How do I tell if my ignition coil is any good? I have everything but spark to get this beast running.
Assuming the ignition system is in the truck and connected..... Remove the coil high voltage wire from the distributor cap, leaving the other end connected to the coil. Remove the distributor cap. Turn the engine over stopping with the points closed. I do this by hand. Make sure the key is off. Open the points by hand and slip a piece of matchbook cover or similar non-conducting item between the points to hold them open. Turn the key on (but do not turn the engine with the starter. Use a voltmeter (a test light will do, but not as well) and measure between the coil + to ground. Should see battery voltage (12 volts DC). If not the problem is incorrect voltage (or none) delivered to coil +. Fix the problem and start over with these instructions. Now measure from coil - to ground. Should still be 12 volts DC. If not the coil is defective. Replace it and start over with these instructions. Now measure from the movable point in the distributor to ground. Should still be 12 volts DC. If not the wire between coil - and the points is defective. Repair it and start over with these instructions. Remove the matchbook cover. Measure from coil + to ground. Should be 6 to 9 volts DC. If less either the coil is defective or a high resistance connection exists between coil + and 12 volt supply. If this condition exists tell me in this thread and I'll add troubleshooting routine. It is more complicated than I care to type tonight. Measure coil - to ground. Should be real close to 0 volts. If not, the coil is bad (open primary winding). Now comes the fun part .... Get a trusting soul (or a pair of vise-grip pliers) to hold the loose end of the high-voltage coil wire about 1/4" away from some nice clean piece of unpainted metal on the engine block. Use a plastic tool of some kind to manually open the points. When you open them you should see a small spark jump at the points and a large spark jump at the end of the high voltage wire. If only the points spark the coil is probably bad (open secondary winding). If the trusting soul lets out a yelp (or you see a nice fat blue spark jumping from the coil wire to the block) the coil is most likely OK. Hope this helps.
Also visually inspect the distributor cap for cracks, corrosion and pitting as well as the wire coming from the coil to the dizzy. BTW, what size engine are you running and do you have the original type points and condenser?
Thanks Thanks for the info Bill. Looks like i do have spark. There was just one problem in the tests. You said Measure from coil + to ground. Should be 6 to 9 volts DC. If less either the coil is defective or a high resistance connection exists between coil + and 12 volt supply. If this condition exists tell me in this thread and I'll add troubleshooting routine. It is more complicated than I care to type tonight. At this point, I have 12 volts. I also noted that the coil was quite warm. I may not have it connected properly - I just use a jumper to the battery for my testing. Is there supposed to be a resistor somewhere in there?
Normally there is a "ballast resistor" in circuit between +12V from the ignition switch and the + terminal of the coil. Typical resistance of the resistor is 1 to 2 ohms. This has the effect of limiting current flow through the coil, reducing heat in the coil and extending point life while running and reduces the voltage measurement on coil + to around 8 volts while the points are closed. Typically the ballast resistor is a white ceramic looking device about 1/2" square and 2" long mounted on the firewall. This resistor is usually bypassed during engine start to provide +12V to coil + for a quicker start. Running with the coil connected directly to +12V will cause it to run very hot and to decrease point life because of the increased current.
Ballast resistor Here's a shot of a ballast resistor that Bill is talking about that I have on my 235. It is required if you're not running an internal resistance coil. Hope this helps.
I wondered where that resistor was supposed to go. I had a box full of wires for the electrical system. Thanks to everyone!
