Anyone know how to test a condenser

6

66miles

Senior Member
Hi all.

Do any you guys know if there is a way to test a condenser using a good old fashioned analogue multi-meter. I sometimes have cause to make capacitance readings at work, using an analogue ohms meter to estimate the length of underground comms cables. I know the theory on capacitors, but not so much about the practicalities of testing an old ford condenser. When I connect the leads of my meter to the condenser (set to read kilo-ohms), I get a reading of 70 to 100k-ohms. I would have thought it should read much higher than this (say in the 10's of mega-ohms for example). When the leads are reversed, there is a mild "capacitance kick", though it doesn't exactly throw the needle off the scale either. Haven't tried this last test yet, but I seem to remember from training school that a capacitor should be able to be charged with a battery, and should then hold that charge for quite a time, if the capacitor is good. If there is leakage within the unit, then the voltage will drop away fairly quickly.

Anyone know if this theory fits with an old ford condenser, or is it time to stop being tight and just buy a new condenser?
 
You can get an idea, use ohms, one lead on the terminal, other on the case...the resistance should increase (via meter voltage) and saturate, like a cap would..then, take a wire/etc, go from the terminal back to the case and discharge it...do it a few times...

Thing is, this would'nt tell you how it will operate when it gets hot from under hood temps/etc...

Ether JWL or GM...mentioned heating it up with a heat gun?...(not sure,...) and testing when hot.

Now...advice given to me via the Barn...saved me 9 bucks:)
 
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Hooking up an ohmmeter to a discharged condenser should send the ohms to infinity as soon as the capacitor is charged by the meter. Any less than that and the condenser is leaking. Or, you can buy this digital multimeter from Radio Shack: http://www.radioshack.com/product/index.jsp?productId=20362256# that measures capacitance and find out exactly what the capacitance of the condenser is. Should be between .2 and .3 microfarads.

Here's a little more information about the effect of too low or too high capacitance on the points and why you may not always want to just replace a condenser that's working right:

A condenser with too much storage or capacitance will cause a build up (metal transfer) on the mounting side of the points. While a condenser with not enough storage or capacitance will cause a build up (metal transfer) on the arm side of the points. Sometimes the right sized condenser will end up in your ignition system and no transfer will occur.

This is why you have to file a used set of points (eliminate the metal transfer) before you can gap them with a feeler gauge. You do not have to file points if you use a dwell meter. The dwell meter takes into account the metal build up, because it looks at the points' electrical measurement rather than the mechanical measurement.

The typical condenser will have between .2 to .3 microfarads. The variation in condenser capacity can vary widely, (most condensers fall somewhere around .25 microfarad). This is why some contact points last longer than others. It is not the points' fault but is the condenser's fault instead.

Look for the metal transfer. If there is none consider not replacing the condenser as it is matched to your particular ignition system. If there is metal transfer replace the condenser as you have nothing to lose in the process.

More here: http://www.farmallcub.com/phpBB2/vie...p?f=11&t=38576
 
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66miles said:
Sounds like for the sake of the price of a cup of coffee, may as well just go n buy a new one!
Click to expand...

You still might want to test even a new one since I've had some new ones that didn't work right and would have saved a lot of headaches if I (or more preferably the outfit that sold them to me) had checked them first.
 
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Henry, I've read most of your posts, enjoyed your pictures, envied your youth and your blind luck.
Now I applaud your post #5 on this thread as the most useful information I've read on the Barn in quite a while.
Now if only I had enough smarts to fully utilize my newfound knowledge!
 
Thanks professor for that wealth of info, particularly on the relationship between condenser and wear of points, as well as the capacitance rating to use. Most interesting. Also I have come across an old-time auto electrician who claims to have an equally old-time condenser tester. He has taken my suspect unit away to connect it to this fantastic mystery machine, and make a determination. I might try n take a sneak peek at this gizmo he uses as I would love to know how it might work. For the price of a good cup of coffee though, I intend to cover all bases tomorrow, and buy a new condenser just the same.
 
You need the Mfg. value of the capacitor in farads or it's range in value as marked on the Cap. (Ford did not mark their ignition Caps. but they did publish their value in Service Bulletins) Then use a dynamic capacitor tester that will measure both value and leakage at the Caps rated voltage. With the Ford ignition Caps. you also have to heat them until they are too warm to touch and repeat the above test especially leakage. There are several digital multimeter's that can read just the value of the Cap. such as a Fluke 87.
The other test using the highest Ohm scale of a analog multimeter will tell you if the leakage is grossly high or the cap shorted or open Ckt. It can not determine what is happening when working voltage is applied.
In instrumentation and industrial use, many of the Caps are also marked with a tolerance such as +/- 10% using a Fluke (or other), if the Cap. measures out of tolerance it is also bad. Also if the Cap. is found to be leaking any of it's internal chemicals, or the integrity of the outer cover is damaged, the Cap. is bad.
 
Could you test it this way?
Connect the leads to a 6V or 12V battery. Hold it connected for say 5 seconds. Then carefully set it down on an insulator, say a piece of glass with the lead not touching the case, of course.
Wait a minute or so and go back and measure the voltage.
Would that work?
 
Bolts said:
Could you test it this way?
Connect the leads to a 6V or 12V battery. Hold it connected for say 5 seconds. Then carefully set it down on an insulator, say a piece of glass with the lead not touching the case, of course.
Wait a minute or so and go back and measure the voltage.
Would that work?
Click to expand...

Creative thinking but here's why it probably wouldn't work.
If you've ever been shocked by a charged condenser you'll realize that the voltage that's shot out of it when discharging is way more than the voltage it was charged with. It's like comparing the air pressure that was used to blow up a balloon with the pressure that's released when it's popped. Not only will the voltage when discharging be way higher than the voltage that charged it but it discharges so fast most if not all meters would not even be able to measure it. Good creative thinking though.
 
66miles said:
For the price of a good cup of coffee though, I intend to cover all bases tomorrow, and buy a new condenser just the same.
Click to expand...

A very common and capable condenser that many (including me) use is the Echlin ECH FA54 sold by NAPA. Because I can go down to the local store and buy this off of their shelf I take my multi-meter with me and test various of their condensers before buying them to make sure they're good and try to get some that are as close to .25 mF as I can get. They're so cheap that I usually have several spares in the trunk. Whenever I have any ignition problem on the road I always suspect the condenser first and can quickly switch it out as the first possible solution.
 
I suspect a lot of guys have already just shot down to their local auto shop and bought half a dozen of these damned things after reading this post. knew I shoulda gone and bought one yesterday. The price has probably just shot up an extra dollar a unit now! Pretty soon they'll be worth the price of TWO cups of coffee.

PS: Alex, I have heard of the voltage test you mentioned for capacitors. Apparently, a leaking capacitor wont hold a charge for very long at all.
 
Just bought a new condenser for the grand total of $15NZ. Now I got something to compare the old one to, I've found one very significant difference between them. After being charged with an ordinary-everyday battery such as you might put in a transistor radio (I used a 9-volt), the new condenser will then emit a small but visible spark when the lead is touched to the case. My old (possibly original) condenser will not do this. It has been confirmed by my local auto electrician already that the old condenser is stuffed. So I am thinking that this "spark" test could be a good way to confirm that an old condenser is not holding a charge. Which is what they are designed to do after all.

Any thoughts as to whether this could be a valid test?
 
Old Henry said:
Creative thinking but here's why it probably wouldn't work.
If you've ever been shocked by a charged condenser you'll realize that the voltage that's shot out of it when discharging is way more than the voltage it was charged with. It's like comparing the air pressure that was used to blow up a balloon with the pressure that's released when it's popped. Not only will the voltage when discharging be way higher than the voltage that charged it but it discharges so fast most if not all meters would not even be able to measure it. Good creative thinking though.
Click to expand...

My thinking was not to discharge the cap through the meter, but simply measure the charge voltage to see if it is leaking down. I know this is a limited test, but if the condenser came from a running car that had quit suddenly, this test might give some confidence in it. That being said, I believe I read that the coil turns ratio is somewhere around 80 to 1. The output voltage of the coil must be several thousand volts, say 8,000 volts. 8000 volts divided by 80=100. This means the voltage on the condenser is around 100 volts. Assuming my basic electronics statement is SOMEWHERE near actual values, testing a condenser that operates on 100v using a 6v or 12v battery will not stress it very much, so it might leak down at a higher voltage. But if it holds a 6v charge for 5 minutes, I would THINK it might be enough of a test to assume it isn't the problem for starters at least, no pun intended.
 
The charge usually leaks down to nill in around 15 minutes after you shut off the power. I generally wait that long before I mess with a high intensity strobe light system on the helicopters. They have higher voltages and larger caps. They'll knock your shoes & socks off for sure.
 
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I found it was near-impossible using any meter available to me, to measure the stored voltage in the condenser, as the meter would drain the thing to zero in a fraction of a second. However, the spark emitted from a charged condenser, while very small, is still visible, and provides proof-positive that a charge was stored. I guess a better test would be to charge the thing, and then wait for say 1 minute before checking for the discharge spark. I used a 9-volt battery to charge the condenser, but if a guy could find a higher voltage DC source, say 50 to 100v, this would be better still, although at those voltages, I would be careful not to discharge the thing through my fingers!
There is NO doubt that my old condenser fails the spark test, but my new one passes with flying colours!
 
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