The OP was last here over a month ago (Aug. 26); everything since that date has been preaching to the choir.

I'm lurking and getting some good info which I appreciate.

I have tried several different regulators recently. My typical voltage looks like this:

Key on - 12.4 volts
Immediately after starting - 13.4 volts
Next 15 - 20 minutes - 12.4 to 12.9 ish
Then it dips to 11.7...sometimes down to 11.5 or even 11.3
Then it will go back to 12.9 to 13.0 for a while, maybe 10 minutes
Then it goes back to 12.5ish and will stay there for the duration of my trip

It's not linked to RPMs and load don't seem to matter much.

I have no idea why it does that.

It's the upgraded alternator from fattywagons with a small pulley that spins it faster than standard.

I have a new Bosch regulator in it now 1 197 311 022. But it has similar performance with almost any regulator I have tried. I have used the Regitron(?) VRB195MA. I have not tried the Bosch 30091 aka 1 197 311 028. Also wondering if the regulators made for the lower amperage alternators might work better. But I don't know what makes these things tick.

I did install a secondary ground from the alternator mount bolt to the chassis.

I don't know if this is normal or whether I'm using the wrong regulators or if there is something wrong with the alternator.

Thanks for your help.

Jim

Voltage is only one half of the equation. Amperage must be considered also. A large ampere draw will result in a low voltage condition.

One poster mentioned the glow plug relay. An aftermarket stereo amplifier can also be a culprit.

Use an ammeter to determine if you have a large current draw with the vehicle running.

Just make sure the ammeter can handle the amps. I blew one trying to read amp draw of a blower motor. the meter had a 20 amp limit. the blower was pulling 25 amps. that was on my w123 years ago. The clamp on Fluke is accurate.

Similar case , I was having an issue after replacing my alternator , the alternator would slowly fail after several weeks of driving . This was a Bosch unit with a 65 amp output that I purchased from Autozone , When unboxing the 3rd replacement it mentioned an a tag on the unit to have the battery fully charged when installing the alternator . It was but I kept getting a low voltage reading , soon after I had a glow plug relay go dead , replaced the relay and haven’t had any issue since , might be this relay staying on , not sure but it was related in my case .I now have a voltage gauge installed on my dash and will be adding one to my other w123s very soon .

I second the relay .

That's great information, thank you. I will check that relay.

Jim

Voltmeter
 

I too like to keep an eye on the system's voltage so I bought a tiny little meter that plugs into the ciggie lighter socket and had two USB charging ports to boot....

The first one I bought came with a BLUE LED display, it cannot be seen in daylight .

About $15 on Amazon, I'm sure plenty more out there cheaper....

Thanks for the help everyone! I replaced the glow plug relay and that seems to have taken a significant intermittent drain out of the system. My voltage is staying above 12 volts with high load at idle RPM and stays 12.8 - 13.2 while driving now.

When my car is warmed up, it charges 13.8 at idle. That is without lights/HVAC.

When cold, does the GP relay cycle until the engine reaches a certain temp? My Dodge with the heat grids does until the engine gets above 60 degrees F.

General comments. When people have been having trouble with there Alternators they seem extremely reluctant to remove the Alternator and take it to some auto parts place for a free test.

Another issue is that when you add a ground wire my opinion is it should go from the Alternator Housing to the Negative Terminal on the Battery.

I had my Alternator off last month to change the rear bearing and slip ring. When I put it together I mad sure the areas the yellow and red arrows point to were clean so there would be good contact with the Voltage regulator.

The same on the Voltage Regulator.

As it turned out it charges well with no added ground wire. If I was to use a ground wire I would get a forked or eye type connector and install it under one of the Voltage Regulator screws (perhaps removing the washer on that one) or you could get longer screw and mount it where the radio anti static capitulate is mounted.

I also did the best I could to clean the 3 connector blades on the alternator that go into the Electrical connector. I wrapped steel wool around a screwdriver blade and moved that around the best I could.

And then of course there is the Alternator Connector itself. The plastic housing comes a part and you can clean and or inspect the wiring there.

If your Alternator Connector is bad you can get generic connectors from the auto part places as Bosh alternators were used on lots and lots of vehicles.
Mercedes did sell the plastic housing and the terminals seperately.

I tried a china brand once, and finally within two weeks had a bosch

You still have problems. 13.2V is not enough to charge the battery fully. When working properly, these alternators typically run around 13.6-13.8V depending on temperature (which still isn't enough for modern batteries, but a big leap forward from 12.8 - 13.2).

For many many years I got a charging rate of about 13.7 volts some times slightly more even though I had cleaned the grounds and terminal contact areas.

When my Battery finally died I did not replace it with one specific to Mercedes and I had to get a different ground cable which I scrounged from some I had made up and left over from an electric wheel chair. After that It went up to around 14.10 volts.

When I rebuilt my Starter I checked out the positive batter cable and I cut a split into where the cable was inserted into the positive batter clamp cleaned out the bluish corrosion and shove it back in and got a torch and soldered it in (something I also did when I made the Wheel Chair Cable. Copper Eye crimped on and then I Solder so as to fill the interior of the crimped collar with solder).
And with all that I cannot remember if the charging voltage went up or not.

Last Month I had the alternator apart and I changed the Bearings, Brushes on the Voltage Regulator and the Slip Ring on the Alternator as well as cleaning the areas mentioned in the previous post and I am getting 14.34 volts.

The Alternator is a rebuilt by bosh north america on and I had around 2008 changed the bearings on it and got no noticeable change in the charging voltage.
This is the Voltage Regulator that came with the care in 2007 Actual Voltage Regulator on my Alternator Bosch 1 197 311 003 EL14V4B Made in Germany

Over the years the cleaning of contact areas and the replacing of the Ground Cable has resulted in a higher charging rated. This last time I did change the Voltage Regulator Brushes and slip ring. However, I have not read of anyone changing the Brushes getting a higher charging rate from that and I don't see how changing the Slip Ring would to that either.

Also it has been over 6 years ago since I changed the Steering Colum Lock which had a new Ignition Switch and that did not change the charging voltage and I have never changed the Charging Light Bulb (the wattage of which can change the charging rate) in the Instrument Cluster.