This 1993 Ford Escort came in with the unusual complaint that the transmission would not shift with the tail/park lights on. From experience it seemed like it would be a faulty ground. I checked all of the grounds under the hood and all were okay. Since my faulty ground thoughts had not panned out I turned my attention to checking to see if all of the park/tail lights were functioning properly. The tag or license plate lights were not working. Opening the trunk found the answer. I do not think the Ford Motor Company intended the trunk wiring harness to be swinging around like this.
A closer inspection of the wiring found that there were broken ground wires behind the hinge area.
You may have to click on the pictures to enlarge them for a better view.
I stripped the harnesses open.
Then proceeded with splicing the wires back together. Note that I went beyond the actual break point in both directions so that the splice connections would not be subjected to movement. If a splice is made at a point of movement the wire will fracture on either side of the splice in a fairly short period of time.
I also had to repair the other wires even though initially they looked okay. A firm pull on the wires at the flex point revealed that the wires were broken inside the insulation. When pressure was a applied the plastic insulation stretched.
The wiring harness repaired and taped back up into a harness assembly.
I attached the wiring to the hinge with a wire tie and routed the harness to the it did not flex at a specific point. Instead it gently moved the whole harness.
I had installed the harness into all of it’s hold down points along the inside of the trunk lid.
It had been obvious to me that something was missing from the right side of the hinge and I found this laying in some junk inside the trunk. The clips were too broken to use it in this repair.
Now the reason for the customer’s complaint of the transmission not shifting when the park lights were turned on. With the grounds broken for the license plate lights there was power being back fed into the reverse light bulbs and then eventually back to the PCM. This false signal was sending the transmission control into limp mode.
The transmission was back to shifting properly with or without the park/tail lights on.