Hi all. I just swapped my 5.4 in my 99 F-150 with a 5.4 from a 98 F-150. Engine runs great, but the A/C compressor was on even with the A/C turned off. So I disconnected the plug to the compressor, and it threw a code..."Fuel pressure regulator open curciut" I do not have wiring diagrams for either year. I had to use the harness from the 98 because the knock sensor plugs were different. Could this be a difference in the wiring?

Thanks

 

Absolutely and positively there is most likely a few differences in those wiring harnesses. Typically when swapping a long block between one year and another the entire ECM and its entire wiring circuit harnesses need to follow the engine.

OR you should be able to swap the sensors etc from the under-repair vehicle with those found on the donor engine, basically re-using those from the damaged motor.

Visit AutoZone and "register" for access to their free viewable and downloadable wiring schematics----compare the two years for wiring paths and most likely you'll see a few differences in how the connector plugs are configured slightly different-----for what real reason only Ford knows dammit.

 

Thanks for the reply. I am certain I have a harness issue. I am using the harness that was on the engine I installed. I still have 2 issues. I have a code for intake air temp and the A/C compressor is pulled in constantly. The harnesses have slightly different numbers. The harness I took off the truck (old harness) is a 12B637 P260G AN and the harness that followed the engine (new harness) is 12B637 P260G AE. I can see that the wire for the intake temp sensor is grey, but the location it should be on the new harness is black w white stripe. I can find no reference the the AN designator on the old harness. Anyone have a clue? I dont.

Thanks

 

Did you check a schematic for the different---remember they're free to view and/or download through AutoZone.

Even small alpha-numeric differences between some parts especially wiring harnesses is the first indication they're not necessarily interchangeable even if the connectors will mate up.

Without checking schematics for both years of the donor truck and the keeper I'm maybe swap the original keeper truck harness in place of the donor truck part and see if that solves the problems.

 

Thanks for your feedback. I did figure it out. The harnesses are actually slightly different. All the plugs are the same, but the wires go into the main plugs in different places. Weird, but manageable.

Now I have a new issue. Last night the battery light came on. The gage indicates everything is normal. This morning I checked the battery at 12.5 volts. when running the alt is putting out 14.6 volts. Isnt that a little high? Could a high alt output turn on the battery light?

 

Typically and in a perfect world output at the alternator would be about 14.3 volts---normal reading, not an issue.

Did you measure the battery voltage with engine running?

Usually the dash battery light comes on when the cluster or PCM sees voltages below a set point. Its also possible the light is on because one of the wires used to sense voltage has lost contact, triggering the light.

Double check all connections to the alternator, look along each one you can see for signs of a short of chaffing.