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Old June 27th, 2008, 12:28 PM
ollieskins's Avatar
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Wade Garrett
1994 defender 90
Join Date: May 2008
Location: NY
Posts: 229
Questions about fuel pump

Hi all,

My truck is at the shop in order to figure out what is going on with humming noise when the key is to the on position. The fellow said he thinks it is controlled by a pressure switch? but he does't want to start anything without a diagram or electrical schematic. Any suggestions?

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Old June 27th, 2008, 12:48 PM
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Hans Haase
The Lab Rat
Join Date: Oct 2003
Location: Long Island, NY
Posts: 2,385
There is no fuel pressure switch in there at all. The only sensor in the fuel system is a fuel temperature sensor on the fuel rail. It's a recirculating type system with a bypass regulator, normally it pressurizes the system and then shuts off after a fixed amount of time if you didn't start the engine. But it runs constantly while the engine is running, any extra fuel pressure is released by the regulator and run back to the fuel tank via the return line. So you're not going to burn out the pump, it's designed for constant use.

I'd start with testing the fuel system relays next to the ECU. There are two silver ones next to each other, one controls the O2 sensors power and the other the fuel pump. Swap them around and see if the problem changes.

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Old June 27th, 2008, 06:55 PM
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D90 SW -- 2.8PS
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Location: Fly over Country
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PS. The "fuel temperature sensor" does not monitor the fuel temperature at all. It is installed in the fuel rail but has no direct exposure to fuel. It is there to tell the ECU if the engine is "hot", ie just got done running, or "cold". The ECU then uses the information to modulate starting the vehicle.

Just thought this would be nice for people to know. I thought it was a nice factoid when I heard it.
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