[RESOLVED] 69 xr7 courtesy lights/door ajar

Hi all. What a fortnight. Have been chasing a short in the courtesy light circuit for 2 weeks now, with much frustration and false celebration. Eventually found a short in the LH door courtesy light, but still cannot get either of the opera lights or underdash lights to work. Background is have put in a secondhand underdash harness from WCCC, everything works correctly except the courtesy lights which blows the fuse every time I put a new one in. Have visually checked everything apart from where wires are wrapped in the main harness. The thing I stumbled across which I can’t explain (from the diagrams) is that the door ajar circuit is being fed power from the courtesy light circuit (which is not what the diagrams I downloaded from WCCC show). As soon as I connect the bullet under the dash for the LH door ajar light the short is back in the courtesy light circuit (have a headlamp connected to the fuse block which has been invaluable). So right now I have one working courtesy light - the RH door light. Bulbs may be suspect but have no idea how to remove the opera ( c pillar) globes).
All suggestions welcome!

You should be able to (carefully) pull the light assemblies out of the sail panels to get access to the bulbs. Then Push and turn the bulbs to remove them from the sockets.

I’m going to bet the short is in the under dash harness. Or at least partly there. The switch in the door when closed makes a path to ground,or an intended short. I don’t have a wire diagram or a 69 but will look at my one for 1970 later to see if I can find anything to help.

Thanks - I thought they probably pulled out but the sail panels looks pretty fragile! I have power at the harness connector going to the LH opera light (and no short in that section) so theoretically just a bulb. But nothing has been simple in this fix…

Thanks Neil. That’s what I’m afraid of - no diagram I can find shows the lights and door ajar sharing a fuse (except for a mustang or possibly cougar standard but harnesses are different). I should also ask if you need to have the instrument panel and light switch connected for the circuit to work properly as I removed them as part of isolating the short?
I can’t see the instrument panel as part of the courtesy wiring diagram, the light switch is there though (I guess to turn the lights on when you turn the switch all the way around). Surely that would only affect the black/blue part of the circuit though…

You can remove the dash cluster and connector and continue searching for a short, so long as the short is not within the dash cluster. There’s nothing in the dash cluster that completes a circuit except for the tachometer, needed to run the engine.

On the XR7, most of the Mustang black/blue wiring is transferred via the XR7 switches to other functions; the courtesy lights are typically black/violet.

Finally success. Correct call on problem within the harness but pretty unexpected scenario. I had noticed that on the RHS harness the underdash light pigtail had been repaired (another one spliced onto the original harness lead). Repair was solid - presumably done by WCCC with crimp/solder and quality shrink tubing. I had previously checked the LHS dash light lead (and door ajar) and both looked good - correct connectors and correct original style wiring colours. This time I digged deeper and found both had been patched in just after they came out of the main loom, hidden by the bundling of the wires at that location. With some cleaning all became clear - they had been rejoined onto the wrong wires - black/orange (underdash light) joined to black/violet (door ajar) and vice-versa. So effectively the courtesy circuit was connected directly to earth whenever the door was unlatched - which of course is every time the door opens!
Relieved to have that one behind me and a little frustrated with WCCC’s repairs (assuming of course they did it and not a P.O.). Note this was a ‘Grade B’ part so I was expecting some repairs but not such a diabolical mistake to diagnose!
Thanks all for the input

My most humble apology, if it was listed and tagged as a grade B harness it was done by me. Not sure how it got past me as I not only repair but double check all wiring in the harness and cross check wires just to keep this from happening. I have been doing this for years and a mistake like this is not acceptable but we all make mistakes and I will take steps to insure it doesn’t happen again. Thank you.

Hi Richard. No sweat, everyone makes mistakes - I’m just hugely relieved to have sorted it out (I was fearing something even worse like having to unwrap the whole harness). And those stripes on black wires are VERY difficult to see, especially when they did stuff like using black/violet and black/blue and black/orange in the same loom. As noted in previous posts very good repair techniques (much better than mine), wish I had your connectors.

One way to stop this problem in the future is to repair all broken leads before checking continuity across all of each circuit. If you find a problem on a wire while checking continuity, just fix that wire (one at a time) so leads don’t get mixed.
That’s my procedure and it has worked successfully for 14 years.

Yeah, I know it sucks when you find a problem after the part goes out the door. It happens, but fortunately very rarely. I’m sure the same goes for you as well.

Thank you for following through with the update.