Discuss Resistance in loom wires in the Auto Electrician Forum area at ElectriciansForums.net

Skiday

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Hello. I am looking into the wiring loom of my 53 year old car and the fitness of the wires and connectors within. I have the two part loom that runs from the rear lights to the fuses under the bonnet out, and have been measuring the resistance of each line. Question is; what would be considered too high? A new cable of the same size and length gives me 0.7 ohms, but I am getting various readings up to 5ohms. I would presume that the higher the resistance the dimmer the light. So where should I draw the line and consider replacing with new wire? Thanks. Andrew
 

Lucien Nunes

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The resistance of wire does not increase with time, unless the copper is completely corroded through by chemical action (in which case your car is probably at the bottom of the sea.) If you are reading high resistances it is probably due to faulty connections and remaking those, onto clean bare copper, will sort out the problem. It is even possible that the connectors themselves are OK but your multimeter probes are not making great contact with them due to surface tarnish. Grip the loom wires with sharp-toothed alligator clips and see what you get. Often with the very low test voltage that a multimeter uses, you will see extra resistance from that oxidation that 'disappears' when 12V is applied. E.g. you have a circuit that reads 5 ohms to the multimeter, but when subjected to working conditions, the oxide film breaks down and only the 0.7 ohms of the copper wire remains.

In a nutshell, you need to exactly localise the source of the high resistance, it's unlikely to be the wire itself.
 

Lucien Nunes

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The simple go / no-go test is the calculated resistance of the conductor, plus say 10% for manufacturing tolerance, plus say 0.1 ohms each end for terminals / connectors. If you test between terminals and find a resistance that is too high, invariably the problem is failed terminations, not the conductor, so one or both ends needs reworking. The only time you will have to replace actual wiring on account of the conductor is when it is too corroded or too short to re-terminate effectively.

But before you condemn anything, double-check your meter lead contact with the terminal and your null setting for the leads. I spend a lot of time doing this sort of test and I would say 80% of the times I glance at the multimeter and see a high reading, it's because I haven't managed to make good contact with the connector insert or terminal.
 

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