Hello,

I'm after some help with my ignition wiring. I'm about to wire in a thermo fan in my '79 F100 Custom. What I'm trying to find is a feed for the fan circuit that is on when the ignition is on (I think I've seen that referred to as Hot-in-run?) but isn't on when the engine is cranking.

I need it that way so that if I restart the vehicle shortly after shutting it down and the temperature switch is still closed due to the coolant still being hot, the thermo fan won't run and draw on the battery while I'm cranking the engine over.

My F100 was assembled here in Australia so I'm not sure if it's exactly like what a US built one would be. I found this thread https://www.ford-trucks.com/forums/1...positions.html which has a good wiring diagram but the ignition wiring doesn't seem to line up with what I've got.

I'm looking at my wiring as it runs down the steering column and this is what I've got.

- Yellow (large cross section wire) I understand this is the main feed for the vehicle electrics.
- Red with a light blue line
- Purple with a white line
- Black
- Red with a blue line
- Green with a blue line

Also, there are number of figure 8 style cables that also run down the column. They're laid against a flat piece of black plastic, I'm wondering what circuit they're for?

Any help is appreciated.

 

Here is the decode. Note that these apply to US-built left-hand drive trucks. Unfortunately, I have no way to tell if this applies to trucks built outside of the US. Not all of the wires you see would be in the column on a US-built '79. But this is the best I can do:

YELLOW: Hot-at-all-times power for the horn. Goes directly to the horn for a non-cruise setup, or the horn relay for a cruise setup.

RED with BLUE stripe: hot-in-START power to the starter solenoid coil and ignition module start retard input. I have not heard of RED with a LIGHT BLUE stripe; Ford rarely uses different shades of a color to identify different circuits, first because it's difficult for people to discern, and second because colors can fade.

PURPLE with a WHITE stripe: brake warning light trigger.

BLACK: most, but not all times is a local ground.

GREEN with a BLUE stripe: I don't recall; can you tell where it goes?

Is this a right-hand drive truck? If so, is your ignition switch on the column? If so, you will see something completely different. On a US-built left-hand drive truck, the only wires running down the column are for the turn signal switch, and the ignition switch is on the dash.

Anyway, as far as your question is concerned, you want the fan itself to run through a set of relay contacts, and the relay coil to be triggered by a hot-in-RUN circuit (your terminology is correct). The best source for this under the hood is a hot-in-RUN tap along the firewall that goes to the carburetor idle stop solenoid. On US-built trucks, it shares a 2-connector pigtail with the alternator stator signal that goes to the electric choke, and is protected by a fuse. Unfortunately, I can't guarantee that your truck would be set up the same.

The best thing to do is to probe different things with a multimeter, then try to trace them to understand what they are, and whether they would work for your application.

 

I am waist deep in wiring mods, so if you post some pictures, maybe I can help. I have 2 harnesses stripped apart, so I can see everything REALLY well

 

Thanks for the replies.

FMC, yes, the ignition switch is on the steering column. It didn't occur to me that US built trucks have the key on the dash.

I've got a 30 amp relay to run the fan, I'm an industrial electrician by trade so I've got an idea on how to wire the circuit, just trying to find the right feed for it.

I'll have a look at your suggestion to pick up a hot-in-RUN feed from the wiring to the carburetor, the wiring in the vehicle has been butchered over time under the dash and the engine bay isn't a whole lot better because it's been converted to run on LPG and the wiring for the gas set wasn't done well. What is the color code for the hot-in-RUN wire?

I'm planning on taking the vehicle off the road next year for a full restoration so I'd like to install a whole new harness.

Ted, if I don't have any luck finding the correct feed in the engine bay I'll take a couple of pics of my ignition wiring and post them. I'm not sure it's going to help too much though, if, as FMC pointed out, having my ignition switch on the column makes things different to what you guys have. And I think you'd be horrified by the jungle of wiring that's under there!

 

Below is a picture of the two-connector pigtail I'm talking about. The WHITE with BLACK stripe signal is the stator signal of the alternator, and is not the one you want to use. The BLUE wire is the 12-volt hot-in-RUN signal that is ideal for this application (that's actually what I'm using it for on mine too).

Since things are very different for the truck you're working on, if you do have this pigtail, you'll want to make sure that it is what you think it is with a multimeter. You'll also want to make sure it's fused - on mine, this is protected by the idle stop (throttle position) solenoid fuse in the fuse panel. Check that it's fused by making sure power goes away with the fuse removed.

 

Thought I'd just quickly post for anyone who is interested.

I found a hot-in-RUN feed under the bonnet. It wasn't a blue wire that FMC advised me about but a plug with a red with white stripe (hot) and a red with black stripe (ground). It has a plug on the end (see pic) but was tied up under the top edge of the firewall.

It's good for 12 volts in RUN and drops to zero when the motor is cranking so it's the one I want.



Sorry about the pic, the camera decided to focus on everything except what was in the middle of the shot!

It may well have been for a solenoid on the carby, it's about the right length and was in that general area.

 

Glad to hear you got it worked out. It sure goes to show that the Australian-built trucks are very different than US-built trucks.