Fuel Pump Fuse Keeps blowing

[7tss396]

I've got a problem with my fuel pump. The car is a dodge with a new gen 5.7 hemi. Under normal circumstances, when I turn the key the fuel pump primes and then I start the car. A few nights ago I started the car and accidentally knocked the negative wire off the battery. The car died. I hooked the battery back up and tried to start again but the pump didn't kick on and prime like normal. I then looked at the fuse block and sure enough the 30A fuse was blown. I then changed the fuse and turned the key to the on position and the fuse blew again. Tried it one more time for good measure and blew the second fuse.

I've never had problems starting the car for the last 2+ years and did nothing to the car other than disconnecting the negative side of the battery while it was running.

I'm not sure where to start. Any ideas?

Thanks,

[7tss396]

Update: I disconnected the fuel pump and the fuse still pops. Frustrated.

[andrewb70]

Update: I disconnected the fuel pump and the fuse still pops. Frustrated.

Clearly there is a short somewhere...

Andrew

[7tss396]

Something as simple as disconnecting the negative battery terminal while the car was running could cause a permanent short?

[andrewb70]

Something as simple as disconnecting the negative battery terminal while the car was running could cause a permanent short?

I wouldn't think so. Start looking at the wiring. Fuses don't just blow...

Andrew

[TheJDMan]

I would suspect you caused some damage to the computer.

[dhutton]

It is possible that you generated a voltage spike when you disconnected the negative terminal. I would try replacing the fuel pump relay. After that it could be the ECM as suggested above.

If you disconnect the negative terminal on an inductive load (relay coil for example) when it has current flowing through it the inductor will produce a voltage spike which tries to keep that same current flowing. These inductive spikes can be large depending on the value of the inductance and the current flowing. Relay coil drive circuits usually have diodes across the coils to suppress the spike. It is possible that this diode shorted out suppressing the spike. The diode may be internal to the relay or possibly in the ECM. I don't know the details.

Don

[dhutton]

It is possible that you generated a voltage spike when you disconnected the negative terminal. I would try replacing the fuel pump relay. After that it could be the ECM as suggested above.

If you disconnect the negative terminal on an inductive load (relay coil for example) when it has current flowing through it the inductor will produce a voltage spike which tries to keep that same current flowing. These inductive spikes can be large depending on the value of the inductance and the current flowing. Relay coil drive circuits usually have diodes across the coils to suppress the spike. It is possible that this diode shorted out suppressing the spike. The diode may be internal to the relay or possibly in the ECM. I don't know the details.

Don

Thinking about this a little more this isn't entirely correct but suffice it to say you likely caused a voltage spike which may have damaged the fuel pump relay or ECM...

Not sure why I can't edit my post....

Don

[dhutton]

Thinking about it a little more I think your alternator may have generated voltage spikes when you disconnected your battery. Sorry for all the rambling above. I blame it on the long weekend and advanced aging... :>)

Those alternator spikes could have damaged your ECM etc.

Don

[blitzer454]

Yes, this is called "load dump" when the battery becomes disconnected while the engine is running. Voltages as high as 100V can be generated by the alternator during load dump.

[blitzer454]

by the way, if you're using an aftermarket fuel pump controller then that's where I would start. Most car manufacturers have circuitry in their electronics to protect against load dump, but I could see where an aftermarket piece of electronics might not have this protection as it's not something that happens every day.

[andrewb70]

OK....so let's say a load dump cause the ECU to fail...I don't understand how that might cause the fuel pump circuit to start blowing the power fuse. Usually a fuel pump trigger wire goes from the ECU to a relay. The relay has a ground and then the power lead to the fuel pump. It sounds like the fuse that is being blown is in the power lead, which I fail to see how that is relevant to a ECU failure. I would start checking that relay...maybe the "load dump" fried it...?

Andrew

[7tss396]

I'm an electronics neophyte. I disconnected the alternator and turned the key. Same result. Hope I didn't fry anything. Got an automotive electronics expert coming to look at it this week. Arg.