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Strange resistance conundrum

PostPosted: June 27th, 2017, 9:31 am
by Mjolinor
I have been trying to fix a starting problem on a car. Not at all interesting one may surmise but I measured resistance -ve terminal to engine block and it was maybe half an ohm, too much I thought with no current flowing so fitted a new lead.

I now have <0.1 ohms so all is good.

If I connect the +ve terminal of the battery to it's lead on the car then I have 15 ohm or -15 ohm from -ve to engine block depending which way round the meter leads are. I tried several meters, Fluke and others and it does the same. My Fluke 25, old and reliable, reads about +-8 ohms.

I cannot get my head round this at all.

Any thoughts?

Re: Strange resistance conundrum

PostPosted: June 27th, 2017, 9:40 am
by k-ww
I think you have a leakage current path thru the battery, this is giving the weird ohmage readings.

[clock in the dashboard, etc?]

Any meter measuring resistance is really measuring the voltage drop across a resistance with a small current supply,
and an injected outside voltage will screw this up.

Take a reading with your meter in the voltage mode [maximum sensitivity] between the battery - & the engine block.

Remeasure the resistance between the battery - and the engine block with the battery + lead disconnected.

Re: Strange resistance conundrum

PostPosted: June 27th, 2017, 3:50 pm
by Mjolinor
9 mv with the battery connected. I already measured that but it really is a hell of a coincidence that it is such a sensible reading. I intend to try again tomorrow with an analogue meter (AVO) but I have to find it first, not used it for years.

I am trying to decide if it is contributing to the starting problem.