While at cruise speed on the interstate today my radio went off (lost power) and the ABS light on the dash came on and stayed on. If I would touch the brake the tach would immediately drop to zero and once I took my foot off the brake it would jump back up to the correct reading. The brakes worked fine. After about 3 minutes the radio came back on and the ABS light went off but then the indicator next to it, which signals a light bulb out, came on and stayed on. Once I completed my trip home I checked all the fuses and lights and they looked OK. I restarted the car and everything was back to normal. No indicator warning lights. What gives?

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Tony

1988 300E Metallic Blue Black/light Gray MB-Tex leather interior

MBCA Member

You should replace the overvoltage relay. Also, You may have experienced a low voltage situation. The brushes in the back of the alternator may be faulty. I would check them or have them checked.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Where is the overvoltage relay located and what is its function?

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Tony

1988 300E Metallic Blue Black/light Gray MB-Tex leather interior

MBCA Member

Tony -

This is too strange. My daughter was driving my '86 300E two days ago and it did exactly the same thing, except that the tach surged and it would not start after she parked it (low battery).

My overvoltage relay is new. Its function seems to be to protect all those delicate little relays and control units from too much volage. It is located behind the battery. It is silver with a fuse on top of it.

I replaced mine a couple of weeks ago in an attempt to get my ABS working and because it seemed to be mentioned in connection with just about every electrical problem. It costs about $45.

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Chuck Taylor
Falls Church VA
'86 300E 5 speed
'95 C220 (wife's car)
'98 Porsche Boxster
Past: '79 280E, '82 300D (18 yrs), '77 240D,
4 250C's

At lunch today I went to a local auto parts store to pick up new brushes/voltage regulator for the alternator. A friend at work said that was his solution to the exact same problem. In any event, when I left the parts store the car would not start. So... I left the car with them to install the new part while I returned to work. I had to leave the car with them overnight since they said they could not get the car to start. They want to do a diagnostic and then see what needs to be done. I am not confident they know what they are doing when it comes to a MBZ. If the problem is the overvoltage relay, I can pick up the part tomorrow and give it a try. Any ideas?

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Tony

1988 300E Metallic Blue Black/light Gray MB-Tex leather interior

MBCA Member

If the car failed to start, you have a low voltage situation and the alternator brushes are where to start. I think the part should be about $80? or so.

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Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

Amazing that now yours has failed to start - exactly like mine. I went to the car tonight and retrieved the battery. It is charging now. Tomorrow I will get a ride to the car, and re-install the battery. Hopefully the car will start so I can get it home and check it out.

I also suspect the regulator and brushes, although I checked it last week and measured 12.5 volts with the engine off and 13.87 volts with the engine running.


Chuck
Tonight the battery showed 11.5 volts. Is that not enough to start the car?

Update -

Car started fine with freshly charged battery. Had 13.9 volts when re-installed. Drove the car home stopping to pick up regulator and brushes at dealer. Voltage now 12.2 and not changing with motor running.

Chuck

As you've figured out by now, funky lights and jumping tach in a W124 (or W201) means a low battery and no charging which is about to leave you stranded. GET GENUINE BOSCH PARTS FOR THIS! Yet another lesson I learned the hard way when the local shop replaced my 300E's alternator 3 times in 6 months. You guessed it, I was stranded 3 times, once 100 miles from home.

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Rick Miley
99 E300 TurboDiesel
00 Jetta TDI

Replaced brushes and regulator and now have 13.5 - 13.7 volts while driving. Thinking I have solved this one. Now back to the disfunctional Klima relay.

Why would the warning light not tip you off that this was happening?

Chuck

After two attempts the repair shop correctly installed the brushes/voltage regulator. Now no more problems. I would have done the job myself but the car would not start in the parking lot of the parts shop. One tip...the brushes are spring-loaded and must be depressed fully to get the part installed correctly. If this is not done the part will not go in and make proper contact. Cost $28 part and $52 labor, plus the cost of a rental car since they didn't get it right the first time and they kept the car overnight.

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Tony

1988 300E Metallic Blue Black/light Gray MB-Tex leather interior

MBCA Member

I have a similar problem with my 93 300CE (W124 coupe).

Battery was low one day, and we had to jump-start the car. ABS light stayed on from then on.

Hopefully its just the over-voltage protection relay. How does one diagnose this part?? Please include detailed instructions, and pictures/diagrams if possible.

Thanks in advance.

Manuel (nsx1164@speakeasy.net)

I had some of the symptoms described here, but mainly on starting as well as central locking problems. Although irrelevant at first I learnt a lesson that might help here ....


I also tested everything and it turned out that I had two cells in the battery that was on their way out. The only way I could test for this was to check the SG of each cell. One bad-ish cell caused a low (but not abnormal) operating voltage whilst the second cell failure caused discharge of the battery on the internal resistance overnight. The starting problem was intermittent so the failure of the battery did not occur to me until the scond cell failed completely.

All voltages checked out OK - the SG on two cells were very low compared to the other 4.

check the SG on the battery - it might lead you in the right direction - not only the charge and unladen voltage.