Hey guys,

After a lot of searching, I'm hoping y'all can help me out!

Here we go...
1986 F-150 5.0 EFI AOD 4x2, 136k miles.
Truck is stock except for exhaust.

Symptom: After replacing the distributor to fix an intermittent miss, the fuel pump suddenly won't prime when key is turned on. I made the rookie mistake of forgetting to disconnect the battery when changing the distributor. Truck will run with carb cleaner so obviously the fuel pump is not getting power. Everything worked fine prior to the distributor change. Put original distributor back. No change. I then noticed that when pushing the key from "on" to "start" the fuel pump will prime. I can get the truck to run by holding the key between the "start" and "on" positions. As soon as the key is released the engine quits. FYI - Both pumps have been replaced recently.

I bought a new ignition switch (not the tumbler) as it still had the original piece thinking this would solve. No change. Truck does not have tilt column, and I've tried sliding the switch manually without the rod to the tumbler attached. No change. It still primes right before starter engages and won't stay running unless I hold it between "on" and "start". I've since replaced the ignition module on the distributor as well. No change. All fuses in the fuse box have checked ok.

After looking at the wiring diagram, I'm thinking there's either a fusable link or resistance wire that's failed. Don't know what to look for under the dash, and hate to start hacking stuff. All wiring is stock and still in factory looms. I did discover that the brown wire with the pink stripe that plugs into the ignition switch (#3 on left side, see photo) is what controls the fuel pump.

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I moved it across the plug and I get fuel pump priming. However, the truck won't start unless I trip the starter relay. I can run a jumper wire from a hot source to this wire and it will start with the key, but don't want to fix it this way as it may end up burning something up.

This has to be something simple, I just can't figure it out. Anyone else ever had this problem? Any help appreciated!

 

Pull the codes. The wiring primes the pump, but the computer needs to verify the engine is turning before it will activate the fuel pump relay. It does this by looking at the signals from the distributor. If you pull the codes, and get one that tells you the signals are missing from the dist, then that's your problem.

 

Hooked up my tester to check for codes and it is not getting communication from the ECM. There has to be a fuse link or broken wire somewhere that I'm not seeing.

I checked the wiring schematic in the Haynes manual and it shows a resistance wire in the ignition system. Any idea if this applies to EFI systems or just earlier carb systems?

 

The resistance wire is only for duraspark II systems(no computer). Take the computer plug off the computer, and then put it back on to re-seat it and see if that does something.

 

If you have to jumper the brown/pk to get it to run then you are not getting power to the Red /Lt green --> Yellow wires.
The Red /Lt green is powered at RUN and the Yellow should have power at all times The brn/pk is powered only at start.

Check that the yellow and any other power wires are connected to the battery side of the starter relay and NOT on the starter side.
Check fuse link M on the yellow wires.

Here is a diagram from '86 shop manual.
http://img809.imageshack.us/img809/943/50ignition.jpg

Note that there is a Res wire in the 50 EEC ignition circuit.

 

Klricks - Very helpful! I will check those wires out. Any idea where the resistor wire is located? I'm guessing it's under the dash...

 

If you look at your diagram(thanks for that) you will notice the resistor doesn't do anything, the white/blue wire bypasses it when the TFI is used.

 

Tested the fuse link(s) including the one you mentioned off the starter relay. All checked ok. I am getting power to the yellow wires up to the ignition switch.

With the key on, I am not getting power to the coil. When I apply 12 volts to the positive side of the coil, the fuel pump cycles as it should. I've re-checked all the fuses in the box just to be sure. Any other ideas?

 

Referring to to diagram.
http://img809.imageshack.us/img809/943/50ignition.jpg
Are you getting power at R/LG wire at the ignition switch. In RUN
Check connector C321 on the W/LB wire. (Located LH fender apron). Should have +12 in RUN.
Splice S401 (Near headlamp switch).
Splice S257 (Near TFI module).

 

klricks - Problem was the connector C321. Connection was corroded. Cleaned and fuel pump primes! My hat is off to you. Very much appreciated!

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