1985 500sel fuel pump relay - PeachParts Mercedes-Benz Forum

fpharryc · #1 11-01-2009, 10:41 PM

I have wasted 3 of my spare fuel pump relays in 2 days. Each of them run about 25 minutes then "burned out". they are very hot when touched after the car stalled.

500sel ran fine before each relay went dead. There is no noise from the fuel pump. Fuel pump and fuel filter are about 30k mile old.

I made a jumper, installed it to bypass the relay, car starts an runs fine again. I am frustrated, of course.

My question is:

(1) Can I just drive with the jumper I made? The fuel pump does not run unless the ignition is on. Is it safe to drive like that longer term?

(2) Can the pump simply bad (draw too much current, shorted) in spite of no noise from it?

(3) Maybe the filter is due for replacement? But the car runs fine?

It is too messy to fool around the fuel system to me. Did that 30k miles ago, honestly not really interested in doing it again.

wbain5280 · #2 11-02-2009, 02:43 AM

Quote:

Originally Posted by fpharryc

I
My question is:

(1) Can I just drive with the jumper I made? The fuel pump does not run unless the ignition is on. Is it safe to drive like that longer term?

(2) Can the pump simply bad (draw too much current, shorted) in spite of no noise from it?

(3) Maybe the filter is due for replacement? But the car runs fine?

1. You can do this for a while.
2. Not usually.
3. Probably.

Better yet, the fuel pump relay connections on the PC board are probably cracked and need to be resoldered. You did mention it gets hot.

dhjenkins · #3 11-02-2009, 08:51 AM

You might consider putting an in-line fuse in your jumper, just in case.

It would also help troubleshoot what the issue is. If it pops a 10A fuse, then there's something definately wrong and you can go from there.

Lenny · #4 11-03-2009, 03:20 PM

I wouldn't drive the car with out a fuel pump relay. It has built-in saftey features that shut off the fuel pump when engine rpm is not sensed (shuts off fuel supply when engine is off). When you say that the relay is hot, is the holding coil burning out or are the contacts carrying too much current. You could have a short in the load side of the relay.

fpharryc · #5 11-03-2009, 09:58 PM

That's what I am afraid of. A short somewhere. Where the short could be? The pump? The wires? I need to measure exactly the current the system is drawing.

mak · #6 11-03-2009, 10:29 PM

The current draw can be measured at the jumper points of the relay.
you may like to remove the wires from the pump and check for current draw again . if there is none ,then most likely the pump drawing too much current. check pump directly for current draw, wiring at end points

dhjenkins · #7 11-04-2009, 08:59 AM

Quote:

Originally Posted by fpharryc

That's what I am afraid of. A short somewhere. Where the short could be? The pump? The wires? I need to measure exactly the current the system is drawing.

Welcome to the wonderful world of wire tracing. Hopefully it's the pump, otherwise it might be easier just to run new wires; you could have a rub anywhere from the power distribution box all the way to the pump that might be knocking against the frame in certain conditions causing a dead short and killing your relays.

If you've got an ammeter or DVOM, like it's been said, you can test at the pump or the relay.

Personally, I'd disconnect the pump from the harness and measure amp draw while appling 12V directly to rule it out or verify it's bad - like I said, there's plenty of things that can go wrong between the relay and the pump.

If you do a lot of electrical troubleshooting, you might want to invest in a power probe. http://www.powerprobe.com/powerprobe/PP2.html You can get them for about $70 on ebay and they'd be worth every penny at twice the price.

fpharryc · #8 11-08-2009, 09:00 PM

Check the current draw by setting up an inline fuse. Upon turning the ignition to "on" position without starting the engine, the current immediately killed My 10A inline fuse.

Changed the fuse to 15A one, the pump kicked on without problem. Turned the engine, the engine started without problem either with 15A fuse. the inline fuse wire is hot. Very hot.

I think it's the fuel pump. Apparently the pump is drawing current that killed my relays.

What do you guys think?

dhjenkins · #9 11-09-2009, 10:06 AM

If you want to test the current for the fuel pump, install the in-line fuse at the pump itself and power it directly from the battery (after disconnecting it from the harness). If it blows then, it's definately the pump, and you've ruled out any possible wiring or other issues that could cause the fuse to pop at the relay point.

Don't get me wrong, it's probably the pump, but it would be a shame to spend all that money and time and still have the same problem.