I could use some troubleshooting advice on my 69 interior lights.
Background: During the restoration process all wire harnesses, bulbs, and connectors were checked for continuity and voltage drop. All repairs were made with solder/shrink tubing. The instrument cluster PWB was replaced. For 6 months, everything worked fine. Then today the inside was dark and fuse #6 was blown. A replacement fuse blew while I was installing it. Full battery voltage was measured across the fuse terminals, indicating a dead short. I have systematically disconnected most everything, jiggled wires, and no change.
I have been using the electrical diagram to troubleshoot, but there seems to be conflict between the fuse block layout and the wiring diagram. Specifically, the fuse block shows a 14A fuse, but the wiring diagram shows a 20A. Not sure if this is a typo or not.
Is a bad rheostat in the light switch a possibility? Any suggestions would be appreciated.
The 14A Fuse and the 20 A Fuse share the same hot post in the fuse block. I had to look at both diagrams multiple times to see what you are talking about with the fuse labeling. I went by the diagram showing the fuse block when installing my fuses. It seems that is correct. Overlooked the error on the schematic. I would physically check the fuse block to make sure a piece of old fuse, wire, ect is not stuck in there or something is not broken. Not easy to access I know. You did mention you checked almost every circuit. You may need to eliminate almost. Cigarette lighter seems like a culprit that needs to be checked/eliminated.
I’m an Electrical Engineer in the automotive industry and I don’t envy you. You need to be meticulous to debug this.
Basically you’ll need to unplug things one at a time to isolate the shorted section. A big one would be to unplug the headlight switch and then check with your ohm meter.
You mention measuring across the fuse terminals and seeing full voltage. Your voltmeter impedance is somewhere between 50,000 and 1,000,000 ohms depending on the meter type. The resistance of the light bulbs are tens of ohms. Compared to your meter impedance any of those lighting loads looks like a short to ground. You need to disconnect the battery and use the ohms function from the load side of the fuse to ground to see what is really happening.
Once you find the plug that makes the short go away, you need to start working down that branch.
Did your clock work? A common failure with the clocks is for the winder contacts to weld themselves closed. Until the winding coil burns out it will draw a lot of current.
Isolate every branch.
When you re-installed the harness, did you route it exactly like they did in Dearborn? Any metal the harness rests against must have a rolled edge. If there is anything that applies pressure to the harness against even a flat piece of metal, vibrations will eventually wear through the insulation.
Thanks halbey and PonyCarMan for your suggestions and comments. Here is the latest:
I started by making some resistance measurements at the #6 fuse socket:
Supply side - infinite (as expected)
Load side - 3.2 ohms, infinite with door switches closed
Removed headlight switch and repeated, now 3.8 ohms on load side
Removed, inspected, and reseated fuse block, remained at 3.8 ohms
Removed cigar connector, remained at 3.8 ohms
Removed trunk light connector, remained at 3.8 ohms
Measured light switch rheostat - 3-13 ohms through full rotation. Switch visually looked OK.
With a resistance of about 3.8 ohms, wouldn’t I expect a current of around 3.2A (I=E/R) ? I attempted to measure the current with my DVM in series with the fuse terminals, but the current exceed the DVM’s 20A limit. So I got out a slow-blow 30A fuse and momentarily connected it for a few seconds through very thin clip leads to the fuse terminals. All interior lights illuminated dimly and the clips leads got warm.
Regarding the clock, I replaced the movement with a quartz module for the same reason you cited - the points on the old clock were pitted and arcing.
Regarding wire harness assembly, I was pretty meticulous about this step. Since there was no evidence that the dash assembly had ever been apart, I assumed it was the original installation, and took photos and video of everything to make sure it went back together the same way. I compared it to the shop manual and other photos I could find online, and it looked correct. It is an AC car, so it is pretty tight in there
I still need to isolate more circuits, but that will require removal of the dash pad. Before I do that, I would like to know if you think my measurements are in the ball park and if you have other ideas of what I should poke at.
Measuring cold resistance of light bulb filaments won’t give you an expected current. The reason is that the bulb filament resistance increases by a lot as the filament heats up and glows. But you should be able to get a pretty accurate current reading by placing the DVM in series and reading current on the 20A scale like you did. If the meter exceeds 20A, that means either you are trying to light up over 240W worth of bulbs (not likely) or there is a short somewhere in the lighting circuit.
I would check the courtesy door lamps. The wire can easily come out of the connection and ground out against the door metal. I would also check where the wire harness passes through the door. A lot of movement and potential stress.
Good suggestion - I do remember taping the connectors to prevent water intrusion, but it is worth a second look along with the harness through the door jamb.
Unfortunately, the cold resistance number won’t be enough different from a short circuit to provide any meaningful info. The bulb filaments look like a short circuit until they get hot and glow.
Thanks - so how do I use the resistance measurements to diagnose my short circuit problem? I am getting 3.8 ohms on the load side of fuse 6, which is pretty closed to a short circuit. Maybe I should just remove all the bulbs, along with clock and cigar connectors, then it should measure close to open.
I agree. I always use resistance checks with all bulbs, switches, relays disconnected. At that point, continuity (I aim for 1.0 ohms or less except for resistor wires) becomes the best way to search for problems.
Yes, that takes the meter lead resistance out of the measurement. But unless you have a higher quality DVM, it’s unlikely you are getting very accurate measurements in the 1 ohm range anyway.