Alternator Puzzler 1987 300E
 

• 1987 300E
• 1-year-old battery
• 3rd re-man alternator + 4th new voltage regulator
• Battery voltage at full charge - 12.4 V
• Voltage indicated at the battery terminals with engine on but no electrical load - 13.6 V
• Voltage indicated at the battery terminals with engine on with all accessories on - 11.6 V and steadily drops until the battery loses all charge
• Bench test of all alternators shows normal alternator, diode and voltage regulator function;
• Diagnostic tester with alternator installed and engine operating without an electrical load shows normal alternator, diode and voltage regulator functions;
• Diagnostic tester with alternator installed and engine operating with all accessories engaged shows normal alternator and diode but a failed voltage regulator function (4th new voltage regulator).

Any ideas?

Alternator
 

Are you replacing the entire voltage regulator assembly. It screws onto the alternator and is replaceable with the alternator in the car. It has new regulator and brushes. I had mine rebuilt and all they did is solder in a new brush. The other brush failed a year later. I then replaced the whole assembly it has been fine since.. When the brushes stop making contact and the alternator stops charging you do not get a battery light because the brushes are part of the circuit that turns on the light. When voltage drops low enough you get an abs and srs light then soon after the engine stops running

Thanks Rob;
All of the voltage regulators have been "new." But nothing is for certain.
Two have come with the alternators and 2 sold individually.
The car still produces the exact symptom you mentioned with the ABS light along with a slow windshield wiper and a flashing rear defogger light.

are you buying Bosch regulators from this site. I always add a bosch regulator to rebuilds. Its hard to afford a rebuilt by bosch

the last alternator I installed yesterday came with what appears to be a brand new Bosch regulator.

I replaced my alternator and noticed the same problem - if all accessories/lights on and car at idle, like at a stop light, then the voltage drops below that of the battery. (Alternator not putting out enough electricity to charge the battery.)

If I place my foot on the accelerator and increase the engine RPM's, then the alternator will put out enough electricity to charge the battery. (Or if I am not stopped and thus the engine has higher RPM's - alternator turning faster).

Or if I turn some stuff off, then the alternator will be able to keep up. (I installed a voltmeter so I can keep an eye on this.)

I think this is the way the charging system was designed. Maybe they figured not all accessories would ever be on at the same time? The alternator does seem to be under capacity to me...

My stock alternator is 80 amps. Some people have installed 100 amp alternators. For those people... Does the alternator put out enough electricity at idle to power all accessories and charge the battery (voltage at idle with accessories on above 12.8 volts?)

Adjusting RPMs does not seem to have any significant effect...
The alternator installed is a 70 amp model.
Maybe it's the diode in the back of the alternator but I cannot find it anywhere.

Can you elaborate on the electrical load you place on the system? A full combined load will draw down the stock alternator. Nearly all MB's of this are have undersized alts.


But beyond that, a few things:

Are you sure the exciter wire is connected to the alternator? This is the much smaller wire. I think its connected to the D+ terminal.

I believe this wire goes up to a dist. block then into the car and connects to the "battery" idiot light. Also make sure that light is good.

Your alternator might be able to internally excite up to a certain load/point.

I have a 143A and 150A alternator installed in my 190E and 300D respectively. Neither have charging problems at idle. The 300D will put down 14V at idle.

Even when I had my stock alternator on my 300e it was enough to power everything plus a 600 watt rms amplifier. I upgraded to the 150a because at idle my lights would dim and I swear my idle would hiccup. The sudden draw was too much for the little alternator at idle.

At speed everything was fine. Bottom line is that something is wrong, either it's not charging or maybe something is drawing too much power when you switch it on; like a short, but not enough to blow the fuse.

Electrical problems suck.

OK, thanks for the suggestions. I have plenty of work to do.
The exciter wire, does that have to do anything with the suppression capacitor on the back of the unit? How would the car be affected if the suppression cap was bad? Anything at all?

I am guessing loose ground connection somewhere. Maybe the ground strap on the battery. Could be loose connection on the hot side also. Check your batter clamps also of course, but that is more obvious (although I have missed it before.) Very unlikely to have a bad Bosch regulator and about impossible to have 4 in a row. For a tester to diagnose "bad regulator" just means the voltage is wrong. If you have a loose or bad connection the voltage will be wrong, but only under load. So really it should say "loose connection or bad regulator."

Hook a digital voltmeter between the Neg. terminal on the battery and the engine block while it is running at fast idle. Should be 1/2 volt or less if ground system in intact. If higher I would suspect a broken grounding strap or bad neg. cable.

X2 on what platt-deutsch said. I would like to add that you can go from the neg battery post (not cable end) to the alternator case. If you see a voltage above 1/2 volt then start moving the meter back closer to the battery. For instance, check from the battery to the block, then to the body, etc. When the problem doesn't show up on the meter, you just passed the problem. So you know the problem is between the last bad reading and the good reading.
If the ground doesn't show a problem, then do the same thing with the positive. For instance, check from the battery positive to the alternator output. If you see a voltage drop there, start going back toward the battery.

87merz:

Further on what James Dean had to say above:

The charge indicator light is in the circuit which appears at the alternator as the small blue wire. That circuit supplies an exciting current that is independent of the regulator current. First test: when the key is turned to ON, does the charge indicator light come on? If yes, OK. If no, the bulb may be burned out, and if the bulb is burned out there will not be an exciting current to the alternator. Second test: if the bulb does come on, remove the blue wire at the alternator, and check for +12V at that wire with the key on. If yes, circuit OK. If no, circuit may be broken between bulb and alternator.

I agree with James Dean. Check the connection and if possible upgrade to a larger alternator. I have the 120amp on my 190 and it cranks out ~14v at idle.