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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53902
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Mercedes ml320: wont start, started..Ran fine

Resolved Question:

Mercedes ml320 won't start, cranks but doesn't start.......first time it happened I let it sit for a few hours went back outside and finally started so they changed crankshaft ssensor for me, 1 week later experienced same problem drove to store , came put car started and died then wouldn't start, I then replaced fuel pump n filter! Ran fine for 2 weeks then the other day went to store and ready to leave it started then died and won't start again!! Mechanic wired pump direct o. Battery and it starts ! Did a test on my fuses and everyone would beep but the fuse pump fuse, but the fuse itself is not blown!
Submitted: 5 years ago.
Category: Mercedes
Expert:  Marty replied 5 years ago.

eurotec :

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?

Customer: Yes it was tested while cranking ....he had some tool that beeped on each fuse under hood, but when placing on fuse for fuel pump it did nothing!! I was told they thought was some bad wires underneath fuse box...I'm at a lost I just don't know where to go from here
Customer: The car won't run at all cranks and will not start!!!! Only way it starts is wiring pump direct from battery which I know because mechanic tried that to make sure my pump was still working!!
eurotec :

Did you replace the fuel pump relay?

Customer: Doesn't even sound like the fuel pump comes on when I turn key to the strt position!! It's like it's not getting power to start , I'm trying to explain this as best as I can you can tell me what my problem is !
eurotec :

If the car only runs when you hot wire the pump you need to test the pump energizer circuit.

eurotec :

Was the fuel pump relay replaced?

Customer: Are u still here?
Customer: I'm not seeing anything ...I cannot even see my stuff I'm saying to you
Customer: He took the relay from another spot and changed it , still nothing !!
Customer: What is the pump energizer circuit ? Where is that
eurotec :

Are you getting my replies now?

Customer: Yes sir...I refreshed ty
eurotec :

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.

Customer: I only rejected it because he went offline......I was not finished if u can see our chat...I had dozed off to sleep and woke back up and he was gone ...
Customer: I will accept it, promise and nay reason I did if u can see I say to last one , I apologized that I dozed off and he was offline so I did not finish is only reason why
eurotec :

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.

Customer: Ok thank you
eurotec :

First your mechanic needs to bypass the fuel pump relay and see if the pump runs.

eurotec :

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.

eurotec :

If the pump runs it tells you that the circuit from the relay to the pump is good.

eurotec :

If the pump does not run with the relay bypassed he needs to test for power on terminal 3.

eurotec :

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.

Customer: my question was ...cause he did most f that what u said and pump ran, how complicated to change fuse box?? He's a great mechanic I have but he's no Mercedes expert and has never really worked on a lot of them, he was just stumped on as to why the fuse wasn't blown but it wasn't getting any power from underneath !
Customer: But if the whole fuse box was would the other fuses be working for my other things ?? It's just the fuse to the pump that's getting no power!!
eurotec :

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.

eurotec :

The fuse box can fail internally on one circuit but he needs to make sure that he has completely tested the circuit.

eurotec :

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.

Customer: Ok! Thank the box can be bad and still allow all the other fuses to work, the onl one not working is fuel pump fuse , I just want to be clear on that ,!
eurotec :

Correct, but read my last reply.

eurotec :

He also needs to test the inputs to the relay that turn the relay on. Those are the other two terminals on the relay.

Customer: Ok
Customer: So do. In your opinion think this is something relatively easy o fix?
eurotec :

Yes, but his final test is to lift the fuse box and test the inputs to the relay from the engine control module.

eurotec :

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.

Customer: Okay thanks I will tell him all this and hopefully he can get it fixed!
eurotec :

You're quite welcome.

Marty and 2 other Mercedes Specialists are ready to help you
Customer: replied 5 years ago.
Do you know where I can get a new fuse box for a decent price ?
Expert:  Marty replied 5 years ago.
It's probably going to be a dealer only item. You can probably find a good used one on or on eBay. Make sure that the mechanic checks the inputs to the relay and to the bottom of the fuse box before you replace the fuse box.
Customer: replied 5 years ago.
Ok thanks ...but if it's nit that what else could it be ...the dealer told me maybe I need a new key but why would that make the fuse where fuel pump not be working ? Anyways thanks for everything!

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