andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,165
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Trying to fix the tail lights on the Delica.
Testing the wiring from switch to lights, I get a continuity reading on the multimeter, but if I pass 12V along the same wire, the bulbs do not light.
However, if I wire 12V directly to the lights, they work.
Why do I get a good continuity reading, but the wires won't carry voltage? I am probably doing something wrong, but what?
Beginning to hate this vehicle now.........
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taurus
Posted a lot
Posts: 1,084
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Ah, I have had something similar. If the original wiring has corroded, sometimes internally, the resistance can cause this problem. It will show the right voltage on a multimeter but the current flowing just isn't enough.
I ended up running a new cable and all was well.
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Hi, Long term use of scotchlocks opens the insulation and allows dampness in to corrode the Copper. Is there one present or evidence of there having been one? If so just splice in a short section of cable to repair the bad bit.
Colin
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andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,165
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Bl**dy thing's full of Scotchlocks!
Think I'm going to admit defeat and make a new loom that connects to the front side lights, that still work!
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If the wire is stiff it's a good sign of corrosion.
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To answer one of your original questions, as far as I know, you can still have continuity through a wire even with only a single strand intact. So it is possible to have wiring continuity, but not a correct voltage supply.
Matt
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Phil H
Posted a lot
Posts: 1,448
Club RR Member Number: 133
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Wiring ContinuityPhil H
@philhoward
Club Retro Rides Member 133
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Indeed - a multimeter will show continuity with more resistance there than a bulb will tolerate. If you want to go fishing for the ability of a wire to carry enough current to light a bulb, go testing with a test bulb (be it an indicator or sidelight bulb) - something that will actually draw some current.
I'd say it's a reasonable guess to start at the scotchlocks though..
If you're going to try and piggy back off the remaining working circuit, you might find it fused too low to cope with a lot more than you might be adding.
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And don't forget that if you remove Scotchloks, there's a lovely split in the insulation left behind. I once had an intermittent misfire that was eventually traced to the split in the insulation left when a previous owners Scotchlok was removed, allowing the low tension wire to earth sometimes.
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To answer one of your original questions, as far as I know, you can still have continuity through a wire even with only a single strand intact. So it is possible to have wiring continuity, but not a correct voltage supply. Matt Agreed, you will get continuity with just a strand, what you really need is a multimeter with Ohms range, if you think you have a fault what your looking for is a high resistance (high ohms reading) which will tell you if you have a wiring/connection problem, ideally you want a low ohms reading dependant on the length & size of the cable, if your unsure find a length of cable and measure that to give you a starting point
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Go to the end of the wire at the taillight and disconnect from the light. Turn on the power and using your meter, see if you have 12 volts at the end of the wire. If you do, then connect the wire to the light and measure again at the connector. If it goes down, then there is internal resistance in the wire and it is not supplying enough current to do the job.
You should also measure the resistance value of the ground between the contact point in the bulb socket and the chassis ground. It should be pretty much zero ohms.
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andyborris
Posted a lot
Freedom is just another word for nothing left to lose.
Posts: 2,165
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Thanks everyone, a few things to check. but I'm now pretty sure that there's a break in the wiring from front to back.
The rear lights work fine direct from the battery, the switch tests OK, but 12V isn't flowing through, I tested using the main connector on the body wiring, but I'm now pretty sure that there's a break in the wiring from front to back.
So, good news, I've found the problem (probably!), unfortunately, the loom is up in the roof under the headlining!
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Had a similar issue with a central locking kit from Maplins (yeah, I know). The wire used to make the loom supplied with the kit was utter rubbish and had sections with just one or two strands, which would bell through but not actually work. The penny dropped when sections that had previously belled through, stopped doing so after being put under load as the strands had acted like an overloaded fuse and melted. Some of the wires had sections with no conductor at all...... just solid PVC.
Nick
Edit: Scotchloks are the devils work
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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