Progress Thread After Action on Short in Dash Lights

This is an after action on diagnosing a short in the dash lights in my 1990 Mustang GT 5.0 Convertible.

Topic: Parking lights and Dash lights were not working.

Discussion: We initially started with a Haynes Repair manual and quickly realized that the Wiring diagrams were not adequate. We sourced a factory wiring diagram and repair manual on Ebay that came in the mail two weeks later. In the mean time, we disassembled the dash using CJ Pony Parts and LMR instructional videos. Upon initial inspection, the dash lights and parking lights were inoperative. We checked Fuse 13 15 Amp (dash lights) and Fuse 4 15 Amp (parking lights) and both were blown. We replaced both and tested again. Same result. We then disassembled the Parking/Head Lights Switch to the left of the Dash. We discovered a non-factory wire that had been spliced into the brown wire that led to the Dimmer Switch. This wire had been installed from a previous mechanic attempting to fix the Dash Lights. We then disassembled the Dimmer Switch and noticed the non-factory wire going to the power wire on the Dimmer Switch. In short (pun intended) the mechanic had attempted to splice power from the Dash to the Parking lights circuit. However, this resulted in both fuses being shorted out. We un-did the mechanic's work, placing all wires back to where they belonged per the Wiring and Switch Diagram. We utilized LMR's photos and a friend's All Data account to determine accurate placement since we did not have the Factory Wiring Diagram at this time. We finished with replacing Fuse 4 and tested the Parking lights with Success.

Two Weeks Later: With the Wiring Diagrams in hand we began tackling the Dash Lights.We first placed an Ohm Meter on the Light Blue with Red Striping found on the Main Light Switch. Per the wiring Diagram, the light blue with red striping is the power wire that led down stream to all illuminated components to include:
the Dimmer Switch,
the Dash Lights Cluster,
the PRNDL illumination (more on this later),
the cigarette lighter light,
the LCD illumination relay which led to
the Radio illumination,
and HVAC controls illumination.
We disassembled the dimmer switch and tested, no issues.
Next was the gauge cluster and tested/inspected the paper circuit board and light bulbs for functionality. No issues.
Using an Ohm meter we then tested each individual component listed above. All components had the light blue with red striping as their power wire as indicated by the factory wiring diagram. All components netted an infinite resistance. At this point we were about to start cutting and splicing. EXCEPT!!!!!

Because this was a manual transmission car we assumed (we all know what that does) that my car did not have the PRNDL illumination found on automatic transmission cars. This was wrong. Using the component locator section of the wiring diagram, we found the PRNDL harness underneath the driver seat up against the center console, underneath the carpet. We removed the seat and necessary trim panels to lift up the carpet and lo and behold we found bare copper in the Light Blue with red striping as well as a melted connector. Somehow over the life of the car, heat had transferred causing the component to melt, causing the short. We cut out the damaged piece and closed the circuit with a single end Butt Crimp, essentially looping the circuit back in on itself. We then added some fiber glass insulator that my girlfriend's dad had lying around from a previous project to protect against further heat damage. We re-assembled everything to the point of being able to test, to include fresh Fuses and everything worked. I don't remember exactly what the resistance was as we were putting everything back together, but through process of elimination we were confident that we had fixed the problem and it had.

Recommendation: Use the correct wiring diagrams for your year and trim of vehicle, and unless it explicitly says so, don't assume your vehicle doesn't have something. The key to success on this job was the accurate technical manuals and wiring diagrams. Without them we would have cut and splice, and jacked it up further without fixing the problem.

Pictures will be forthcoming.
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