there are plenty of (amazing) posts out there for how to fix your tailgate window to go up and down again. It's got an amazing fanbase out there. So, I won't go repeating what others have written, the info is all out there for "how" to fix it. I just wanted to offer some insight about "why it might be acting funny". On my Bronco, for example, the tailgate switch works fine, but the dashboard switch didn't make the motor do *anything* except once in a while, you'd hear a muffled "brrr" and then nothing. No movement. I carefully stripped a small 4 mm section of the purple wires at the tailgate, the ones that control the motor from the dash, and I wasn't surprised to find that instead of nice shiny copper, there was blackened copper. It got too hot, or something. Overloaded and started burning out the copper. I'm not professional electrician, but I know blackened copper is never a good thing. Then I measured the resistance from the dash switch to the now-exposed wires. 0.6 ohms, approximately. then I measured the resistance (static, non-inductive) across the motor terminals. coincidentally 0.6 ohms as well. That means (if I remember my electricity 101 correctly), the battery is dropping 6v across the resistance of the wire, and 6v across the motor. 6v isn't enough voltage to get the motor even turning! I'm assuming the resistance on the wires coming to the tailgate key switch is a lot less, since it's able to turn the motor.


Thus my next steps will be to clean up all the contacts on everything, and re-try. Especially will check the junctions in the driver's kickbox, and under the truck (I think that's where junctions occur?). if THAT doesn't work I'm thinking I will have to lay a new wire run from the dash switch to the back of the car. Good thing I have all the interior out of the truck already!

 

I'm a little embarrassed. Here I thought it was the bad copper on the way to the tailgate. turns out it was the dash switch all along with bad (corroded) contacts. I guess test #1 I should have done is connect one purple wire from the dash switch to ground, and the other to +B and see if the motor turns. it did. that tell me my wiring is okay. But the corroded contacts on the dash switch will cause a voltage readout at the tailgate location to read +B, but when current actually flows, there will be a voltage drop across the dash switch contacts which causes the motor not to function.

I saw no way to repair the dash switch, so I ordered another one.

 

I hate it when that happens...


You should come figure mine out for me.
Had everything functioning perfectly then mothballed it for awhile. The need to start driving my '78 again arose and now it's cold and snowing and rear glass is at half mast!
first time it was a corroded wire near bumper, second time it was a corroded splice behind tailight. Now teststing indicates I have power to the t-gate, but none to the dash switch, also the rear key switch isn't usable because the key does not fit. To get it to go up or down do I ground one of the purple wires or take it to B+12vdc? Also where is the power to the dash switch fed from? I could have a corroded connection behind the fuse block but haven't had the time to tear into it to check. This is just one of several gremlins I need to evict.