rising to 12-12.5 volts at idle with AC. The startup voltage will rise to 12-12.5 after approx 3 minutes as I think the glow plug relay (newly replaced bosch from fastlane) stays on until that time and then cuts out--- a warmup feature I think)

Contrary to all the posts I have read, I have never seen a full 14.5 volts charging at idle on either car. Belt tension is firm.

Turning on the A/C + Blower seem to knock off a good 1 volt on my car (i.e. 13.5->12.5) etc.

Here's what I think might be the problem:

Six months ago I had replaced the glow plug relay with the kit from fastlane. it seems to have a feature that keeps it glowing on for 3 minutes....its a warmup feature probably good for cold climates but it causes a massive current draw dropping your voltage to 10-11 volts at idle (especialyl with AC, lights) for the first 3 minutes. After it cuts out the voltage drastically improves!

...wonder if anyone's had problem with these "newly improved" glow plug timers....especially in the peak of summer when you need your full battery capacity.

This is further coumpounded by short distance driving which causes the battery to just keep getting drained trying to supply current to the timer x 3 minutes.

Anyway, dug up the old relay (there was nothing wrong with it as it turns out I had a loose connector to the relay that was causing the light not to glow. Installed it back in, and the voltage has greatly improved to 13v idle and 13.75 at 2000 rpm (that's on a partially drained battery from the problem earlier today). This is much better than the measly 11 volts I was getting with the "upgraded" relay.

Is this a known problem with the updated glow relay? I would recommend sticking with the original relay (does partshop or fastlane have the original stock one?) in hot climates and city driving with AC on (like my car does).

Anyway, will go drive around some more and see if this clears up....unfortunately I had bought the same relay for the other 300SD about a year ago too...it too seems to have the same problems on startup/warmup voltage.

[BobK]

Check this if you have not already: Near the battery is a terminal block. Three terminals. Two are large gauge wires and one smaller gauge. With the negative terminal of the battery disconnected, one at a time take the screws out and shine up all of the connections on both sides. I ran into a problem similar to yours and it turned out that the connections had a black coating on them. Some strange form of corrision. Cleaned all the connectors with stiff wire brush, put them back together and battery charged fine after that. Agree your short trips are not helping matters

[JimSmith]

300sd2000,

The symptoms you describe reflect a starter motor that is aging and suffering from poor insulation resistance. Lots of words for you need a Bosch rebuilt starter. I highly recommend the Fastlane units.

As an electric motor ages, and Diesels are particularly difficult on starters, the insulation on the wires that the battery current flows through loses it's ability to resist current flowing through the insulation to an adjacent wire or the metal structure of the motor, which is grounded. Insulation resistance is adversely affected by temperature, so, when the unit gets hot from ambient temperatures, or, underhood temps after running when it is hot out, a weak insulation can begin to significantly leak current whereas the same unit will perform ok when it is cold. So, with a short in the starter it is no wonder your voltage droops so severely.

A rebuilt starter from a local rebuilder will normally have all of the replaceable mechanical parts replaced. They will sometimes have all the replaceable electrical components (voltage regulator, your capacitor that failed, and, if the guy is really thorough, the diodes in the rectifier) replaced too. But they will almost never rewind the machine. So, the real problem is not fixed. Since this procedure is usually adequate for gas cars (their starters don't see nearly the abuse a Diesel starter sees) it is accepted as the norm for most rebuild shops.

The rebuilds by Bosch include insulation restoration and the machines are really in like new condition. The ones from Fastlane are very favorably priced and you get them really quick. Changing them out can be a chore though. If you do it yourself, disconnect the battery! I have personal experience that I will never forget on the subject. Good luck, and I hope this helps. Jim

[pmckechnie]

First, 13.5V is about normal for Euro built cars (MB,BMW,Audi, etc).
US brands usually run about 14.5. Asain about 13.5. Batterys last a lot longer if charged with 13.5.
This is how I diagnose a starting/charging problem. One of these test will require a 100+ amp meter, which you probably won't have but maybe you can get some shop to check it for you.

1. Start with a fully charged battery.

2. With engine running, and no loads (no A/C, lights, etc) check the voltage at the battery. Should be about 13.5 or so.

3. Check the voltage from the back of the alternator to the battery ground. Should not be over .1 or .2 v higher than the battery voltage in step 2.

4. Turn on the A/C on high fan and the head lights.

5. Check the voltage at the battery and alternater as in step 2 and step 3. The voltage should be about the same but will be lower than with the A/C and lights off. If the voltage is more than .2 or .3 different when checking at bat and alt then there is a resistance in the wiring between the battery and alternator.

6. Now with the engine still running and the A/C and lights still on, check the voltabe between the bat neg and the alternator case. Should be less than .3 volts. If not, there is a ground problem between the bat and the alt. Also check between the bat neg and a good body ground. Again should only be .3v or less.

7. Now comes the part most people don't have the equipment to check. We need to check the amperage the alternator is supplying. Both idling and 2000 rpm. A good MB alternator will supply at least 50 or 60 amps idling with A/C and lights on and 85 or maybe a little more at 2000 rpm. Now this is the tricky part. If the voltage is low (less than 12.5) at idle and the amperage is low, either the belt is slipping or the alternator is weak. If the voltage at 2000 RPM is less than 13.5 and the amperage is low, the alternator (or regulator) is bad. If the voltage is less than 13.5 and the amperage is greater than 80 amp at 2000 then the battery is bad or there is to much amperage draw caused by something in the car.

I know all if this is very confussing, but believe me, this is how it must be done. (I do this every day. Paul's Auto Electric}
Most mechs don't know how to do this be cause of the electronic principles involved.

One thing I see the most is belts that are too lose. They must be very tight. To check, With engine on, and A/C and lights on, check the voltage at the battery. Now stop engine, tighten the belt some more and start the engine and retest. If there is any change in the voltage, the belt was, or is slipping. Believe me, I see this a lot even from other shops.

The other thing I see a lot is defective rebuilt alternators. When an alternator is rebuilt, they test them at 2000 or 2500 RPM. If they supply the proper amperage and voltage they are considered "good". They do not test at idle speeds. This gets a little complicated because of the different sizes of the pulleys on the alternators and engines. 600 rpm engine speed may be 2000 rpm at the alternator, or it may only be 1500 or any thing else for that matter.

If you can do these test and reports back, maybe I can guide you
in the proper direction. Even if you can do steps 1 thru 6 above and let me know maybe we can sort this out without you having to find some one with the amp meter.

Please excuse the poor typing and spelling. I don't claim to be a scollar.