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Electricity question

JPSmit

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Posting here instead of the Spridget board as the more input the better. I am confused. In the diagram below I have two wires and one dual female connector (Bullet connectors of course). (If you are interested, at the other end is the brake light switch - currently disconnected)

With the wires unplugged I get 12.10 V on one wire and 0 V on the other.

If I plug them in I get 6.06 V from the connector.

I can only presume this means that something is now connected in series but I just can't visualize it.

any thoughts? thanks all!

wiring.jpg
 

George_H

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Yes, something must be in series. Also you must have some current flow to get a drop in voltage. Although the flow may be small.
Also, poor connections can be thought of as resistors. Add some current flow and the voltage will drop. I hope this helps. George
 

somewhatspritely

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So what is happening from your description is that there is an electrical resistance in the path of both wires. So when they are connected together the two resistances act as a voltage divider giving the 6v reading. It may be that both sides have a really high resistance, you'll still see some voltage measurement with a modern meter.

From your description it sounds like the 12v wire has come from the fuse box and goes to the brake light switch with the bulbs the other side of the switch, is that right?

It that is the case I'd suggest using a bulb with croc clips to test whether the 12v wire can actually light the bulb. The trouble with voltmeter readings is that the high resistance of the voltmeter means that you are measuring the voltage with almost no load, which doesn't tell you if the connection is good enough to supply a useful current at the full voltage.
 

JPSmit

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Thanks for this - Last night finally got it working. If I'm honest it may have been working - ish - all along. I took it all apart and checked each individual component all of which worked. I then took a closer look at the piece that went from the harness to the reverse switch and it was seriously degraded. Fortunately I have a big box O wires from the Vauxhall and the colours match (and the gauge is just a little thicker) so, made a new small harness and all works fine. I have never plugged in the brake failure warning light circuit as for some reason it causes the starter to run on. That was part of the circuit but, no longer. So hopefully today tape it all together and go for a run.
 
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