W210 voltage at idle - PeachParts Mercedes-Benz Forum

VW1300 · #1 07-10-2016, 03:18 PM

I was checking the diagnostics on the AC panel a few weeks ago and saw it read a voltage of about 12.8-13.1 volts at idle. This seems kind of low so I replaced the regulator, but it hasn't changed. I recall I changed the regulator about 4 years ago and it was around 13.6 at idle afterwards.

Does this seem low to you guys - and if so, can anyone advise on what to check next?

Thanks,

tjts1 · #2 07-10-2016, 04:52 PM

The CCU read out is about 0.5v below what you'll see at the battery or + terminal under the hood. Measure your voltage there.

That said, I think the 90a alternator on the 97 is too small for the application. Keep in mind that at idle the alternator can only output about 1/2 of its rated amps so 45a. Now throw in afterglow which can be be ON up to a couple of minutes after start up (12a x 6) and it'll take a long time for that 90a alternator to recharge the battery. 98 and 99 E300 got 115a and 150a alternator from the factory with a OAP pulley. I'm running a 150a alternator with 14.2v at idle (13.7v on the CCU) even with lights and aux fans running.

Also the slip ring in your alternator could be worn out and ground straps might need cleaning.

VW1300 · #3 07-10-2016, 05:05 PM

Ok, thanks for the advice. Will pull up back seat and measure there to see how it compares. I'll also put the CCU panel up tomorrow on the way to work to see what it shows while driving for a while. Will report back my findings.

Re: slip ring - is this repairable or does it mean a new alternator?

No doubt 90a is a wimpy alternator for this application but it is what it is...

tjts1 · #4 07-10-2016, 05:08 PM

Yes the slip ring is replaceable along with the bearings and diodes but its pretty involved. Check for a groove in the slip ring. Depending on what tools you have and time you might be better off getting a rebuilt alternator.

97 SL320 · #5 07-10-2016, 06:30 PM

This slip ring is perfectly serviceable from a wear and coloration standpoint. I would not turn it down as that gains nothing. However I would deduct the ledge wear from the min acceptable brush length. The brush path is supposed to look slightly blackened, this is carbon brush transfer that provides lubrication.

There is a whole black art when it comes to electrical device brushes.

Also note the difference in wear between the two. I'd expect the slip ring with the most amount of wear to have the least worn brush. This is due to the direction of current flow. One brush / slip ring is current in and the other current out.

jay_bob · #6 07-11-2016, 08:50 AM

Hmm

My E300 reads 12.9 to 13.1 all the time no matter what on the CCU.

Next time I'm back home I will clip my Fluke 87 on the battery, enable the min/max recorder, and go for a drive to get some data.

dieselbenz1 · #7 07-11-2016, 09:02 AM

I thought when checking voltage on the ac panel on the 210 series it consistently was .7v lower than actual battery voltage.

VW1300 · #8 07-11-2016, 09:53 PM

FOLLOW-UP:

1. Drove to work with CCU diagnostic on - read 13.3V steady
2. Got home and hooked up multimeter to battery terminals while CCU diagnostic was on...sure enough, it read 13.8 when CCU read 13.1 at idle.

OK, this time I'll remember the CCU reads 0.7V or so low. Thanks to all who replied.

On the other hand, on Sunday I also replaced a broken auxiliary fan belt and it really does make a difference with the AC on these 96-degree days.