OK, I have a probably simple question, but it gets confusing every time I try to describe it.

I have a 1977 F100. While debugging the AC, I found a cut harness. Looking at my donor trucks with the same big AC housings, I found the 1977 to have this extra mystery harness. Unfortunately they were all cut as well. (The only one with an intact mystery harness already had been unplugged and the dash partially stripped, so that wasn't much help other than telling me this mystery harness does exist, that it had a 3 wire female plug, and that I wasn't crazy.

My 1977 truck has the deicing sensor and wires already plugged up separate from this mystery cut harness.

My 1978 and 1979 AC donor trucks do not even have this mystery harness.
I do not know if 1973 - 1976 donor trucks have this mystery harness or not because all my 73 - 76 are non air.

Where does this missing harness plug in? What does it do. What do I have to go out and find to plug it into!?

While typing this out and describing the pictures, I begine to see the function of the mystery harness and 3 wire female plug (it runs a relay or sylanoid from the blower switch on the heater control assembly...right?, but I do not see the relay or sylanoid it would plug onto. So still need help figuring out where the missing relay or sylaniod is.

HELP!!!!!!

HAH! the pictures worked.

This is the cut harness on my truck. The wires are, a big solid orange coming hot from outside, a red with black stripe that splits and goes to both the heater controll assembly above the radio and to the plug next to the blower motor resistor above my hand in the picture, and a yellow with red stripe that also goes to the heater control assembly.



This is the deicing unit on my truck. Everything looks AOK except for the extra orange wire not shown in the book or in my 1978 and 1979 trucks.



Here is the smoking gun. A good factory mystery harness and plug on a 1977 donor truck. Where does it plug onto!?!?!?!



A close up of the 1978 1979 firewall plug without the mystery harness.



A close up of my 1977 truck with the extra orange wire in the fire wall plug which starts the whole mystery harness getting fire.

 

I don't know the answer but my '76 F250 SC Ranger XLT has the same plug dangling under the glove box. I thought it went to the blower motor resistor, but my fan works on all speeds just fine. I will follow the thread and see if an answer turns up. The size of the wires makes me think it is/was for an option of some kind but other than AC I can't think of anything requiring that size wire.
Good luck and I feel your pain.

 

I looked closer and found some kind of relay or seleniod in the junk pile I swept out of that 1977 donor cab with the same harness and intact plug. It plugs in to the harness plug prefectly and the prongs were bent over which matched the stretched tape on the harness near the plug. So I feel it was plugged in when the guys yanked the air conditioning box before I got the cab. I pulled the mystery harness with good plug and the thing I found in the floor litter. To add to it probably being the correct part, I found part of a smashed passenger side optional floor light which originaly went on the bottom of the dash under the glove box. It was stuck to the mystery relay/selenoid. I looked in the dash and found the rest of the optional light still plugged in. So I feel this is a smoking gun. I'll picture everything for you so you can see what the part looks like. I think you need to get one for your truck! As far as I can tell itcuts your blower motor down or up when needed and saves it from burning out, or something like that! So don't do what they did on my truck and cut the plug off and splice the 2 big wires together! It worked for about a year while I had the truck and then I had to dig thru the dash and AC to replace the blower motor!!!! The mystery relay/seleniod looks very similar to your horn relay. Wouldn't that be funny if it was the same thing! Anyway, I'll snap some pictures before putting it all back together...boy I was made when I saw that busted optional floor light! It would have been cool!

 

First, I know this is an old thread.
Second, my question is almost identical to this thread.


I have the same question regarding this "mystery harness/plug/resistor". I too have a 77 f100 with AC. When I got the truck the ac components were all missing, except this "Mystery harness/plug/resistor" which is part of the AC only additional harness needed to operate the ac system.


I, like the original poster, took the AC components only from the 79 f100 and installed them into the 77. I did a careful swap and plugged everything in correctly from the 79 to the 77.


I used the 77 original wiring harness and the ac only wiring harness (which includes "mystery harness/plug/resistor") which was in my 77 truck . When I went to turn on the system. Nothing worked. I then went and installed the AC only harness from the 79, that is specific to the ac. This harness does not have the "plug/resistor?" Picture below.





After I swapped the electrical over to the 79 "AC only harness", it worked fine. Does the 79 have this resistor somewhere?? Should I be concerned in not using this?


Is this resistor the "Blower Motor Resistor C604"? from the wiring diagrams?


Thanks and sorry for all the questions.