The Stovebolt Site Forums The Shop Area Engine Shop coil resistance

What should the resistance be for a 12volt 235 coil with points setup? I'm having trouble with my truck just dying as I'm driving it. I suspect the pertronix ignition as being the problem and want to change back to points to see if it helps. The coil I have with the Pertronix measures 1.6 ohms and a resistor in line. I have a spare 57 235 that the coil measures 3.6 ohms with the resistor in line. Which one should I be using. They both say 12 volts on them. Dave

What is the Pertronix part number?

A simple Ohm's Law calculation tells me that before such things as inductive reactance get figured into the mix the 1.6 Ohm coil will flow 7.5 amps at 12 volts- - - -WAY too much for a set of points to handle. At a a normal charging system voltage of 14.5, the coil will flow just over 9 amps and the points will be crispy critters in a matter of minutes. The 3.6 Ohm coil will flow 3.3 amps at startup, and 4.3 amps during normal running- - - -much better, since the max a set of points is supposed to carry is 5 amps.

There are two types of Pertronix ignition systems- - - -the failed ones, and the ones that are going to fail soon.
Jerry

Tim I'm not sure of the part# as it's about 10 years old. but it is a Pertronix 1. Jerry, should I use the 3.6 ohm coil with the resistor? Dave

I think so. Is the resistor external, or built into the coil? Normally, a GM "12 volt" coil is supposed to have an external resistor. "Internal resistance" 12 volt coils are for Fords. Voltage at the coil + terminal with the engine running needs to be around 9-10 volts. With the resistor bypassed during cranking you should see almost full battery voltage - - - -around 11.5-12 volts, usually. A good way to check primary voltage during cranking is to take the high voltage coil wire out of the center of the distributor cap and ground it, then crank the engine for a few seconds and monitor the voltage at the coil.

It is an external resistor. I'll install the parts this weekend and do a voltage check like you suggest. Thanks for the info. Dave

Jerry, The engine is in a 49 truck with the foot starter. This system doesn't use a higher voltage during cranking does it? Dave

It should, but you'll probably need to install a foot start switch with the small terminal on the side. Rock Auto has them at a reasonable price. A wire is run from the small terminal to the (+) terminal of the coil, and bypasses the resistor to provide full battery voltage during cranking. That switch was used on foot-start 12 volt 235/261 applications, so it's the best way to do things when a 12 volt ignition system is used, even on a conversion of a 6 volt truck.
Jerry

Ok Thanks. Dave

the best way to fix it is take out the pertronix and use the points. I have tried pertronix in my mustang, and my truck, the mustang failed the first time I used it, the truck failed on the way to the dump. After finally getting my wife to go with me and my daughter, I got to stand out in the rain learning how to navigate the bus system in our town. I had all the parts to put the points back, in a bag in the glove box. But had taken it out earlier for some reason, and forgot to put it back in. Then I got to go stand in the rain a little bit more swapping everything back. In 35 years the only time the truck failed to make it home was after I UPGRADED the ignition. I have had gas pouring out of the carburetor, I have had pushrods fall out, due to exhaust valves sticking open because of a poor but expensive rebuild, but it always made it home. There is no amount of festering with points that will ever be as much trouble as that day. I asked the question directly on 2 different forums. Of the replies I got, it was about 60% failure rate. Not to mention it didn't run any better. If for some reason you feel you really NEED to replace the points, use langdons HEI.

Brokenhead, well said. Pertronix left me stranded in my 62 impala at Longbeach Washington with my family of five on a sunday. A passerby gave me a ride down to an old style grocery store down the road, he said they had some car parts. I have only been lucky twice in my life, the day my wife said I do and when I laid eyes on an old dusty box on the shelf with a gm set of points inside. Owner said those points had been on the shelf since the sixties and it was 1994,lol. He even had a match book so I could gap them. I'll stick to points or a GM Hei any day too. They can have every and any aftermarket ignition. IMHO.