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Cab swap 89 to 91 fuel pump compatibility

 
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Old 05-01-2017, 10:13 PM
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Cab swap 89 to 91 fuel pump compatibility

I'm in the middle of a cab swap. Found out something interesting.

Donor is a 91 F250. Recipient is an 89 F350. Both are gas. Both use the circular connector on the firewall for the wiring harness.

Plugged it in after the replacement cab was installed and found everything seemed to work with the exception of the frame mounted fuel pump which had no power. The in tank pump worked. The F350 is single tank and the F250 was dual tank. There is an 8 pin plug with 2 jumpers used in the single tank trucks in place of the tank switch and this was properly installed.

Of course, the issue became that the 89 truck used the frame mounted fuel pump and the 91 had the FDM's in the tank. The 89 powered the frame mounted pump via a pink/black wire off the switched side of the inertia switch back through the firewall connector. The 91 had no need to power an external high pressure pump and this circuit was eliminated. To add a little intrigue, Ford decided to repurpose the pin location and added a grey/white wire which turned out to be the high beam indicator circuit. As the engine compartment was now the 89 harness and the cab had the 91 harness, this had to be rearranged to make the frame mounted fuel pump circuit and the hi beam indicator circuit compatible.

I solved this by sourcing an appropriate sized wire with the correct female pin for the circular firewall connector from a scrap harness. I unpinned the grey/white wire from the cab harness connector, replacing it in the connector with the salvaged wire. I then spliced that wire into the switched side of the inertia switch which is located just above the firewall connector inside the cab. Ford changed the color of this circuit from pink/black in 89 to red/yellow in 91. Now the frame mounted fuel pump runs as intended. The hi beam indicator can be put back into service by splicing the grey/white wire removed from the firewall connector into the high beam circuit, (green/black) also at the firewall connector.

Hope this is helpful should anyone be swapping cabs between a FDM equipped truck and a frame mounted fuel pump truck.
 
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Old 05-02-2017, 11:26 AM
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Great info. Thanks!
 
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Old 05-02-2017, 03:44 PM
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LOL I ran into that about 10 years ago when I swapped a 89 cab harness with a 91 engine bay harness in my 89 F250

I figured out the fuel pump issue, but a few weeks later I discovered the high beam indicator issue......


Check every electrical function, I recall another electrical gremlin but cannot remember now....
 
 
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