1985.5 SVO Fuel pump

Hello
I was wondering if any could school me on why my 85.5 has two fuel pumps.
I understand the in tank pump is a low pressure(Lift pump) and the frame pump is a high pressure.
I was told that it works off boost pressure, which doesnt make sense to me BUT.
The problem
If I jump across the fuel relay, I can run the low pressure pump, move my wire to another terminal on the plug and run the frame(external) pump
shouldn't they run together, they are on the same hot wire on the wiring diagrams I have found.
ANY Help would be appreciated

I have had a couple of 85s with two pumps like yours and I have had 86s with just the intank pump. I thought the 85.5 would have the same configuration as the 86 but I guess not. Sorry I can't help with your question however.

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Originally Posted by stanger19899ddr

...I was wondering if any could school me on why my 85.5 has two fuel pump...

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If it's truly a 85.5, it should have only 1 fuel pump. Do you have dual exhaust and the aero/flush headlights, or...single exhaust and recessed sealed beam headlights?

The dual pump system powers both pumps at the same time (note the in-tank pump harness in the dual-pump system has a 1 Ohm resistance wire). There is no pressure/electrical control of the pump(s). The Fuel Pressure Regulator, aka FPR, does all the pressure regulation. Both pumps are on full power all the time they are powered (except as noted above about the 1 Ohm resistance wire to the in-tank pump).

The single, in-tank, HP pump system didn't go across the board until MY 1986 in Mustangs when ALL Mustangs got EFI in some form.

There is one fuel pump relay (under the drives seat) that sends power to both pumps.

Where are you jumpering? Are you powering on the relay coil circuit? Or jumpering individual wires that connect to the relay on the harness side?

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Originally Posted by MikeFleming

The dual pump system powers both pumps at the same time (note the in-tank pump harness in the dual-pump system has a 1 Ohm resistance wire). There is no pressure/electrical control of the pump(s). The Fuel Pressure Regulator, aka FPR, does all the pressure regulation. Both pumps are on full power all the time they are powered (except as noted above about the 1 Ohm resistance wire to the in-tank pump).

The single, in-tank, HP pump system didn't go across the board until MY 1986 in Mustangs when ALL Mustangs got EFI in some form.

There is one fuel pump relay (under the drives seat) that sends power to both pumps.

Where are you jumpering? Are you powering on the relay coil circuit? Or jumpering individual wires that connect to the relay on the harness side?

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I always learn things here-good to know.