General Tech Condenser test?

[NutmegCT]

Please remind me - if you're having ignition problems, one thing to test is the condenser.

Remove the condenser. Use VOM on condenser case and wire. Should show 0 resistance. Reverse VOM leads and it should show 0 dropping to infinity.

Am I right in thinking that if the VOM shows infinity regardless of the VOM leads position, the condenser is bad.

Right? (gol dern it ...!)

Thanks.
Tom

 

[Gliderman8]

Set your meter on ohms and see if it will charge up. If it doesn't charge close to its rated capacity then it is no good.
edit: if it takes a charge, set your meter to vcdc and put the leads back on and see if it slowly discharges.

 

[NutmegCT]

Thanks Elliot. I set the VOM to ohms. The needle barely budged from infinity. Reversed the leads - needle still just barely budged.

What does that mean? It's an ignition distributor condenser - I don't know what it's supposed to be rated at.

Thanks.
Tom

 

[Gliderman8]

discharge it my bridging the leads and then try to charge it up again using the ohms setting.

 

[NutmegCT]

You mean the condenser leads? or the VOM leads?

Here's what I did:

1. set VOM to x1K ohms; bridged VOM leads to show 0 ohms.
2. touched VOM leads to condenser (body and wire)
3. VOM barely moves from infinity
4. reversed VOM leads on condenser; VOM barely moves from infinity

 

[Mickey Richaud]

You can't really test a condenser (capacitor) with a VOM, unless it reads microfarads/nanofarads. A good condenser, should, I believe, read high on a resistance scale; a shorted one will read zero.

Pretty sure that's right; maybe someone else will chime in.

 

[NutmegCT]

Mickey - when you say a good condenser should read high on VOM scale - you mean it should read near infinity? or near 0?

Tom

 

[Gliderman8]

It will take some time for the capacitor to charge... keep the leads on and see if it continues to climb (charge)

 

[Mickey Richaud]

Should be near infinity. Try it on several scales to be sure.

 

[Mickey Richaud]

Looks like my memory was fuzzy (surprise, surprise!)

 

[NutmegCT]

It will take some time for the capacitor to charge... keep the leads on and see if it continues to climb (charge)

Kept leads on it for two minutes. Needle moved a tiny bit above infinity. Did the same thing with a brand new ignition condenser I bought an hour ago - exactly the same result. Needle remains close to infinity.

So the old and the new are showing the same thing. They're both good? or they're both bad?

 

[NutmegCT]

Thanks for the video Mickey - that's actually what I've been trying. My analog VOM moves slightly away from infinity, and stays there. Doesn't drop back to infinity.

But it does exactly the same thing on the *new* condenser I just bought. So ...

 

[TexasKnucklehead]

OK, I'll try to explain this a little.

First a word about what a capacitor is. It is a electrical component capable of storing an electrical charge. Like a battery, power goes into it at a certain rate, and can be drawn from it at a particular rate. A capacitor's ability to store/give a charge is rated in FARADs. The TR3 capacitor is rated at 0.2microfarads. The capacitor in the video is large (farads, and physically) in comparison, by a factor of at least several thousand.

A Volt-Ohm-Meter is also an electrical device, generally used to measure somewhat passive circuits. The ohm-meter has different scales, but always applies voltage/current into the circuit being measured to determine "passive" resistance. When you use an ohm meter to measure a capacitor, the voltage being applied from the meter causes the capacitor to charge, and appear to change it's resistance. Swapping the leads causes the same thing to happen on the scale, because it first discharges the capacitor, then charges it with the opposite polarity. Different scales of your meter might provide different results. If you have a cheap meter with a "diode" checker that beeps, it employs the highest voltage source, and you can hear the capacitor charge and discharge.

Just because a "new" capicitor acts the same as the old, does not mean anything about either capacitor. Your test equipment is not acting the same way the device is intended to operate -or maybe it is and both capacitors are faulty, but I doubt it. Try another meter and/or another resistance scale.

 

[TR3driver]

Steady state, the reading should be infinity. Unfortunately, how fast it changes depends a whole lot on the individual meter; there is no standard for how much current it supplies to measure resistance. So about the best you can do is compare to a known good one, using the same meter (on the same scale) and procedure: Short the terminals, connect the meter then watch how quickly the reading rises to infinity. Usually, using the highest available resistance range (eg 20 Megohm) will produce the slowest rise (for a given capacitance).

Still only a very crude test.

Mickey, note that "infinite" resistance on an analog meter (like the one in the video) is on the left hand side of the scale. Also, automotive ignition condensers have very little capacitance, compared to the motor starting capacitor in the video. An ignition condenser should be very close to 0.22 microfarads, while motor starting caps are usually somewhere in the range of 50-500 mfd.

PS, Oops, I see Texasknucklehead wrote basically the same thing while I was distracted. Sorry for the duplication.

 

[NutmegCT]

Thanks gents. Randall, you say "... then watch how quickly the reading rises to infinity."

As you say, infinity is on the left of the scale on my analog VOM. So are you saying when I touch the leads it should immediately hit 0, then move left to infinity?

Edit: I get the same readings whether using the x1K scale, the x100, and the x10. Barely moves above the infinity reading and stays there. On both the old and the new condensers.

Sorry for the dense-ness.
Tom

 

[Mickey Richaud]

Still only a very crude test.

Yup - that's why I said you can't really test one with a plain VOM]

Mickey, note that "infinite" resistance on an analog meter (like the one in the video) is on the left hand side of the scale. Also, automotive ignition condensers have very little capacitance, compared to the motor starting capacitor in the video. An ignition condenser should be very close to 0.22 microfarads, while motor starting caps are usually somewhere in the range of 50-500 mfd.

PS, Oops, I see Texasknucklehead wrote basically the same thing while I was distracted. Sorry for the duplication.

Yold ya my memory was fuzzy! :blush:

 

[pdplot]

I got a better idea. Condensers are cheap. Buy a new one and stick it on. If the problem doesn't go away- it's either (a) not the condenser, so look elsewhere, or (b) Very unlikely - you bought a bad condenser. If (a), put the old condenser in the glove compartment as I did and now you have a spare.

 

[NutmegCT]

Yup - already tried the new condenser.

Old condenser: engine ran fine for 100+ miles. Yesterday started missing, got so rough you couldn't accelerate while driving. Actually died coming back home. Wouldn't start again. Cranked but wouldn't fire.

Bought new condenser. Put it in, cranked the engine - started perfectly. Then on acceleration, got rough, then just died. Same symptom(s) as the old condenser.

Also tried a different coil. Same thing - no improvement.

grumble grumble

 

[TR3driver]

As you say, infinity is on the left of the scale on my analog VOM. So are you saying when I touch the leads it should immediately hit 0, then move left to infinity?

Sorry, it's been so long since I used an analog VOM that I forget things Normally yes, I would expect to see the needle flick towards 0 and then fall back. But it appears that your VOM is not very sensitive and so you may not be able to see the flick.

Seems to me, the best thing to do here is to install the suspect cap on the engine, and see how it runs. If it is bad, you'll know it. The engine will either not run at all, or run really bad. Last time I had one fail, the engine just dropped dead (like the key was off) and refused to even fire while cranking.

Or find a real capacitance meter. I see HF has a DMM that includes capacitance for $20. (Think I've got that model, I'll have to try it out on a condenser.) Or here's an amusing kit that turned up:
https://www.seeedstudio.com/depot/capacitance-meter-kit-p-268.html

Mickey, yes, I was trying to agree with you. Even a capacitance meter isn't a complete test, as you might have a capacitor that breaks down at high voltage.

 

[NutmegCT]

Thanks Randall. See my post above yours. I've tried both the old and the new condenser (capacitor). No permanent improvement.

As we've covered many bases on the condenser test question - maybe I should start a new thread on the "rough engine, won't start" question. The engine is on my ol' M-B, not my TR. Posted here, as there have been "condenser test" topics here in the past.

I was supposed to leave on my big trip on Monday. Same day this problem began. Surprise surprise.

Onward through the fog.
Tom