The Stovebolt Site Forums The Shop Area The Electrical Bay Coil Resister Issue

1953 3100 with a 1954 235. I've upgraded to a 12v system.

I drove the truck back from the body shop a few weeks back, and everything was ok. I did realize that oil was leaking out of the top of the coil, and found out that the wires on the key switch were wired incorrectly keeping battery power always to the coil. I fixed that.

I went to start it again a week or so later, and it seemed like it was running on 3 cylinders. I've read the posts on this forum about the coil resister, and so wanted to confirm my findings before going ahead.

I ensured that the points were closed, distributer cap off.

Battery voltage = 12.3
Voltage before the external resister = 12.3
Voltage after the external resister = 12.3
Voltage after the coil = 12.3

Am I correct to understand that the resister is bad?

Thanks,
Steven

Leaking coil = bad coil.

In your readings above, it appears the points were open since theres no voltage drop across the resistor nor coil.

To properly test your ign resistor, make sure theres no voltage to the ign circuit, disconnect both wires to the external resistor, change your VOM to read ohms in the lowest scale (like 0-10, 0-20 etc) then read across the resistor.
You want to see "some" resistance. Around 1.3 - 3 ohms is OK, over 3 ohms is bad and under 1 ohm is bad.

The coil should also read "some" resistance across the two low voltage terminals, the exact reading differs but in your case you need to replace it anyways since it's leaking.
The oil in a coil helps to cool it and once a coil heats up it's internal resistance will change which effects it's operation.

Good luck
Dave

Resistor and coil are bad.

Volfandt is correct about the oil for cooling, when these blow flaming oil goes everywhere. If leaking, it probably will not blow, but it will overheat and burn up inside.

The purpose of an ignition resistor is to increase the life of the points by reducing arcing. 6v can live without, 12v they will arc much more.

Hope this helps
Mike

Mystery solved. As I am in Italy at present (rebuild of an old American car is not for the faint of heart here!), I finally found an auto electrician (as here they divide the specialties) that was willing to listen and put up with my less than perfect Italian. He helped me with the 12v conversion by supplying the alternator, coil, and ballast resister.

I measured the resistance of the ballast resister this morning....0.6 ohm.

I went back to him. He asked...where did you get this ballast resister...I said from you...He says...it isn't a ballast resistor, but a radio suppression whatever. So.....now a bad coil too.

I have found a new coil here for purchase, but no ballast resistor. 'he' says that it won't hurt anything to leave the ballast resistor out of the mix. Ideas?

Coils aren't vehicle-specific. If it makes at least 20,000 volts on open circuit in the secondary, it will make a stovebolt engine run. Magneti Marelli is the Italian equivalent to Delco. Find a 12 volt coil for a Fiat, or some other Italian car with approximately the right resistance and you'll be good to go. If you're going to run without a resistor, you'll need a coil with about 2.5 ohms of primary resistance. Otherwise, find one with about 1.5 ohms in the primary, and run a 1 ohm 100 watt resistor as a ballast. You want to keep the current flow across the points when they're closed below 5 amps, otherwise they will pit and burn quickly. 12 volts divided by 5 amps is 2.4 ohms. Whether you get that with a coil alone or with a coil/resistor combination is up to you. Cold starting will be more reliable with a coil/resistor combo and an appropriate bypass circuit.
Jerry

Thanks Jerry, I'll check out the resistance of the Marelli, this is the one I found at the parts store. I've already tried finding resisters for the fuel gauge and the heater, and had to order them from the USA.

Jerry,

I still don't know the resistance of the marelli that I found, but assuming the worse, I also found a ballast resistor that has 1.5 ohms of resistance. Do you think that would be ok?

Steven

As long as it has enough wattage capacity to handle the current load, you should be OK. Watts = Volts X Amps. A resistor that drops 3 volts at a 5 amp current flow will need to have a minimum of a 15 Watt rating. A 25, 50 or 100 watt 1.5 Ohm resistor would give you plenty of fudge factor to prevent overheating.

With the engine running, you should be able to measure about 3-4 volts by putting a voltmeter to both sides of the ballast resistor. The voltage applied to the coil (+) terminal should be around 9 volts if everything is working right, assuming a 12 volt input from the ignition switch. At full charging system voltage of 14, the voltage drop across the resistor will be a little higher, maybe 4.5-5 volts.
Jerry

Thanks Jerry, I'll hopefully find out more tomorrow.

Jerry, Are you sure the man needs a 1,000 watt ballast resistor? That thing would be as big as a loaf of bread.

No, I said a 100 watt resistor would be several times over-capacity. The ability for a resistor to dissipate heat determines its reliability, which is why ignition resistors are way over-engineered. Those huge ceramic heat sinks draw the heat away from the nichrome resistance wire.

OOPS! just found the typo and corrected it! Thanks!
Jerry

Ok. I bought the coil. I measured the resistance of the Primary coil by setting up for an Ohm reading, and placing the leads on the + and - of the coil. I got a reading of 3.6 ohm. Is that possible?

I spoke to a mechanic that was in the parts store (everyone is an expert), and he said that it would be strange that I would need a ballast resister with this coil.

Any agreements?

13.5 volts (average of starting/running voltage) divided by 3.6 ohms = 3.75 amps. You should be OK without a ballast resistor with that coil. The only problem might be during a very hot or very cold start, where starter current load on the battery will drop its terminal voltage down near 9 volts (it happens). That will reduce current flow through the coil primary and result in weak/no spark when you need it most. That's why GM used a low-resistance coil and a resistor in the first place, for reliable starting with low battery voltage, with the resistor bypassed. Keep the engine well-tuned and the battery cable terminals clean, and you should be OK.
Jerry

Thanks Jerry. Just so I don't burn up the coil again. This will give me time to get a proper coil/resister combo from the US when I am there next.

Just be sure to double check that the ignition switch is OFF every time you shut down. The only times I've seen a coil overheat to the point of puking oil was after several minutes of "switch on/ points closed/ engine not running".
Jerry

That is exactly what happened last time. Thanks.

Got the new coil installed, but it wouldn't start.

So, I went back to ground zero. Pulled the plugs,( I swear I didn't put them in tight, but I almost broke my wrench removing them ), cleaned and checked the gap.

Checked the points and gap. I couldn't see them opening AT ALL. Then I found that the locking screw was loose (which I was very

careful to tighten the first time around), and so, reset the gap.

It started. My timing light just arrived tonight, and so will fine tune it later.

At least it IS running again. Now to sort out those turn signals, and fuel gauge!

To resist or not to resist. Upgrading everything on my '48 to 12V. Question, is the only reason one would add a ballast resistor to increase point life?

Mostly. May also increase coil and condenser life given that they wouldn't have as much load.

As I stated above, GM chose to use a coil that functions correctly with about a 9V input. That assures a hot spark during hot and/or cold starts with lots of load on the battery. To prevent point damage during normal running a resistor is used in series with the coil primary winding. Points can handle a maximum current flow of around 5 amps, but they're much happier with 3.5-4 amps. Choose a coil primary resistance and a resistor that keeps the primary current somewhere in that range, and bypass the resistor during starting. How you do that is subject to all sorts of "opinions"- - - -those re like noses- - - -everybody's got one and they're usually smelly!
Jerry