Distributor Condenser Value (Is .7 Too High)

[DB26]

Hello everyone,

 

After doing some research, I’ve found that the average recommended distributor condenser value is between .2 to .4 microfarads. 
 

My 6 volt ‘26 Dodge has an original North East condenser. It measures .7 microfarads. Is that value far enough away from the recommendation to be a problem for me? 

[Joseph P. Indusi]

DB26:

Seems like it might be too high but it might also be leaky considering it is that old.  Some meters may test for leakage but you can connect one lead of the capacitor to one lead of a 100 volt or more DC voltage source.  Then connect one end of a DC voltmeter to the other lead of the voltage source.   Then connect the other lead of the DC voltmeter to the other lead of the capacitor.   If you get any DC voltage reading at all after  a few seconds then the capacitor  is passing DC current or leaking.

Observe correct polarity when connecting the DC voltmeter.

Good luck.

Joe, BCA 33493

[Rusty_OToole]

How do the points look? If they are not burned the condenser is doing its job. If there is a mound of metal built up on one side or the other the condenser is too strong or too weak, I forget which is which.

[DB26]

4 hours ago, Rusty_OToole said:

How do the points look? If they are not burned the condenser is doing its job. If there is a mound of metal built up on one side or the other the condenser is too strong or too weak, I forget which is which.

I’ll take a closer look tomorrow. The condenser was bought NOS off of eBay and hasn’t run that many hours. I’ll keep an eye on the points Over time and see if it starts to corrode the points. 

[Frank DuVal]

Do you have the old condenser? If so you could measure that one and compare.

[TerryB]

What type of meter did you use to measure it?  Can you measure another capacitor, one that’s marked, so you can verify your meter’s accuracy?

 

Many capacitors have a very wide tolerance range which works fine in most applications.  If you find a marked capacitor it might have a tolerance of 20% marked on it.

[DB26]

11 hours ago, Frank DuVal said:

Do you have the old condenser? If so you could measure that one and compare.

The old one (which is a traditional round one that someone mounted on the outside of the distributor) measures .29 microfarad. 

 

1 hour ago, TerryB said:

What type of meter did you use to measure it?  Can you measure another capacitor, one that’s marked, so you can verify your meter’s accuracy?

 

Many capacitors have a very wide tolerance range which works fine in most applications.  If you find a marked capacitor it might have a tolerance of 20% marked on it.

I have a craftsman autoranging multimeter. I think it’s fairly accurate. I have used it to do antique radio repair, and all the new capacitors I use measure at what the stated value is. 

[Frank DuVal]

Wow,  .29 to .7 is over twice the value. Not unusual in electrolytics, but it is unusual in typical paper capacitors. 

 

How is the leakage on the new and old one, i.e. does your Sear DMM give a resistance reading for both?

[TerryB]

Sometime capacitor values can be difficult to read with a hand held meter so that was the reason for my question.  Here is a little info on ignition from an old mechanics training manual.

 

[DB26]

1 hour ago, Frank DuVal said:

Wow,  .29 to .7 is over twice the value. Not unusual in electrolytics, but it is unusual in typical paper capacitors. 

 

How is the leakage on the new and old one, i.e. does your Sear DMM give a resistance reading for both?

I will check resistance next. 
 

If the .7 value proves to be a problem, I suppose I could rebuild the capacitor and put a modern one inside the original “can”

 

Here are some photos of the points still installed on the distributor, looks like I need to take action. 

 

 
 

[Bloo]

8 hours ago, DB26 said:

I have a craftsman autoranging multimeter. I think it’s fairly accurate. I have used it to do antique radio repair, and all the new capacitors I use measure at what the stated value is. 

 

For what it's worth, a capacitance test on a multimeter will mislead you on old capacitors/condensers. If you have an old fashioned capacitance bridge, or know someone in the antique radio world who does, that will tell you the actual capacitance. It can also test for current leakage at a higher voltage.

 

Any current leakage that a multimeter can see will show as extra capacitance, and you can't tell which is which. If the leakage does not occur at the voltage the multimeter tests at, it just won't show up at all. You'll think the capacitor is good when it isn't.

 

A few decades ago current leakage was the most common way for a capacitor to fail. Not so much anymore.

 

[Frank DuVal]

1 hour ago, Bloo said:

A few decades ago current leakage was the most common way for a capacitor to fail. Not so much anymore.

 

Well, since we are talking about checking old stock capacitors, it still is the number one way to fail!😉 We are not talking surface mounted components here....😄

 

Leakage should show up with DMM if the resistance range is high enough. I use to test leakage with a VTVM (Vacuum Tube Volt Meter), and although the internals ran at or over 150 VDC, the resistance test was just a 1.5 volt C cell......

[Bloo]

14 minutes ago, Frank DuVal said:

Leakage should show up with DMM if the resistance range is high enough. I use to test leakage with a VTVM (Vacuum Tube Volt Meter), and although the internals ran at or over 150 VDC, the resistance test was just a 1.5 volt C cell......

 

The trouble is, it often doesn't show at 1.5V. At a higher voltage the leakage resistance goes down rather dramatically.

 

Now put the VTVM, a high voltage power supply, and the capacitor all in series. Set the power supply to something close to the working voltage of the capacitor, and the VTVM on the highest DC voltage range. After the initial spike, flip the VTVM voltage range down lower until you see the voltage not dropping any more. If you know the internal resistance of the VTVM (usually 11 megohms) you can calculate the rest......

[Frank DuVal]

Yes, that will give an exact ohm reading of the leakage. No need for a fancy setup to get test results of pass/fail, the VTVM test did just fine for years on the bench testing capacitors for my radio and TV repair business. That nice capacitor checker with the eye tube sat gathering dust (as it still is) on a shelf.😁  In most shops that tester was used to charge capacitors and leave them on the bench for the next guy to pick up.....

[DB26]

I think I have an old vtvm in storage somewhere. But for now, all I have is the DMM. 
 

I removed the condenser from the distributor and did some isolated testing. 
 

Capacitance reads .807 out of the car. 
 

I put my DMM on ohms and the condenser started at 39 Mega ohms and immediately started to drop. I lost my patience with it at 11 mega ohms and turned the meter off. But it wouldn’t stop dropping. 
 

Do you think I can buy a modern capacitor and stuff it into the old can? Here is the capacitor in question:

 

 

 

 

 

[Bloo]

Yes. Figure out the exact capacitance needed first. Drive the car enough with a capacitor you might use, and see about metal transfer on the points. When you know the value that makes points last then do it. It looks soldered together. The antique radio people call that a "bathtub cap" and people have restuffed them.

 

 

[DB26]

26 minutes ago, Bloo said:

Yes. Figure out the exact capacitance needed first. Drive the car enough with a capacitor you might use, and see about metal transfer on the points. When you know the value that makes points last then do it. It looks soldered together. The antique radio people call that a "bathtub cap" and people have restuffed them.

 

 

I will do this. Good thing I’ve got a few spare sets of points. 
 

Myers Early Dodge has a modern replacement. I can buy theirs, measure it, and see how that works out. If it’s good, I’ll buy the appropriate value cap and restuff the original. 

Edited May 3, 2020 by DB26 (see edit history)

[pmhowe]

Fred Winterburn recently posted this on the MogGroup site which is a site for Morgan sports car owners/lovers. You may find it helpful.

 

 "I just posted this on the Porsche 356 registry as a lot of my customers for 6V CDIs frequent that site. I do not intend to post it on any other forum. 

 

Folks, A customer asked about a better condenser and I told him what I had suggested a few years ago in the form of a ceramic capacitor that could be soldered into a gutted condenser shell and then potted with epoxy. Quite a few people with various cars now have used this capacitor to make their own more reliable condenser. This ceramic capacitor outperforms the original paper/foil condensers of similar capacitance but appears to be now obsolete. I ordered 4 more that will do me the rest of my life. DigiKey still has them but the price has doubled from 3 years ago. I looked for a substitute but couldn't find anything that wasn't horrendously priced. This little capacitor is a good one if you want to grab a few while they are still available. It is 1000V rated and 125 degree C rated. A polypropylene high voltage capacitor might work just fine but the maximum temperature rating is usually 110 degrees C at the most. I know this little ceramic cap will take the abuse. The part number is Kemet C350C224KDR5TA I have one inside a Lucas shell for my Morgan and will do the same for the Mallory condenser on my Volvo 1800. Condensers are not stressed if using a CDI, but they take a real beating otherwise. Fred https://www.digikey.ca/product-detail/e ... -ND/789646".  He showed the attached picture. I have used his approach successfully with my Morgan (12V system) and Cadillac (6V system).

[DB26]

I did an experiment today. I took my torch and opened up the can of my old capacitor to see if it can be easily rebuilt. It can. Here are the results:

 

 

[DB26]

And here it is empty

 

[Bloo]

Ceramic capacitors can indeed take heat, but their capacitance changes with voltage (and heat). While that isn't necessarily a problem for something like an ignition capacitor, if you were just trying to match a uF rating without doing actual testing in a car it could lead to unpredictable results. Ceramic capacitors with a dielectric rated c0g or NP0 will show less of the problem, but will be physically much larger than a capacitor with the x7r dielectric as shown in the Morgan info.

 

A film-and-foil capacitor is probably best, but it is true they cannot take near the heat a ceramic capacitor can. Polyester/Mylar can take more heat than Polypropylene. True "film-and-foil" capacitors are probably better suited to an ignition system than "metalized film" because they can handle more current. Epoxy encapsulated capacitors can typically take more heat than ones in a plastic housing. Always read the datasheet for the capacitor you are planning on using. The vendor (mouser.com, newark.com, digikey.com. etc.) will usually have the datasheet posted.