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Old 07-23-2008, 10:17 AM   #1
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Default Won't start - Fuel pump fuse keeps blowing

It turned out to be the fuel pump fuse... but replacing it doesn't fix anything, it just pops again just after the car starts. I can hear the fuel pump come on for a couple seconds when I turn the key forward, and then after the engine starts, I see the flash of the fuse blowing.

This all began after I jacked up the right rear to look at the brakes. In addition to a floor jack, I had a jack stand (w/ small piece of wood on top) just forward of the RR jacking point for safety -- but only allowing the weight of the car on it for a few seconds. By any chance do the fuel pump wires run there just on the other side of the plastic underbody cover? Maybe a wire is cut or crimped? Or did the action of extending the suspension all the way dislodge the fuel pump or wires? Any ideas? The fuel pump _did_ come on and pressurize the fuel line... but after starting the engine, the fuse blew. so...what could that be? Any ideas are appreciated. Thanks!
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Old 07-23-2008, 10:33 AM   #2
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Default If you can, unplug the pump and see if it still blows....

But if it runs even for a couple of seconds it's more likely the pump not the wiring.
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Old 07-23-2008, 03:07 PM   #3
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Default You lifted the car close to the fuel filter. You need to check that area for damage.

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Old 07-23-2008, 08:09 PM   #4
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Default you mean on the other side of that flat plastic cover? OK... I'll check it out, thx

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Old 07-23-2008, 08:09 PM   #5
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Default OK, I'll try that too... and post the result for tomorrow. thanks!

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Old 07-23-2008, 10:39 PM   #6
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Default Results - Unpluged the fuel pump, fuse does not blow. Plug it in and it runs for 1 sec, then blows-

just after the car starts. It blows right away. I checked under the car and nothing is damaged. I also checked the fuel pump positive wire and voltage looks good both when the key is first turned, and while the starter is turning, as long as the the fuel pump is disconnected. Also checked for a short to ground and the wires show open -- but the fuel pump terminals show .01 ohms. I'm not sure if this is normal or not...but maybe the fuel pump is going bad and shorting when it runs and blowing the fuse. Any opinions on that theory? Is this typical of a worn out fuel pump. Or is the fuel pump relay somehow causing this short? Does anything else get power from the same circuit? Help!
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Old 07-23-2008, 10:42 PM   #7
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Default Checked... there is no damage.

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Old 07-24-2008, 01:00 AM   #8
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Default nope... it didn't blow. see results above

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Old 07-24-2008, 05:45 AM   #9
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Default Yikes, that's low resistance...

A few comments...

0.01 Ohms is a difficult thing for the average DMM to measure since that is less than the resistance of the ohmmeter leads and connectors alone. However, if you take 0.01 Ohms and do the math, I=V/R, then the pump is trying to draw 12/.01 Amps, or 1200 Amps! (Obviously not gonna happen)

I'd check your reading again, but it sounds like the pump is shorted just from the tests you've done so far.
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Old 07-24-2008, 10:05 AM   #10
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Default I think it's normal... you're right a DMM (like a cheap one) probably can't measure it accurately...

That's one difference between impedance and resistance... Once the motor begins to spin, it's current draw is obviously not 1200 Amps. The true resistance/impedance can't be measured that easily. I'll have to hot-wire the Fuel Pump and measure the voltage drop across a wire of known resistance and then I can calculate the actual current. Maybe the Fuel Pump relay is shorting to ground. I have to at least isolate the problem. I have some more tests to try.

Thanks for the input. More results later tonight.
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Old 07-24-2008, 10:05 AM
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2001, 20055, a4, a5, a8, audi, blowing, car, fuel, fuse, pump, s4, starting, starts, wont


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