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Hey everyone,

I recently got a 1986 Mercedes 300e. I got the car for free and just got it running recently. The car always had trouble starting when cold. It would stutter and stall out for awhile and when I did get it started it would idle really rough until it was warm and then it idled and drove really well. Today I got in after just replacing the battery since the old one was shot and it won't roll over and it doesnt seem to be getting any power at all. The car was running yesterday with the new battery. I tried jumping it and as soon as I hooked it up all the lights came on and when I tried to start it I heard a click and then everything shut off again. I removed the jumpers and put them on again and I got power again and as soon as I tried to start it, it popped and everything lost power again. My initial thought is a starter relay or something like that. Does anyone have any better ideas?
 

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1989 W124 260E
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Pull out the altrernator relay its held in with just 2 philips screws .Take it out and check the length of the carbon brushes. They should be 3/4 in length .Good one on the left .
CIMG6541.JPG
 

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I tried jumping it and as soon as I hooked it up all the lights came on and when I tried to start it I heard a click and then everything shut off again. I removed the jumpers and put them on again and I got power again and as soon as I tried to start it, it popped and everything lost power again. My initial thought is a starter relay or something like that. Does anyone have any better ideas?
This is similar to what happens with a bad connection at the battery terminals or within the battery. Have your new battery load tested by the folks you bought it from and make sure your terminal connections are clean, tight, and are stout (no "aftermarket terminal connectors where you bolt on clamps over the cables). Report back on this and the previous suggestion to check the alternator regulator.
 

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2003 G500, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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I doubt your symptoms are from the voltage regulator or the starter itself. Totally agree with tuttebenne that this is typical for a bad battery connection. You should also check Fuse 5 just to be sure it's okay as this fuse powers the starter circuit.
 

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1990 Benz 300E 2.6 (mine) ; 1994 E320 Wagon (wife's) ; 1993 300CE Cabrio, needs some TLC
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the battery - terminal has one big fat wire that goes directly to chassis ground at the strut tower in front of the battery.

the battery + terminal has 2 fat wires, the fatter one goes directly to the starter, and the other goes to the X4/10 terminal which is mounted on the side of the strut tower facing the battery and under the grounds. X4/10 has 3 terminals, 2 large ones are connected together, and are where EVERYTHING in the car gets its power from, the 3rd smaller terminal is the alternator D+ circuit.
 

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Nathan did say he had fitted a new battery ,,.and the next day it was flat . The relay is a reason for the battery to go flat over night . Cost nothing to check it out .
 

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This may be a nuance, but the OP hasn't said the battery was dead. He described a problem that could be a dead battery but could also be something else. One-day-old batteries are rarely totally dead (no lights at all) the following day. First step would be to validate connections and condition of the battery before suspecting it has a drain or a charging issue IMO.
 
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