Technical what is points resistance?

A buddy of mine recently gave me a Sun dwell tach unit. The unit has several functions, low rpm, high rpm, dwell and points resistance. The unit is in great shape and I understand the first three functions but what the points resistance is is a bit of a mystery to me. I was using it last night on my buddy's 63 Nova to tune it up and wow is it better than a feeler gauge. Anyways when I selected the points resistance feature the needle goes way into the "bad" zone on the gauge. Thing is it has brand new points and a condenser in it so I don't understand what's up? Timing is set to 8°btdc and dwell at 33° (its a 6 cyl. 194 btw) just like they should be. Runs good but there is some hesitation when you kick it down all and all running good. Anyways can some one please demystify points resistance for me.

 

great question. Inquiring minds want too know.

 

I ignored that feature on my tester, and nobody took away my birthday. If the distributor cam has been cleaned and lubricated, and the points are new, they may have a film on them. When they are closed, pry them open and insert some clean news paper and draw that through them to clean them. News paper that has only been read once or twice is very good for this, no shop rags allowed.

 

I believe, if you search around here, you find a desertion by GMC Bubba on coil and point resistance. As your points are a switch, the more resistance you have, the more arc you will get as they make and break. They will still work however, they will wear much faster. Check out GMC Bubbas thread - good stuff.

 

Basically tells you if your points are burnt and need replacing or filing. I always had better luck renewing the points, not filing.

 

I dollar bill also works well for cleaning points.

As for resistance I don't know anyone who ever used that feature and I have been doing this on my own for over 40 years. Any number of reasons that one would get a bad reading, but dirty points is the culprit that I would look at. Cheapo points will always give you a bad reading as well.

Anyway if it is running fine why worry about it.

 

That feature on the tester is basically an ohm meter, so it's reading through the contacts on the points. If the points are new, then no reason to worry about the reading. I've always just used my ohm meter to check used points.

 

That was my hunch. Thanks much for clearing that up for me! As always y'all are true grease gurus! Thanks much for the info!

 

You always get the bad reading until the points are closed. Turn the motor or distributor until you get a reading then make the diagnosis.

 

Points, at least some kinds, develop a film of tarnish with age...meaning brand new ones can have it to the level of no useful contact at all. A bit of polishing brings them back to life. Points is use can develop resistance through dirt, stray lubricants, or just burning.
I don't have a gauge that shows that, but I would think that good crud-less points would show right around zero resistance.

I once rebuilt a '35 type Ford distributor with NOS points...it was long ago, but the points must have had very little or no resistance because I set it up on the 3-volt KRW fixture...
I found it in my ba*****t, still unused, maybe 30 years later...I put it on the tester and got NO light at all on a the 3 V bulb. A quick cleanup got them back online. Will give it another 30 years in ba*****t and post an update...

 

And another question answered before I got a chance to ask it!

I would have ***umed that a point resistance reading would NOT have been made with the engine running. I think you'd want the engine off with the ignition switch in the "run" position and the points closed and want to make sure that the rubbing block on the points isn't contacting the distributor cam. Maybe the ignition doesn't even have to be on?

 

Aha! That makes sense. Admittedly the engine was running when I ran the test. Makes sense that you would get a F'd up reading that way! It's all clear to me now. This is why I put a points conversion on my 57! Thanks guys for unveiling the mystery!

 

You guys got it right.
The point resistance meter was used back in the day when all cars had points and we tuned up the ignition quite often. As part of a tune up you would turn the engine to a position where the points were closed and look at the scale, a good reading showed good contacts .
Like someone said above most just replaced the contacts every tune up.
Problem is that often they were replaced with poor quality parts and we throwed away good stuff to install bad or weaker parts......