In regards to my previous post(300E FUEL PUMP), I pulled the relay from behind the battery and the connectors looked like one maybe two were burnt. Is this the only way to tell if a relay is bad,is there a test that you can perform or just change the relay?

While it is difficult to directly test the relay, its function isn't so difficult to analyse.

The simple way is to bypass its function. This involves jumping between the 87 and 30 terminals on the car. If the relay is the problem this should run the pump (it will do it instantly; without the key being on - be aware). With it running the car should start if this is your hypothesis.(fuel supply being the problem).

NOTE: the fact that the fuel pump runs and the car starts DOES NOT fully indict the relay as it has many inputs to be functional. All this test does is prove the hypothesis that power to the pump will run it and that with it running the car will start.

Turns out that the relay is such a common problem (almost always bad internal solder cracking) that doing this test probably infers a bad relay.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Thanks Steve,
I was getting ready to ask you,"according to the service manual, my car(1986 300E)calls for relay#002 545 92 05. What's in the car now is 003 545 17 05. Unless I'm reading this wrong,how significant is this difference.

I own '84 380SL, two years ago the pump just quit. I pulled out the controller from behind the glove box, opened it & resoldered all connections - works great since.

I have been trying to troubleshoot an idle problem and the procedure in the MB CD basically checks all the inputs to the relay and not the relay itself. It seems that if the inputs are all good, then it must be the relay.

Steve G
'85 500SL (Euro)

Does my 84 SD have such a relay? I drove my 83 SD with a pretty dead battery and no charging system, and the car didn't die. My former 91 Volvo 940 wagon wouldn't run at idle with a dead battery, so it must have had an electric fuel pump also. I've read that the M617 engines can run with no electrical system... wondering if this is true.

Robert W. Roe
1984 300SD 171K mi Optima 800U battery

[This message has been edited by Robert W. Roe (edited 09-14-2000).]

Yes Robert, it is true that diesels will run without electrical power. The diesel fuel combusts through high compression, whereas gasoline combusts through a combination of lower compression and spark. You will need electrical power to power the glow plugs when starting though.

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Mike Tangas
73 280 SEL 4.5

I get a totally different number when I look it up: 002 545 27 05. MB has so many numbers for those relays. What is important is that it say 6cyl and give a reasonable rpm for cut-out (around 6000 on 6cyl).

If is were a 4cyl it would cut out way too soon as it would see the 6cyl as going 50% faster than real.

I replaced my fuel pump relay with a spare that I had and the car started. Upon inspection, the old one has a burned solder joint. I'm going to re-do it and then try it again.
The spare, (working), relay did not have exactly the same pins as the failed relay, but the car works anyway. Why?
Both relays said 6350 KVS which is, I gather, the cut-out rpm. Could I get higher rpm with a higher KVS? I don't really need more rpm, just wondering. Still, I do hit red line rpm pretty quickly in first gear....

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VR
1967 250SL