Touchy W123 Voltage Regulator - 3 Warning Lights on at Same Time - PeachParts Mercedes-Benz Forum

pwjeep · #1 09-13-2008, 07:35 PM

The fuel resevoir, brake AND pad wear indicator are all on at the same time. When I press down the parking brake the brake warning light gets brighter. (when I release the parking brake the light gets dimmer) By the way, the master cylinder was full, the brakes were fine and the fuel was at 3/4+.

The first time this happened after I zapped the battery with an electric spark arc with a crummy battery disconnect (green thumb screw which I dumped) unit. This happened when the connection was not tight and the engine was running. My fix then was to replace the alternator voltage regulator with a new one. It seemed to work fine (no warning lights lit) until got out the jumper cables and I jumped another battery with the car running. Yup, I zapped the battery with another electric spark (doh!). No, I didn't put the negative jumper cable on the engine to ground it and the positive cable on the positive terminal.

I then got a used regulator from a friend and changed the regulator again, hoped that this solved the problem. However, beware of good looking voltage regulators from unknown sources. (unknown condition of used unit). I thought I was on the right track, but I still had the same 3 warning light issue.

I again bought a new voltage regulator and installed it. TA DA!!! the problem is solved.

So the lessons learned are: (1) don't jump another car with battery cables hooked up directly to the battery when your car is running. (2) hook one jumper cable to the engine block or some other ground. (3) don't trust used electronic stuff unless you know for sure that it was working when it came out of the donor. (4) the diodes in the voltage regulator are really touchy and don't survive electrical shocks, (5) don't use the cheesy battery shut off unit with the thumb screw.

My replacement time has improved from a sweaty 2 hours to about 30 minutes. So I guess I got better.

Jeremy5848 · #2 09-13-2008, 09:07 PM

The last model W123 (1985) has an over-voltage protection relay hidden behind the right front (passenger) seat's right kick panel. It's possible that you zapped it during one of your misadventures. Several electronic circuits get their power from it. The later W124 models also had this thing and the W210's infamous "K40" relay is a descendant.

pwjeep · #3 09-16-2008, 10:28 AM

Here's another chapter in this saga...

The lights are back on. (see original post) I checked the Overload Protection relay. It is located to the right of the glove box above the right kick panel. Even though it looked OK (fuse was fine), I swapped it out with another relay. The warning lights were still on.

We checked the cluster and found that one of the ribbon printed circuits had broken and there was melting going on. So we soldered the wire and insulated it for a quick fix and replaced a light bulb that looked suspect. The warning lights were still on.

Next we swapped out the alternator with a known good used unit. This time all the warning lights went out.

Now I have to find a good fuel gauge / oil pressure / Temp gauge module to put into the cluster to replace the fragile repair we made.

Many thanks to W. Roy Hunter for his help and the late hours we worked to hopefully resolve this issue.

pwjeep · #4 10-12-2008, 08:32 PM

The next part of this saga was that the used alternator we installed had a little issue. I heard a rattling noise over and above the usual and customary diesel noise. W. Roy Hunter and I diagnosed the issue as a bad alternator. Granted the unit we installed was used and you always take a chance, but this was certainly not expected.

What we found, aside from some angst about having to replace the alternator again, was that the shaft that holds the alternator pully had somehow become corrupted. By that I mean the pulley, even though the retaining nut was tight, would wobble on the shaft which caused more wear. Eventually, we figured that the shaft would be totally rounded and we would have an idler pulley which would not rotate the alternator at all.

So I had a spare 350SDL alternator. We removed the heat shield and the extention studs because they weren't needed and interferred with other stuff. We removed and replaced the worn regulator and I'm good to go... For now.

Oh yes, I have good voltage and all appears to be working with no warning lights showing.

npretnar · #5 02-07-2012, 09:02 PM

Is the Overload Protection Relay ONLY on the '85 W123's?