C240 won't start
 

M 2001 C240 decided not to start last week. The car was cranking just fine, but there was no fuel pressure.

I checked the fuel pump relay and it works.

Fuse #4 in the trunk looks good.

The fuel pump was not turning on with the key in the "on" position, so I ordered a new fuel pump and installed it. The car ran fine for a few days although the fuel gauge showed 1/2 with a full tank. 2 days later it died again. Fuel pump not turning on again.

I put in a new unit (fuel pump and fuel sending unit) today and its still not starting. the fuel gauge is now stuck on empty as well.

I checked the wiring to the sending unit and it seems to be showing voltage for a split second when I switch the ignition to "on" but then it disappears.

Fuel pump is working when hooked up to a battery.



Running out of ideas.

Greg

WDBRF61J91F090553

Both #4 and #53 are good. The fuel pump relay is working as well when tested with a spare battery.

Unfortunately, my fuse diagram doesn't number relays, so I'm not sure where #87 is.


Would a faulty crank position sensor shut off the fuel supply?


Edit: Circuit 87 relay is in bay K if I'm not mistaken, and it functions properly.

Bad Crank Sensor will shut off fuel but usually you'd get trouble first in hot conditions.

And in my case, cranking for a long time tripped the P0335 code

No previous issues. No codes.

What surprises me the most most is that it worked perfectly fine for a few days after swapping the fuel pump and then it died again.

Well, I found the problem. The Sam unit in the trunk was actually melted in the back where the wires attach. Something caused the plug to get hot and created a short. I replaced the unit and it started right up. The only issue left is that the gas gauge still shows empty. :confused:

I read on the "other" forum that new fuel pumps are not compatible with some old instrument clusters. You may need to put back your old fuel pump back and if you need to just swap the motor of the defective pump with a good motor from the new pump.

The fuel level is determined by resistance, yes? I'm curious as to what the difference is between the new and old pumps.

[Fuel level] sender units are attached to both fuel pump as well as fuel filter. The sender unit can be swapped from the old unit to new unit.

Unfortunately I have not kept the old pump assembly.

What worries me is that the fuel pump relay is still getting nice and warm when the car is running. Are they normally pretty warm after a drive?

yes, but should not get hot enough to melt the plastic or burn wires.

My next question is do I have to hunt down an early production unit and install that, or can the computer be reprogrammed to play nicely with the new one?

When I spoke to the dealer, they said they only carry the new style pumps, so they must make them work in the early 2001 production models somehow...