I have a 1991 F350 351w C6. I'm having an issue with the ignition switch. By itself, it won't start. I have 12v to some accessories, but not my ignition switch. I have 8 wires in the harness to my ignition switch, but I don't know what they go to. I have purple/white, red/light blue, 2 gray/yellow, red/green, brown/pink, black/green, yellow, and another yellow. If I provide outside 12v to either of the two yellow wires, I can start the truck. My question is does anybody have a wiring diagram of the ignition switch harness? Does anybody know what I need to do to get 12v from the battery to the yellow wires? I just need to know what the wires are and where they go. On a scale of 1-10 I'm about a 4 for electrical ability but I do pretty well with it. Thanks for any help!

 

Here's a picture of the harness. Can anybody tell me what these wires are?

 

ill tell ya what they are buddy. they are cut out of the truck...

 

in all seriousness, i dont think you needed to cut that out. id be willin to bet that your issue is simply the ignition switch. my 91 had a little issue with its switch before i sold it, and last time i talked to the guy i sold it to, he said it was to the point that if he hit a bump it would either shut of all the accessories and the truck would run, or the accessories would stay on and the truck would die. then he blew the clutch. SO, hes gonna fix em both. my step moms 89 had a similar issue. just get a new switch and ill bet that will fix it.

but rewire your harness first...

 

Haha. Freakin Trey. I didn't actually cut it out. That's a second harness that came with the donor switch I picked up at the JY today. I replaced the switch today and that didn't help. I got a diagram from subford today so in the morning I'll get back to work on it and post any findings.

 

CHECK YOUR FUSES!!! please tell me you did that...?

 

Of course! Haha. I'm not gonna post something unless I know I'm not gonna look like an idiot.

 

alex, im afraid theres nothin you can do for that...


hehehehe.

 

Here's the schematic for a 88. It appears the 91 uses the same wire color
code at the ignition switch.

At the battery, the Red cable goes to one of the lugs on
the fender mounted starter solenoid.

At the same lug is a 16 gauge fusible link (called Fuse Link M) which turns
into a Yellow wire. That Yellow wire becomes 2 yellow wires at splice S101.

One Yellow wire from Splice 101 goes to the Splice 208 where
it becomes 3 Yellow wires. Two of the Yellow wires provide 12 volts
to the ignition switch.

 

check the fusible link mentioned above, when that burns you lose power to the ignition switch.

 

They don't really look burned to me, but they don't look so great either. Should I just go ahead and replace them myself? They all seem to have what looks like RTV squeezing out the bottom of the heat shrink. Is that normal? I can post pics if necessary.

 

The fusible link is a piece of wire that melts when excessive current
passes thru it. You said you applied 12 volts to the yellow wires at
the ignition switch and the vehicle started. That indicates the fusible
link is probably open.

The Yellow wire is circuit 37 Y. The Yellow wire connects directly to the
Battery via a fusible link so you should have 12 volts at the two Ignition
Switch Yellow wires even with the ignition switch off.

Page 124 shows how to test the ignition switch.

 

Perfect. Thank you. I'll let you know if that was it.

 

Ok. Update. So I decided to open up the harness on the passenger side fenderwall from the starter relay toward the passenger side headlight. Found all the fusible links and checked them for continuity. Current was travelling through the fusible links, so that ruled those out. I decided to start checking further down the wiring harness and found a random butt connector that was linking an 8 gauge wire (yellow/black dotted lines) to a 12 or 14 gauge wire (yellow). The smaller gauge wire had fallen out of the butt connector. I reconnected them and turned the key to "run". POWER!!! However, the truck would still not start. No response came from the truck when the key was turned to "start" except that the dash lights would dim a bit. Then while lifting the steering column back up to its normal position, I accidentally arced to a relay for the brake controller. No big deal, but a moment later I turned the key again and the truck started right up! Why? I don't know. I drove it around for a little bit to give it a feel and saw that the ammeter in the dash display was showing barely over 8.5v. The battery itself tests at just under 12v with the truck off. It never rose above 8.5v in the 15 minutes the truck was running. I turned the truck off and attempted to start it back up. Nothing. Why would it not start now? I still have power when the key is turned to "run". The dash lights and chime come on and the fuel pump runs for a second then stops, but when I turn the key to "start" the dash lights dim and nothing happens. Sometimes there will be a whirring sound like a stuck electric motor coming from the engine compartment, but not always. What could it be now?

Edit: Also, I hooked the multimeter up and tested all the pins at the ignition switch harness. The readings were a little bit all over the place, but none of them were anywhere above 3v. WTF? They actually seemed to be dropping steadily every time I tested a pin. I held the multimeter in one of the pins for a while and it steadily dropped from about 1v to .85v. Where should I be looking now?

 

do the good ole push button start