I took the P5B out on Tuesday. It was raining a little and I turned on the side lights. My son who was driving behind me, told me afterwards that my right tail light was flickering on and off. I checked the bulb it was okay. I removed the rear light cluster, cleaned all the connectors and removed all the corrosion from the mounting screws and generally gave everything a good clean. One of the foam gaskets was disintegrating so I made a new one ready to re-install the cluster. I bench tested the lights, everything worked fine. However, when re-installed on the car - no tail light. I checked the earth connection but since the indicator uses the same earth connection and it works, that wasn't the cause.
If I disconnect the feed that comes from the switch (bullet connector in the boot) and hot wire 12 Volts to the tail light wire the bulb lights so again it's not an earthing problem. When I switch on the tail lights and measure the voltage coming from the switch I only see about 8 Volts. This ought to light the bulb dimly but I suspect there's not enough current to light the bulb. This suggests a high resistance connection on the red/purple wire from the switch.
Looking at the wiring diagram, the left and right tail lights are just wired in parallel but my car has a park light facility which isn't on the wiring diagram. So I'm not sure where to look to trace this fault. All suggestions welcomed.
Sounds like you’ve got a rotten wire or corroded connection somewhere, if you check the voltage with everything connected I suspect you’ll lose your 8 volts. I’m not all that familiar with the wiring in a P5 yet so don’t know where the joins are or any common places for problems, but if you just work through it methodically starting at the switch & making sure you have your full 12v coming out of it & just check at every joint in the wiring from there back over, without a diagram in front of me I doubt there’ll be many joints. My suspicion would be where the lamp wires plug into the vehicle harness. If you test it with a test lamp rather than a meter you will have more success😉
Last Edit: Jul 30, 2020 21:26:24 GMT by stirlingmg
All the issues I've ever had with the electrical system of my MK3 3 coupe over a period of 30 years ownership (rebuilt from a basket case) and used as a daily driver for 20 years has been to do with either bad earths/grounds or high resistance connections at the switches, fuses and bullet connectors.
All electrical connectors need to be cleaned to bright metal/alloy or whatever the connector material is...including the fuse block.
Earth/grounds are especially critical. Where ever an earth wire attaches to the frame scrape the metal clean of paint and attach to clean bright steel. Don't be precious about removing paint at ground connections unless you have a penchant for erratic electrics.
Recheck the main earth strap at the battery and at the frame to the engine block.
Most switched connections on these cars are through the positive/active wire not the ground as in modern ECU controlled cars (excepting the horns)...so adding additional earth connections doesn't affect the ancilliary function but will definitely improve it.
Do add additional ground connections at indicator points and between parts of the frame and body where welting is used (high resistance points). These are old cars now with lots of dirt (high electrical resistance), between the joints frame and bolted-on panels.
If the car has been fully restored, then this can be just as big an issue because there's undercoat and paint between every panel join!
PS. Ensure the bulb wattage is the same at every indicator.
Yes thanks, I checked that. The switch and its connection to fuse 5-6 are okay. Other side of the fuse I have two wires, one to front side light (working) and one to the tail light. Both wires enter a wiring harness bundle, there are no bullet connectors visible. So the break or intermittant fracture in the tail light feed is 'somewhere' in the wiring harness going from the bonnet to the boot. The simplest solution seems to be replacing that wire. I have ordered some cable with the correct colour code and I will run it from bonnet to boot. In the meantime I have wired the non-working tail light to the working one on the other side. Now both illuminate and I'm street legal. All I have lost is the rear parking light.
Yes, thanks Phil. I did find the red/black wire from the fuse box and a bullet connector to the wire going along with others through a harness under the sill plate to the boot. It was where you said it would be in the A panel trim on the n/s. It wasn't a pull apart bullet connector, it seems to be a crimped type so I couldn't inspect it internally. I did break into the wires both sides of the bullet and bypassed it but that didn't cure the problem. However the tail light has now started working. I don't know if there are any other connectors when that harness gets to the boot. I have been tugging on wires to trace them so may have disturbed things. I suspect the problem is not fixed - just gone away for a while but at least I now know where to look. I will test drive tomorrow to see if the flickering returns. If it does and I don't find an obvious cause I will just replace that red/black wire with a new one.
Yes, 1972 P5B coupe. And yes, you are right they are not crimped. They are normal bullet connectors but boy was it tight. Probably due to corrosion. I've cleaned the inside of the bullet with a round needle file and wire wooled the inserts so should have a good connection. Just maybe that was the cause of the problem, I hope so.