coil voltage measure

Fri Dec 14, 2012 6:13 pm

Good Day All,
I last posted about a scored cylinder, I honed and replaced the rings and brought the compression from 95 to 110 and I don't think that the rings are fully seated yet. That being said, the lower compression wasn't my problem. For some reason I tend to think worst case for many of my problems. I had a miss, seamed like one cylinder was weak, did a compression test and #1 was considerable less than the others so I concluded that it was the source of my trouble so I tore it down and the rest is history, search forums for "scored cylinder" for the rest....I also concluded that the score was related to a leaky rad and when my brother was using the cub it had overheated causing the score so I replaced the rad, purchased from C&G, great people buy the way...But I digress...
Anyways as it turns out my trouble was burned points the hole time :oops: I didn't file or clean the points before I installed them.
OK here's my question. I have searched for coil resistance and voltage and can't seem to get an answer. When measuring the coil voltage on the distributor side, with engine running, I cant get a measure with and analog meter, it seems that high voltage is coming back through the negative side of the coil, is this normal? With a digital volt meter I can't get a reading of the battery voltage either :? Is there something taking voltage away from my plugs? Would this contribute to the burning points? The plugs and wires are new as well.I also swapped coils with the same result...
Any comments are greatly appreciated.
Sean

Re: coil voltage measure

Fri Dec 14, 2012 6:24 pm

You probably won't get a reading while the engine is running. With the points opening and closing your going from hot to ground on the same wire.

Re: coil voltage measure

Fri Dec 14, 2012 6:25 pm

I was reading some "back issues" here on the forum, perhaps this or a similar post can help you and answer your questions.
viewtopic.php?f=1&t=16903

Re: coil voltage measure

Fri Dec 14, 2012 6:35 pm

Thanks, I should have said I have a 69 with a 1 wire alternator conversion. This also doesn't refer to the erratic voltage reading at the battery side. With a Digital meter I get the same reading at the battery as the distributor...

Re: coil voltage measure

Fri Dec 14, 2012 6:52 pm

If you turn the engine over slowly with a crank while measuring the voltage you will see the voltage at the distributor terminal of the coil going from full battery voltage when the points are open to zero volts when they close. When the engine starts running this change is happening 60 times per second at full throttle, to as slow as 17 times per second at idle. In addition, the capacitor is giving a reverse kick reversing in an alternating current affect, that also occurs in the form of a ringing pulse. Which if you are not into electronics means nothing to you except for the fact that a meter cannot correctly read the voltage there. It requires an oscilloscope that can stand voltage spikes.

Re: coil voltage measure

Sat Dec 15, 2012 4:23 am

Cool, so what I'm trying to measure is to see if I'm getting the recommended 6 to 9 volts on the distributor side of the coil...so what your saying, I think, that what I'm seeing is normal....

Re: coil voltage measure

Sat Dec 15, 2012 5:42 am

If it’s a 12v conversion it may need a ballast resistor in line with the coil, full 12v may burn up your points. If it is a coil with a built in resistor and they are still burning it may be time for a new condenser. The condenser is a small capacitor attached to the point’s side of the coil that helps to neutralize the sparking that occurs when the points open and shut

Re: coil voltage measure

Sat Dec 15, 2012 6:36 am

Its not a 12 volt conversion from what I know, I think the 69s were 12 volt and the condenser is the same age as the points. I know there was an external condenser that was not hooked up so I did install one there to the positive side of the coil....

Re: coil voltage measure

Sat Dec 15, 2012 8:10 am

SeanW wrote:Its not a 12 volt conversion from what I know.
SeanW wrote:Thanks, I should have said I have a 69 with a 1 wire alternator conversion.
It's almost 100% certain it's 12 volts. Minimal chance that it's a 6 volt alternator. What is the nominal battery voltage?
SeanW wrote:I know there was an external condenser that was not hooked up so I did install one there to the positive side of the coil.
Was there a condenser installed inside the distributor when you filed the points? The condenser was originally installed inside the distributor. However, it is possible and quite functional to remove the condenser from inside the distributor and install at the coil.

Take your voltage reading with engine off, ignition on, ignition points open - at the distributor side of the coil or wire terminal on side of distributor.

Re: coil voltage measure

Sat Dec 15, 2012 8:22 am

Its charging at 14volts + or -; will do on the other test and will get back. There is and always was a condenser on the inside of the distributor, I just remember that rusty condenser bolted to the side of the coil but not hooked up, and its been there as long as its been in the family, 35 years....

Re: coil voltage measure

Sat Dec 15, 2012 10:13 am

A condensor fastened to the side of the coil was intended to reduce radio interference from ignition static on the AM band. It was a virtual necessity on cars if one was going to enjoy the radio. On tractors, it might help for radios in nearby houses or vehicles. That condensor does not play a part in actual operation of the ignition system.

Re: coil voltage measure

Sat Dec 15, 2012 10:28 am

Maybe the coil got "borrowed" from an old car and the radio noise suppression condenser happened to come along with the bracket.

Re: coil voltage measure

Sun Dec 16, 2012 5:42 am

Always something to learn here, thanks. Snow storm here today so I'll be staying inside....

Re: coil voltage measure

Sun Dec 16, 2012 2:15 pm

Sean:

Ahhhh I see Torbay is in the middle of a bit of snow? Radar looks active.

Here in Dieppe it was -13 this morning, warmed up to -6 right now. A little snow on the ground, but the wind is kinda brisk. Week looks good to get some stuff done in the barn. Your week looks ok but just a bit on the chilly side with snow/rain and wind chills in the - single digits. Enjoy the week :D

Re: coil voltage measure

Sun Dec 16, 2012 2:40 pm

Thanks for the update Rudi, 20+cm (8") here this morning....a couple of hours cleaning up but I guess it is winter or just about now, Christmas like....
Sean