Is there any way you can test a condenser.

[John Mason]

I have a couple of old condensers that I keep as “Get you home spares” is there any way you can check them to see if they are indeed any good. If there is a way how?

Thanks in anticipation
John Mason.

[Chris KC]

It's possible to test with an analogue ohmeter set on highest scale. Disconnect condenser then discharge it by grounding the terminal to which LT lead attaches. Touch red lead to terminal, black to casing, should see a small jump of needle then decay towards infinity. Hold leads in place for 20 secs. Reverse the leads and there should be a bigger jump this time, again decaying to infinity. Less than infinite resistance indicates a short.

n.b. I don't know the first thing about electricity - these are crib notes I have stored against the day when I might want to do it myself! You will find video clips on youtube if you search.

[Renaud]

n.b. I don't know the first thing about electricity - these are crib notes I have stored against the day when I might want to do it myself! You will find video clips on youtube if you search.

I do Chris!    You are perfectly right.
A capacitor has an infinite resistance in DC (Not in AC of course). The ohmmeter (DC) charges it when trying to measure that infinite resistance. When reversing the wires the ohmmeter sees a discharge current and finally zero current again hence infinite resistance.
Some testers, even cheap ones can measure the capacity. But your stored method is perfectly enough and valid for our purposes.
About capacitors, modern ceramic (chips) ones should be used now when possible because much better than old style.

[John Cornforth]

Hi

May I blow my own trumpet ? I wrote an article about capacitors (condensers) for the Dorset A7 Club which you can find here:

http://www.da7c.co.uk/technical_torque_a...citors.htm

An ohmmeter test is better than nothing, but won't show up electrical leakage/breakdown at high voltage. Capacitors can also develop high internal resistance, which degrades their performance but is not straightforward to measure. Best to carry a spare which you have verified by fitting it to an engine to check that it works OK. This applies to new ones too.

[John Mason]

Thanks to everyone who has given advice on this subject. In particular to John Cornforth with his article in the Dorset Club web pages that I have read fully. This helps me as the two old condensers I have I keep as get you home spares.

John Mason.

[Parazine]

An ohmmeter test is better than nothing, but won't show up electrical leakage/breakdown at high voltage.  Capacitors can also develop high internal resistance, which degrades their performance but is not straightforward to measure. Best to carry a spare which you have verified by fitting it to an engine to check that it works OK.  This applies to new ones too.

Absolutely correct John, if you have a look at my "Restoring a Top Hat" thread, latest entry, you'll see that I've been having trouble with a condenser. An ohm meter test showed nothing at either normal temperature or when heated with a hot air gun (the ignition was failing after a few miles). I guess that the capacitor was breaking down under voltage stress, becoming progressively short or open circuit after a few minutes running. After not being used for a bit, it "healed" and became operational again.

The quality of some of the new capacitors available causes concern, as you say, best to have a few tested spares under the seat if you are a driver.

[Mick Hanna]

When I worked on an American Airbase in their service station, the mechanics used to charge up the condensers and toss them over for me to catch - a natural reaction. The discharge was very powerful - got me every time!

[Bob Culver]

As John has observed some of the mains based testers were fearsome. One involved application of mains volts, then checking for a spark on discharge. Did not always get one, depending on where in the AC cycle the contact was broken, but any a fairly sure test. Engines should not be run or cranked with an open circuit coil secondary. High voltages in pri and seconadry develop.

[Colin Wilks]

A quick way to identify if the capacitor is the problem at the roadside was explained to me by Nick Turley.

Remove the lead from the centre of the distributor and remove the distributor cap.

Ignition on, points closed, place the lead from the coil near a convenient ground. Waggle the points open and closed with a non conductive prodder and you hope to see a continuing flow of sparks. A failing condenser will show a few sparks which die away after a couple of seconds or so.

I like John Cornforth's idea of having a known good item on fly leads. A nice quick fix.

[Bill Dixon]

If you know an electrician ask if they can check the capacitor/s with a megger on the 500 volt range, if it/they show some leakage throw them. Donot keep as you could use in the future and be mislead.