1972 280SE 4.5L- What does relay behind battery control? - Mercedes-Benz Forum

 
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post #1 of 7 (permalink) Old 11-12-2006, 01:03 PM Thread Starter
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1972 280SE 4.5L- What does relay behind battery control?

1972 280SE 4.5L- What does relay behind battery control?-relay-picture-2-mercedes.jpg1972 280SE 4.5L- What does relay behind battery control?-relays-mercedes.jpgI've been trying to troubleshoot a problem of suddenly not getting 12V to my fuel pump. Several threads made me think it was controlled by the rectangular relay behind the battery. However, from looking at the plug the relay attaches to, there are 6 round slots, and the upper right and lower left (positions 1 & 6) do not have a metal pin sleeve insert. A brown wire goes to #5, a blue with white stripe goes to #3, a purple with white stripe goes to #4, and a big blue and small blue wire goes to #2. If you look at the bottom of the relay that plugs into this, the upper right pin is 87A, the upper left is 87, the middle right is 86, the middle left is 30/51, and the lower left is 85. So, no matter which way you plug the relay in, the 87 pin goes into an empty hole. So, whatever this relay is controlling, I think the circuit is closed until 12V is applied. Therefore, I don't think this is the relay for the fuel pump.

Does anyone else have any experience with this model??
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post #2 of 7 (permalink) Old 11-12-2006, 01:35 PM
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It is not a fuel pump relay. It is deemed a "change-over relay' and seems to be for A/C, electric windows, seat belt warning, and/or turn signal. Mercedes number is 000 542 5919.

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post #3 of 7 (permalink) Old 11-12-2006, 03:39 PM Thread Starter
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Same question that I have on my other thread- Any idea which relay might give voltage to the pump? Since none of the wires off of any of the other relays I checked showed continuity to the (+) at the pump, there must either be another device between the relay and the pump, or there is another relay/fuse somewhere that I haven't seen, or there is a break in the wire between the relay and the pump. Any suggestions?? And, do you know if there is a schematic/photo anywhere out there in the vast Internet that I can buy that shows the electrical layout for this vehicle?
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post #4 of 7 (permalink) Old 12-29-2006, 05:15 PM
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Did you manage to find a schematic for the wiring system? If not, I might be able to help...
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post #5 of 7 (permalink) Old 02-20-2007, 04:18 AM
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I could use one. .

I've been trying to find the cause of an intermitent misfire that develops in my '72 280SE 4.5L after running like a dream for a day or two. Initially, it presented as a points problem, so I put in a Petronix ignitor. That first approach had the same result as pretty much everything I've tried (over the past year) since, which is that it appears fixed, then horror as the problem returns.

It goes from champ to chump & back with no rhyme or reason. It seems like a short or component failure, but I've replaced more wires/components than I care to list, & am at wit's end.

As prevalent as these cars are, why can't I find a shop manual? Not something generic, or for a similar model/yr, but this exact car?
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post #6 of 7 (permalink) Old 02-20-2007, 11:51 AM
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Have you cleaned/replaced the two sets of trigger points inside the distributor? they, over time get fouled with crankcase oil fumes.
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post #7 of 7 (permalink) Old 02-22-2007, 04:02 PM
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Yes, I tried that before installing electronic ignition.

Where do I get a good manual/wiring diagram for this car?
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