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I realise my testing of some old condensers a year or so back was probably not very conclusive. I remember sticking them on a digital meter and waiting for a drop to 1 after a short period. I put those back in the box as ok!
Reading up and coming across this thread I now see that its a bit more difficult than that. But all the advisory seem to be with analogue meters. What should I be setting and looking for with a digital meter? Setting max ohms setting but which terminal to which part? And is this enough just for weeding out bad ones, accepting I need to try things on a car to get a real idea.
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Bob, pleased to see you are back online. Some of us were becoming concerned.
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I fear that Bob's contribution dates back to the original discussion, so May last year.
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Hi JonE
For the full story on condensers, read my article on the DA7C website. However, to summarise, early type condensers made with paper tend to fail short circuit especially under voltage stress. If really bad you will see a finite resistance with a modern digital ohmmeter, once you have waited for the reading to settle. Probes either way round, doesn't matter. Any that don't show essentially infinite resistance can be chucked. Any that pass MIGHT be OK
Modern condensers made with plastic suffer from high series resistance due to poor manufacturing technique. This sometimes shows up only when hot. No easy way to test other than by substitution in a working engine. An ignition setup with a condenser failed in this way can be brought back to life by bridging a good condenser in parallel.
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John - thanks; I have looked through your interesting article. But - your reply - what does that infinity equate to... with a lesser meter's reading? I realise it's just getting a hunch rather than any definitive but have to start somewhere.
I put it to maximum scale, and on some, the reading just continually climbs slowly over 20 seconds or so. (either positive or minus dependent on the probe position)
On others, the climbing number suddenly reverts to '1'. This is the bit that I'm unsure as to whether its good, or bad.
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19-07-2022, 01:06 PM
(This post was last modified: 19-07-2022, 01:15 PM by Adrian Payne.)
Hello JonE
If the meter reads "1" then you have exceeded the range set. So if you get a "1" on the 100MOhm range then the leakage resistance must be greater than 100MOhms.
If the meter climbs on the highest range but does not reach "1" then the reading will be the resistance.
This is all just only at the meters own regulated voltage, probably 5 volts.
Hope that helps.
Adrian.
Edit: If the meter reads minus then the capacitor has been charged opposite to the way you are now reading it. Just short the cap and start again. The Magneto Guys suggest over 10M is ok.
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thankyou - that does. So they (the reverting to 1 ones) are in the 'might be ok' pile! I was doing essentially the right thing, but not understanding why.
