non start fuel pump not running

[Alan Fallow]

have a 4matic 3.0 auto merdedes. It started cutting out. When you turn on the key you sometimes got the fuel pump running as normal but then it stopped all together. Mercedes dealer said it was the fuelpump relay behind the battery beside the relay with the two fuses on top. I bought one i think £90.00 but the same problem. Ignition and engine turns but the fuel pump does not run at all.

 

[Malcolm Brown]

You seem to have eliminated the relay as the problem. Here are some thoughts on checking the other bits. Crocodile clips, wire and matchsticks are required.

Try removing the relay and bridging pins 7 & 8 in the socket (short length of wire wedged in with matchsticks). This should run the pump constantly. If it doesn't run, you either have a broken pump or faulty wiring.

You can check the pump by running a 12v source (say from the battery) direct to the +ve terminal on the pump (the brown wire is earth; the other - not sure of colour - is +ve) and checking whether the pump operates. If it doesn't, additionally connect the pump's earth terminal to a good earth somewhere on the car or to the battery -ve terminal. If the pump still does not run, it is broken.

If it does run, then you probably have faulty wiring somewhere. The first place to check is the connections to the relay socket. After that, you'll need to work out how the cabling runs from relay to pump and where the other relay connections arrive from - which means a wiring diagram will be required.

Also, bear in mind that your immobiliser and/or alarm could have been wired to interrupt the fuel pump power supply. I have had a very similar prob to yours that turned out to be a faulty alarm.

Happy hunting!

 

[Alan Fallow]

thanks malcolm
the only problem is the relay numbers there is no 7 or 8 it is 50 30 87 etc i will try and bridge anyway
thanks alan
ps i removed imobiliser still inop

 

[Malcolm Brown]

Pin 7 = ISO 87, pin 8 = ISO 30 - so it's 87 & 30 that you should bridge.

 

[Alan Fallow]

malcolm
bridged pin 30 and 87k i got a click from the engine somewhere no pump then bridged pin 30 and 87v no pum but a click somewhere else.
I am bafled

 

[Bruce_Jeffares]

Fuel pump not running

I had exactly the same problem last week, identical car too.

My local 'man' advised removing the relay and putting a jump lead between pins 15 and 87; worked a treat. It is now on a week's trial before I get a new relay.

I know this differs from other advice; I do not pretend to fully understand the intricacies etc. and would not argue with anyone. Results are very good though.

Hope you get yours fixed !

Bruce

 

[Bruce_Jeffares]

Fuel pump not running

I had exactly the same problem last week, identical car too.

My local 'man' advised removing the relay and putting a jump lead between pins 15 and 87; worked a treat. It is now on a week's trial before I get a new relay.

I know this differs from other advice; I do not pretend to fully understand the intricacies etc. and would not argue with with anyone. Results are very good though.

Hope you get yours fixed !

Bruce