The Stovebolt Site Forums The Shop Area The Electrical Bay Ballast resistor/ coil?

Just curious and not quite sure how to check bothe the resistor and coil for acceptable use? 1955 235' 12 volt 1/2 truck.

Do you already have the coil and resistor and want to know how to test them, or are you looking for recommendations on what coil and resistor to use?

Sorry, I guess I didn't make myself clear. I have been running bothe the resistor/ coil for well over 10 years now and thought I would perform a check on them to see if there within specs. The truck here lately has been running a little off? (Radical)

Testing a running system is very simple. With the points closed, engine stopped, and the ignition switch in the "run" position, there should be around 9 to 9.5 volts at the coil (+) terminal if the system is working "as designed". On the distributor side of the coil with the "points closed- - - -switch on" a voltmeter should show virtually no voltage- - -less than 1/2 volt, maximum. The less the better on the distributor side. Higher voltage indicates dirty or burned points. During cranking, the coil should be getting very close to full battery voltage if the resistor bypass system is working correctly. Have fun!
Jerry

While cranking were talking the (+) side of the coil should have close to battery voltage or the (-) side?

Someone else can chime in, I don't recall the specifics. Once the engine is running, shouldn't there be a cdrop in voltage from the resisitor? Isn't that the point of having it?

While cranking the + side of the coil should have battery voltage.

When measured with a voltmeter (a slow acting measuring device), the - side of the coil will be varying between zero volts when the points are closed, to battery voltage when the points are open.

If measured with an oscilloscope or your finger, the - side of the coil is much more interesting. When the points close, current flows through the primary winding of the coil creating a magnetic field that is transferred to the secondary winding of the coil. When the points open, current flow in the primary winding stops, the magnetic field collapses and the secondary winding produces the high voltage pulse that eventually is transferred to a spark plug. That same collapsing of the magnetic field causes a pulse that can be measured at the - side of the coil, sometimes measuring 200+ volts for a very short period. Picture below is worth a thousand words. Blue oscilloscope trace is voltage at the coil - terminal. Red is current measured at the same place.

Yes, a voltmeter (a slow measuring device) will read somewhere around 8-9 volts at the coil + lead on a running engine with a resistor. Actually, the voltage will be varying 30 times a second on a 6 cylinder engine at 600 rpm as the points open and close. When the points are open, coil + will be at battery voltage. When the points are closed, coil will be less than the 8-9 volts the voltmeter is reporting.

Some of the better quality digital voltmeters are virtually useless around a point type ignition system. They react to the rapidly changing voltages Bill refers to, and just produce gibberish numbers on the digital readout when the engine is running. I use an old Simpson analog multimeter with a sweep needle for situations like that. Ditto for generator/regulator adjustments with the old mechanical point regulators!
Jerry

Ok, here are my numbers as follows. Battery is at 13.32 volts. With points closed I am reading 7.0 volts on the (+) of the coil and .49 on the ( - ) side. While cranking the coil is reading 8.75 volts on the ( + ) side of the coil, not sure if these here numbers are acceptable or not?

Sounds pretty good- - - -once the charging system kicks in, that "battery" voltage will probably go up to 14+ volts once the drain from the starter gets recharged. That will translate to around 9 volts at the coil positive post. Your downstream voltage could be a little lower- - - -try removing the points and polishing them with a pencil eraser. It has just enough abrasive built into the rubber to clean up any oxidation without putting any scratches on the contact surfaces.
Jerry

So sounds like no replacing of eighther the coil or resistor? Might have to look at the carb. a bit, it seems like it will start up and run for a bit, then once it starts warming up it starts sputtering, that's why I thought it could be in the electrical..

Remove the air cleaner and look into the carb inlet. When the sputter starts, I'm going to guess you'll see a little bit of liquid gasoline dribbling from the venturi. (the restriction halfway down the carb body). That indicates a leaky float valve, and/or fuel pressure that's too high. It's like a toilet tank float valve that fails to shut off and allows water to leak into the bowl constantly. The cause could also be a carburetor float level that's set a little too high, or a float with a pinhole in it that allows it to partially fill with gasoline.
Jerry

I don't like the .49 volts. Too much resistance between the coil - and the ground provided by the points.

Continue testing by ...

Remove the distributor cap.
Make sure the points are closed and the ignition switch is in the run position.
Leave the black lead of your meter on the same ground that you used for the previous measurements.

Touch the red lead to the distributor end of the wire that attaches to coil -. Record voltage reading.

If the distributor has a terminal on the outside that passes the coil - connection into the distributor body record the voltage on the inside end of that terminal.

Record voltage on the wire where the wire connects to the points.

Record voltage on the points spring where the wire connects to the points.

Record voltage on the moveable point.

Record voltage on the fixed point.

Record voltage on the plate that the points mount to.

Record voltage on the outer case of the distributor.

Your problem could be any one or more of these places.

Or, you could just replace the points and hope for the best.

Does anyone still have a points file?

One source of voltage drop in the primary circuit that's often ignored is the distributor housing ground. A piece of high-flex wire like the kind used for voltmeter test leads connected from the distributor housing to the engine block will improve the ground circuit- - - -which can be pretty intermittent as the housing is moved by the vacuum advance. Normally, the ground is provided by a copper contact on the bracket that connects to the octane selector, but that can be damaged or worn out by decades of movement.
Jerry

You can still buy a points file from chain auto parts stores. I bought one a year or so ago from Advance.

Save the point file for really tough toenails. If you can't clean up a set of points with a pencil eraser, replace them. There are plenty of original equipment Delco-Remy points on Ebay, not the offshore clones from the local FLAPS.
Jerry