With the key turned to the accessory position, what should be powered? In my truck, I get the Alt light, fuel gauge, temp gauge, stereo, and if the transmission is in reverse, back-up lights. Is this correct? Why would the back-up lights be powered in accessory?

I was wondering why somebody said my backup lights were on while I was parked, listening to the stereo. It took me a while to figure out it out.

 

With the age of these trucks and the potential number of previous owners, there is no telling how things were "repaired".

I suggest reviewing the wiring diagrams below and start on "hack patrol".

<A HREF=http://i55.tinypic.com/21jw50l.jpg>74-78 Wiring Diagram Pt1</A>
<A HREF=http://i52.tinypic.com/35luc8n.jpg>74-78 Wiring Diagram Pt2</A>

 

Three previous owners; the guy who bought it new, his grandson, and the guy I bought it from who had it all of three months, long enough to replace the heater core and clutch. I've had the dash apart and all the wiring looked unhacked and factory original. Everything 'cept the switch that was added for the second fuel tank.

 

Fuel gauge, temp gauge, and stereo should be on in accessory, so should the wipers.

ALT light and backup lights (when in reverse) should NOT be on in accessory, only RUN. Sounds like hacked wiring or a bad ignition switch.

 

Forgot to mention that the heater fan and turn signals also work on accesory.
Below are pics of the plug that goes to the ignition switch (do the wire positions look right?) and the terminals on the switch. If you look closely, you can see that two are discolored. They look like they got really hot. Maybe the switch got cooked and is the problem? Or is that just wishfull thinking on my part as that would be the easy fix.

 

I can't tell off the top of my head if the wire positions on the back of the switch look correct; for that you'll need to consult the factory wiring diagrams: 1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net

I didn't do it myself since you didn't list what year truck you're working on, so I'll leave that exercise to you. Note that it's very unlikely that someone would intentionally rearrange these and therefore I'd suspect a bad ignition switch, but I've seen previous owners do dumber things, so who knows.

The fat RED with GREEN stripe wire coming from the back of the switch is the hot-in-RUN ballast resistor for the coil and it looks like it has burn marks on it. It's tied with another wire that looks like it got hot, as you suspect. This often happens if the coil shorts out, or someone tries to wire a load off the coil positive wire under the hood. I can't say if this is related to your problem or not. You should try and inspect that circuit as much as you can. Since this is a hot-in-RUN circuit, if this wire is burned open anywhere along the path, it could explain your issue if it's touching the accessory circuit somewhere down the line. However, that's a little bit of a stretch.

One of the spade terminals on the back of the switch looks like it has suffered electrical arcing; I can't tell from the picture which one.

 

The heater fan and turn sig are supposed to work on accessory, so no problems there.

 

Turn signal yes, heater no. The heater is intended to be operated only while the engine is running for two reasons:

(1) The blower motor draws significant current and would drain the battery without the alternator charging.

(2) The heater controls are vacuum operated. Without the engine running, there is no way for the plenum to direct air flow based on the heater panel selector switch.

Therefore the blower motor is powered from the hot-in-RUN circuit only. If you're seeing something different, your wiring has been modified or your ignition switch is bad too.

 

Two of them appear this way; yellow and the black/green (pic below). I would try to take some better pics and examine the wires more closely but I put it the ignition switch back in the truck so I can drive it. I want to thank you for your help. I've never really had to deal with electrical issues in a vehicle before and really don't know what the hell I'm doing.

So, according to 1976 Wiring Diagram the wires in the plug for the ignition switch are correct. I will look for another switch and see if that cures my problem (fingers crossed). Are they still available from Ford? Aftermarket? Or is it a junk yard item?

 

YELLOW is the main 12-volt power feed from the battery; this gets routed to the different circuits via the ignition switch. BLACK with GREEN stripe is the main accessory feed. These are the two heaviest-current circuits going through the switch so it's not surprising these are the terminals that you're seeing arcing. You'll want to check for similar burn marks on the underside of the connector in the corresponding locations.

The ignition switch is reproduced and easily available at most parts stores. Not sure if it's still reproduced by Ford, but a high-quality unit from Napa, etc will do just fine.