Can't seem to find the problem

[John Stone]

I worked on a friends Model A all day yesterday. It runs fine when cool but when it heats up, it cuts out, back fires and no power. Pulling the choke out helps. The car was converted to 12v negative ground. Here is what was changed to known good stuff. First a condenser and then a complete distributer, carburetor, some MMO to the gas tank, gas cap in case it wasn't venting, spark plugs and coil wire. He said he had the coil checked and it was good and I checked the spark and it was good. Every change we made seemed to work for 2 miles and then the old problem started again. Finally put on a new coil and external resistor and it fixed it. Drove the car for 8 miles and it was fine. Since it was 102 here yesterday it was a good day for a test. The coil fix ruled out vapor lock. He called last evening and it started again after driving about 2 miles as he and his wife went to dinner. After dinner it ran fine going home but the temperature had gone down.

I am a 6v guy so enlighten me on the conversion to 12v. How much resistance should there be from the coil resistor?

Should the voltage drop to 6v at the other end?

I believe he is getting 12v to the coil causing them to fail. He is bringing the car back this morning so we can start over again. I am going to check the wire very carefully. I haven't pulled the dash panel to look at the wiring in there.

John

[Patrick L.]

Regardless of what some may say, points[ as well as condensers] do not like 12v..Depending on what coil your using depends on where the resistor should be located.. 12v probably shouldn't hurt any coil.. If you know your using a 12v coil I'd stick the resistor in the ground line,, for a 6v coil I'd try it the hot/feed line..Voltage at any set of automotive points that I've ever come in contact with should be run on 5-7v..

[John Stone]

Coil is 6v with resistor forward of the plus side of the coil.

[Patrick L.]

A couple other things, I guess.. What kind of points is he using[ modern or original] ?? I'm just wondering there is a wiring issue with the upper plate[modern] or a condenser issue[ I've found mounting screws loose on originals].. Do you have good fuel flow?? How far past the fuel line carburetor ferrule does the supply line extend??[only about 1/16"].. Just trying to rule out the easy stuff..

[Patrick L.]

OK, so if your coil is 6v and marked + and - [some aren't], the coil should wired same as the battery and resistor in coil feed line..

[John Stone]

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Thanks for the thoughts. It has original points and condenser which both were changed with the new distributer. Fuel flow has been checked both cold and hot. Line is trimmed off nicely so the easy things have been tried. As I said above, the new coil fixed it for a while. One observation is that the coil was hot to the touch and there was evidence that the old coil had leaked oil. Of course at 102 outside, everything is hot to the touch under the hood. LOL

[Dick Carne]

If I understood your comments correctly, I'm confused by running a six volt coil on a 12 volt system. I have converted my A's to 12 volt systems, but run 12 volt coils with internal resisters. I have also changed to "modern" points and condensors, the same ones that Ford ran in many of its cars in the early 1970's, and are still readily available at most auto parts houses. My nephew and I both run 12 volt systems on our A's, and both of us put well over a 1000 miles each on our trip to French Lick last month -- neither of us made any adjustments to point settings or experienced any engine-related electrical problems during the entire course of the trip.

If indeed you may be running a six volt coil, I would imagine that the additional heat build-up in the coil might well be the culprit. Just my initial thoughts, but if you are not running compatible voltage, that would be my first place to look.

Best of luck to you. Please let us know what you determine the problem to be - I'd be interested to know. Thanks.

[Bruce Lancaster]

"coil checked..."
When coils deteriorate, they give symptoms like this...fine when cold, then rapid deterioration with heat, fine again when cooled down. If you checked the coil for spark during the no-go time, in my experience, you will still get a spark every time the coil is grounded and released--but it will be thin, yellow spark rather than the fat, blue one you want, and does not do the job.

[John Stone]

6v coils can be run on 12v with an external resistor which is what we have. My question is about the value of the voltage after going through the resistor. Should it be 6v? I still think 12v is getting to the coil some way. Thus the wiring check when he gets here.

When I checked the coil initially, it shot a blue spark at least 3/4" evertime I would break the points. It must be breaking down when really hot.

[Bob A.]

Maybe the heat has nothing to do with the problem. If this car is running a repo. ignition switch, start there by jumping it and see what happens.
When I first got my car it would run fine then cut out with a backfire. Turned out to be a bad ignition switch. Nothing like getting stuck on the road and making a quick repair to get home.

[Patrick L.]

Your coil should be fed with 6v. .. Do you have any way of checking that?? Even an old test light will work.. If the electrical system is + grd then the + coil lead should go to the distributor.. - grd, - coil to dist.

[MAG]

John, the "static" voltage will be the same at each point of the ignition circuit as long as the points are OPEN and ignition switch ON. In this case , points OPEN, Ign. switch ON, 12v will be on the input to the coil primary, 12v. on output of the coil primary, 12v on input of the resistor, 12v on output of the resistor, 12v on the movable point contact.

When the points CLOSE, 12v will be on the input to the col primary, 6 v on output coil primary, 6v on the input of the resistor, 0v on the output of the resistor, 0v on the movable point contact. This scenario places the resistor after the coil. The electrons in this circuit do not care if the resistor is before or after the coil. It wiil work fine either way. If your resistor is before the coil, simply change the sentence accordingly.

The voltage readings are based on the values of the coil primary and the resistor and assume that the resistance values are the same in both components.

Bear in mind that the voltage changes mentioned are happening very fast with the engine running and cannot be accurately measured with a volt-ohm meter. With an O-scope they can be easily measured and seen. Not running, the voltages would be as indicated. But dont't leave it that way very long as the coil and resistor will heat up rapidly and can cause damage to them. Hope this helps.

[John Stone]

Here is the latest. Did the points closed/ignition on test and got the right voltage on the sides of the coil so I eliminated that one. The thing I did notice was the points had closed and that could have caused his latest problem. They weren't closed yesterday when it was running fine. It was a new distributer and I had lubed the cam but for some reason they closed. Oh what fun I am having. LOL

Test run is being made even while I type.

[Patrick L.]

Just curious, how much did they close??

[John Stone]

I didn't measure them. They were close.

[John Stone]

Test run is over and it ran fine. With all the other things I found, the closing of the points just acted like the initial problem. Now I am convinced the initial culpret was the "Made in China" coil. I tested it and it showed bad when heated.

I have learned a lot about 12v conversions and what to expect when measuring resistance and voltage. Thanks guys. Hope I never have to use that knowledge again. LOL

Thanks again
John

[marc hildebrant]

John,

Curious...when tested "bad" when hot, was it an open circuit ?

Marc

[John Stone]

The spark got weak and intermittent. Obviously something was breaking down under heat. I'll let the coil expert tell us why.

John

[ford1]

to simply things buy an internally resisted coil, a 12 v internally resisted coil will have around 9v make sure the condensor is for a 12 volt system, ford and chevy in the 50's ran 12v with resister with no problems, if the coil is hot to the touch its either getting to many volts or its shorted out internally, and if its sweating is probably already shot, a coil will be warm but never hot to the touch, if you insist on running a resister get a ballast resister they start out at 12v for a hotter spark when starting, they then bleed backto 8 or 9 volts

[John Stone]

Yes, he does have a ballast resistor. I think it was used on Mopar in the 80's. As I said before, I hope this is the last time I play with a 12v conversion. I've learned more than I ever wanted to know. LOL