new coil vs old coil

Bobber · Post by **Bobber** » Sat Mar 12, 2016 11:51 am

Confused, is this coil bad? I read where the coil should read .5 to 1.0 OHMS on the lowest setting. I have about 12.9 OHMS on the lowest setting, Am I reading it wrong or have the wrong setting? My new coil reads about the same. new coil

old coil

What is happening is after I drive it sometimes it won't start again. I don't have spark. I thought it could be the ignition, but I pulled wire #12 from the switch to dist. and have 25volts. It runs great, sometimes it just won't start. Condenser/points are new and set at 20 thousands. I followed the trouble shooting part in the manual. Any thoughts?
Thank You.

Post by **wesk** » Sun Mar 13, 2016 11:44 am

This is not the correct way to read secondary resistance. Move that test probe off the + pole and place it on the metal coil case. Now tell us what the reading is. Also when you have selected the 20K position on the meter dial the reading in your window then becomes 10,980 Ohms.

This reading is 14, 900 Ohms. Again you should use the metal coil case not the + post for secondary resistance readings.

This is reading primary resistance. This is the correct way to read primary resistance.

This is the correct way to take the readings.

Also since you say the coil gets you started when cold but won't when hot is why the test I show is always done on a cold coil first and then you heat the coil with a heat gun and take readings again.

Bobber · Post by **Bobber** » Sun Mar 13, 2016 12:31 pm

With the coil on the bench, there was no reading when the test probe was on the metal case and the other test probe on either pole. When I placed the test probe on the high output and the side of the case, there again was no reading. The only time I got a reading was when touching both poles.
Thank you.

Post by **wesk** » Sun Mar 13, 2016 12:34 pm

Using my illustration tell me the reading between B & C?

Bobber · Post by **Bobber** » Sun Mar 13, 2016 12:41 pm

On the 20k setting, I get 15.07 OHMs.

Bobber · Post by **Bobber** » Sun Mar 13, 2016 12:47 pm

In the manual it tells you to check coil, condenser and or starter switch by using a known one to work. Well now I have a new one and I switched them out. Then I replaced my starter switch. I think the starter switch was the problem. It hasn't exhibited the same problem since. I was still interested in the proper way to test my coil, Thank you

Post by **wesk** » Sun Mar 13, 2016 12:47 pm

If you review the testing notes in my illustration your secondary reading is within tolerance and your primary reading is way out of tolerance. Have you compared hot and cold readings?

Bobber · Post by **Bobber** » Sun Mar 13, 2016 1:34 pm

very little change. I ran the heat gun on it for about a minute, a little too warm to hold too long.

Post by **wesk** » Sun Mar 13, 2016 1:36 pm

Then I would say since your primary resistance is way too high you need a new coil. Generally excess heat will drive a resistance reading higher.

Bobber · Post by **Bobber** » Sun Mar 13, 2016 1:46 pm

I did change it out, and it does start every time after it warms up. I did change out the starter switch just cause it was sticky when turned. Wait till you read my next post. BTW the fuel lines are all new, tank was flushed, but there was some fine debris that was found in the fuel filter.

Thanks

cabinfever · Post by **cabinfever** » Sat May 20, 2017 10:08 am

Wes, I am still trouble shooting why I cannot get my M38A1 to start after re-torquing my head bolts. (It ran just fine after the replacing the cylinder head)

Could you correct/revise the picture URLs in your second post of this thread? Thanks!

In case you're wondering, I do have spark, but maybe its not a strong spark. So, I thought I'd check the coil and point gap.

Post by **wesk** » Sat May 20, 2017 12:02 pm

I have no idea where those photos came from and they are not where they were anymore so I cannot retrieve them. Based on their tags you don't need them anyway. The 4th photo shows all you need to know to take proper readings.

This is not the correct way to read secondary resistance. Move that test probe off the + pole and place it on the metal coil case. Now tell us what the reading is. Also when you have selected the 20K position on the meter dial the reading in your window then becomes 10,980 Ohms.

This one you do not need. It shows the wrong way to do it.

This reading is 14, 900 Ohms. Again you should use the metal coil case not the + post for secondary resistance readings.

This one probably showed a meter reading and again the wrong connections.

This is reading primary resistance. This is the correct way to read primary resistance.

This one would have shown the connections at bottom of my 4th photo.

cabinfever · Post by **cabinfever** » Sat May 20, 2017 1:37 pm

I just read the resistance according to your photo. All readings are cold. (its 42º right now).

A to B: 6.2 ohms

C to Grd: no conductance (in other words, the VOM got no readings set at both the 20K and 200K scale)

I assume there is no reason to take hot readings and that I need a new coil. Is that correct?

Post by **wesk** » Sat May 20, 2017 2:02 pm

Correct.

cabinfever · Post by **cabinfever** » Sat May 20, 2017 2:55 pm

Thanks, Wes.

The insulation on the wire that goes from terminal B to D was all crunchy and had some major bare spots where the brittle insulation had fallen off. Do you suppose this wire could have been the culprit that destroyed the coil?