Troubleshooting Charging System - Page 2 - PeachParts Mercedes-Benz Forum

Diseasel300 · #16 07-02-2017, 10:00 AM

Quote:

Originally Posted by Mxfrank

There's a lot more going on than you think. The regulator is temperature compensated. The nominal regulation voltage is 14.25-14.50V at STANDARD TEMPERATURE (25C), as can be clearly seen in the chart I posted. But the voltage target decreases .03 degrees for every ten degree increase in regulator temperature. This isn't magic. The zener diode in the regulator has a non-zero temperature coefficient, which is the cause of the voltage drop. This is done deliberately to compensate for internal battery resistance, which changes with ambient temperature.

Actually the zener diode drifts with temperature due to its germanium construction. When these cars were built and that alternator designed, it was still the industry standard and temperature drift was a fact of life. They're still used today because they're cheap and they're very reliable, but they aren't accurate or temperature stable. Modern designs use an embedded junction zener diode which is thermally accurate around .01% vs 3-5% of the standard zener diode. The other reason the output voltage drops with temperature is because the regulator does not compensate for winding resistance changes as they increase in temperature. The result is that for a given excitation voltage, the output voltage drops as temperature increases. Remember: the regulator is designed to be cheap, reliable, and accurate "enough".

I design voltage regulators for a living and have been in the field for over 20 years. Engineers in my field spent over 50 years trying to get rid of zener diode temperature drift for the very reason shown here: regulation accuracy drifts with temperature. The EJZD ended those problems and has become a standard component in anything that even pretends to be temperature stable. An alternator isn't that critical, a backup generator connected to an operating theater is.

Mxfrank · #17 07-03-2017, 09:31 AM

Quote:

Originally Posted by Diseasel300

Engineers in my field spent over 50 years trying to get rid of zener diode temperature drift for the very reason shown here: regulation accuracy drifts with temperature. The EJZD ended those problems and has become a standard component in anything that even pretends to be temperature stable. An alternator isn't that critical, a backup generator connected to an operating theater is.

We aren't talking apples vs oranges, but maybe macintosh vs golden. A generator that supplies delicate electronics in a conditioned space isn't fulfilling the same mission. The primary purpose of an automotive alternator is to charge the battery.

A starting battery is a specific chemical system which lives in an area subject to wide temperature variations. If voltage is too high relative to ambient temperature, the battery will eventually gas out and die. So the regulator has to adjust for variable under hood temperature. As it happens, a voltage drop of 3mv/degree C is about perfect for charging a flooded lead acid battery. The temperature coefficient of the zener may be a disadvantage in some situations, but here it's used specifically for temperature compensation. It's not sloppy design, it's clever. As an aside, a lead acid charger wouldn't work well for LiIon, NiMH, or NiCD, each of which has it's own charge characteristics.

optimusprime · #18 07-04-2017, 05:34 AM

The problem is you charging rate is over maximum .Should be 14.5 anyhing above the manual reads -
Regulator faulty. Open circuit of exiter or diodes in alternator ..

Diseasel300 · #19 07-04-2017, 12:51 PM

Quickie update:

Had a chance to inspect the alternator this morning. External plugs are in great shape and very tight grip on the spade terminals on the alternator body (was a BEAR to get off). Bearings nice, smooth, quiet, and creamy. Slip rings still in very good condition, no grooving or scoring. Regulator brushes worn down to ~1/8" in length. Brush faces are still in good shape, smooth, no scoring, no copper drag. It is remarkable that the alternator is still in that kind of condition. Ugly on the outside, but still great inside.

Given the wear on the brushes and the charging issues, I'm going to replace just the regulator and see where it gets me. The rest of the thing seems to be mechanically sound, fan is good and tight and there's no slop or play in anything.

Car's sat for nearly 24 hours now since last driven and the battery voltage is still 12.64V. I'd say that rules out battery. I'm reasonably impressed that the voltage is as high as it is given the issues I've had with charging voltage.

Will replace the regulator and report back.

funola · #20 07-04-2017, 01:12 PM

So your brushes were worn and making poor contact, probably that's the only thing wrong with the regulator. I'd install new or longer used brushes and it should be good as new. A used regulator that's been working for so many years is fully burned in and is better than a new one IMO.

Tip: The brushes cannot be un-soldered with a regular iron because it was "welded" with some sort of high temp solder and must be drilled out.

I have done that a few times with great results. i.e. I have never bought a new regulator.

Diseasel300 · #21 07-04-2017, 02:37 PM

Already ordered a new regulator this morning. I'd rather not fuss around with finding the proper brushes for a 31 year old regulator. Knowing my luck I'd change the brushes and the transistor would burn out the next day anyway!

optimusprime · #22 07-05-2017, 06:00 AM

More like the high output is due to internal problems , and not the regulator .Poor regulator will make the output low not high .

Diseasel300 · #23 07-05-2017, 09:37 AM

Quote:

Originally Posted by optimusprime

More like the high output is due to internal problems , and not the regulator .Poor regulator will make the output low not high .

The output on this alternator *IS* low. Charging voltage should be ~14V. Mine won't get above 13.5 and is below 13V most of the time.

Diseasel300 · #24 07-10-2017, 08:45 PM

New Bosch regulator arrived today and has solved the problem.

Before replacing it, I went for a drive (about 20 minutes) to get the car hot and charge the battery. At idle with nothing on in the car I had 13.35V at the battery. Kicking on the A/C dropped it to 12.65V and was falling quickly.

Post regulator replacement, I took the car for the same drive - to recharge the battery and get the regulator to the same operating temperature - and came back to check voltages. 13.92V at idle with nothing on, 13.87V with the A/C on at idle. Turning on the headlights as well dropped to 13.65V.

I'd say that's proof-positive that the old regulator wasn't cutting it anymore. It went in the trunk as an "emergency spare".