Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
Hi, the fuel pump fuse only gets power when you crank the motor or when the car is running. Was the fuse tested while cranking the motor?
Did you replace the fuel pump relay?
If the car only runs when you hot wire the pump you need to test the pump energizer circuit.
Was the fuel pump relay replaced?
Are you getting my replies now?
Ok, before we go any further I see that you rejected Michael's help yesterday. I will be glad to give you advice on this but it is not fair to the expert if you work with him and then you reject the answer. I can tell you what needs to be checked but your mechanic needs to do some tests in order to find the problem. If you can assure me that you will click Accept for my help I will continue with this question. Otherwise it is not worth my time to try and help you.
I did see that, and I understand. Perhaps you should have waited for Michael to come back online or accepted his help so far and opened a new question. The experts only get paid when you click Accept. Let me explain what needs to be done from here.
First your mechanic needs to bypass the fuel pump relay and see if the pump runs.
In order to do that he must remove the relay and place a fused jumper lead in terminal 3 and 5 of the relay plug.
If the pump runs it tells you that the circuit from the relay to the pump is good.
If the pump does not run with the relay bypassed he needs to test for power on terminal 3.
If he has no power on terminal 3 then the fuse box is bad. If he does have power on terminal 3 and the pump doesn't run when the relay is bypassed it also means that the fuse box is bad.
It's fairly easy to change the box. Just disconnect the battery first then unscrew the box then lift it up and disconnect all of the connectors and plug them into the new box.
The fuse box can fail internally on one circuit but he needs to make sure that he has completely tested the circuit.
If the pump runs when the relay is bypassed like I said that tells you that the fuse box is good from the relay, through the fuse and out to the pump.
Correct, but read my last reply.
He also needs to test the inputs to the relay that turn the relay on. Those are the other two terminals on the relay.
Yes, but his final test is to lift the fuse box and test the inputs to the relay from the engine control module.
If he is getting the correct inputs to the fuse box but power and ground is not traveling through the box and energizing the relay he needs to replace the fuse box.
You're quite welcome.