I had to pull the FSS wire to kill the engine in my beloved 1991 F350 dually crew cab the other day. After I installed a new ignition switch on the column, I decided to check the voltages on the connector before continuing, or putting the key in, and discovered that the RUN R/LG (Red/Light Green) wire was hot, as well as the other wires connected to it throughout the engine bay. The FSS wire is hot, as is the glow plug relay. In fact, every R/LG wire appears to be hot. Thinking something was backfeeding 12 V into the circuit, I pulled the connectors to the alternator, the regulator, the TECA relay, and the glow plug relay, Nothing made a difference. I don't plan on buying parts when I don't know what's wrong; While pulling the FSS wire killed the engine, it didn't disengage the glow plug relay, so the plugs ran the batteries completely down, and they won't charge all the way back up. I figure I already need to buy new glow plugs and batteries. Does anybody have an idea where this errant 12 V could be coming from? I have owned this truck since day 1 (30 years), it has only 90K miles, and there haven't been wiring fires or such to damage the harness.

 

my first guess is you did not get the new switch adjusted properly.

could be issue with the actuator for the key Cylinder too.

https://www.fordf150.net/forums/viewtopic.php?t=63901

I just recently posted a link for this

 

I read the actuator adjustment instructions before I installed the switch (in fact, I read the posting that you reference), and used them when mounting my switch, but, the critical point here is that I have 12 volts on the RUN wire (R/LG) at the connector before it is connected to the ignition switch. The terminal next to it (B/PNK?) is also at 12 Volts. This should not be, and I can't figure out where that voltage is coming from, as it's not from the regulator, the alternator, the glow plug relay or the transmission relay. What else is left?

 

OK I guess in the beginning I missed that you still had power on the Red/green with the connector disconnected from the Ignition switch.

I would disconnect from the GP Controller to see if that solves it.... the only other thing I can say is the Red/Green wire has melted to an Always hot Wire some place in the harness

Next I would disconnect from the Bulk head to Isolate if the problem is in Cab or engine compartment.

 

here is a Schematic that might help in your tracing

 

Hey Lone Wolf, a big hearty Thank You for you assistance! I was intimidated by the size of that giant bulkhead connector, but your suggestion prompted me to disconnect it, and that isolated the problem to the engine compartment, as I still had 12 VDC on the FSS wire.

I finally found the culprit, in the harness that contains the wires feeding the glow plug relay. One of the glow plug wires feeding the relay had gotten hot enough to melt insulation on itself and a blue wire next to it, and I suspect the blue wire is the FSS solenoid wire, as it was backfeeding 12 VDC throughout the entire ignition circuit. This glow plug was not the factory wire, as I had replaced the two wires feeding the Glow Plug Relay from the harness connector to the relay with 10 AWG automotive wire after the pins in the connector had gotten hot enough to melt the connector, as well as some insulation. I figured I was safe with the 10 AWG, as it is heavier than the factory wire. I'm wondering if the 30 year old glow plugs (but only 90 K miles) have increased in resistance sufficient to draw enough extra current to over-burden the wires. I have ordered new ZD-9 glow plugs and a new Duralast glow plug relay, as the relay cycled a number of times before giving up the ghost after I fixed everything, and the Wait To Start light never came on. I also will buy new batteries, as this was the last straw for them. I have learned that 12 VDC on the ignition circuit of these trucks for 12 hours can damage a fair amount of stuff!

BTW, is there anything special I need to know to get old glow plugs out, and the relay? If so, I'm all ears (or eyes, as the case is).

 

Reps given Glad I could help.....

on removing those old plugs use a 10mm 6pr deep socket and take your time and use WD-40 or similar on em and work em out easy Carb Cleaner works good to break up carbon build up... last thing you want to do is snap one off.

I kinda figured it was melted wire, pretty hard for it to be anything else BTW I use Corrosion X on those bulkhead connectors when I put em back together.... I'm on the Coast so it's a big deal for me.

 

What coast? Thanks for the tips. BTW, I had replaced the glow plug relay feed wires so many years ago I had completely forgotten I had done that. Have other people had troubles with these wires and connector pins overheating? I'm wondering how large AWG I should use when I replace these wires, or if I need to find something with a better rated insulation.

 

Gulf Coast Texas

I think my feed wire to my GP relay is either 4 or 6 welding cable... I changed it years ago too .. I don't use the controller just a Stancore Relay and a manual switch.