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Repeat fuel pump Failure - '04 Mercedes C240

posted May 13, 2016 18:12:42 by Lars.T
I replaced a fuel pump on a car that came back and am trying to figure out what I could have done better. Originally the car was towed in, I went out to the car and drove it in the shop. Checked the car for a week - fuel pump relay, crank sensor, but was never ale duplicate the complaint.
The Customer came to get the car and of course it wouldn't start- no fuel pressure. I quickly jumped the relay and it still would not start. I Checked for power at the fuel pump and found 12 volts.
I am a scope rookie so I thought this would be a great chance to see a bad fuel pump pattern. It was a choppy line right around 1 amp. I was positive this was a bad fuel pump. I replaced the fuel pump and filter, car ran for a month and was towed back.
Again I was able to drive the car in, checked power to the fuel pump - ok, checked for voltage drops on the positive and negative side - everything checked good. During my checks I noticed the ground wire in the connector to the fuel pump was not a "shiny" as the other wires, I removed the terminal and sure enough it looked like it had been hot. The insulation was shiny and the connector was discolored. Now I know why the amperage on the circuit was low. But my question is how could I have found this voltage drop in the ground? When I back-probed it, it checked ok, because I was not checking the bad part of the ground circuit. If i measured it from the positive side, the voltage drop from the motor invalidates the test. Should the low amperage have been the red flag? Just want to know how I could have found the problem using scope for voltage.
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6 replies
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Dylan said May 13, 2016 18:48:56
Should the low amperage have been the red flag?


Yep. It would be a red flag for me :-) I always check connectors.
Where's the connector located? Right at the fuel pump?
Belgium, Europe
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Lars.T said May 13, 2016 18:58:25
Yes, right on top of the fuel pump.
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Lars.T said May 13, 2016 18:58:25
Yes, right on top of the fuel pump.
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Dylan said May 13, 2016 19:15:20
Most people would've changed the fuel pump. Since you back probed it, you didn't loosen the connector and find the real problem.
Belgium, Europe
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Dylan said May 13, 2016 19:23:42
But you changed the pump so you disconnected it then. One to keep in mind and have a closer look.
Belgium, Europe
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Inge_Jeppesen said May 19, 2016 16:00:50
Keep in mind, the heat from the bad connection could have damaged the internals on the pumps connector. So it would most likely have needed a new one anyway.
Norway
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