The Stovebolt Site Forums The Shop Area Engine Shop Low voltage at coil

Im trying to track down a periodic miss under load with my truck. I don't find anything wrong on the fuel side. What I have discovered is that I'm only getting between 3 and 4 volts to the ignition coil. Runs fine at idle but the low voltage is weird. Both primary and secondary windings in the coil tested good. It is not shorted to the case. The ballast resistor also tests OK. It's getting about 14 volts on the input side and putting out 11 to the coil. Might be a little high, I don't know. Anyway, only 3 to 4 is actually making it to the coil. I haven't checked the wire coming from the ballast resistor to the coil but that's the only thing I can think of, like maybe there is a break somewhere and a lot of resistance. Will be checking that tomorrow. Anyway, I just wanted to see if any of you had any ideas about what might be going on or can recommend further tests. Thanks.

Try using an alligator clip jumper lead from the ballast resistor output to the coil + terminal, bypassing the existing wire, and take a test drive. That is a quick way to diagnose the circuit, without having to chase down both ends of the wire. It sounds like you've got some corroded terminal connections, or maybe a bad wire. What year is the truck? In the late 1960s and early 70s, Chevy used a long nickel/chrome resistance wire with fiberglass insulation instead of a ballast resistor. That wire is stiff and shiny, and it doubles back on itself inside the wire harness. It's actually about 8 feet long, between the ignition switch and the coil.
Jerry

Thanks, Jerry. The truck is a 56. I rewired the whole thing myself 15 years ago. It's just common automotive wire but one of the ends looks pretty kinked. I'll know more tomorrow.

I'd trouble shoot this problem like this:

Remove the distributor cap.

Crank the engine over and stop with the points closed.

Leave the key on.

Measure voltage from battery + to battery - Call this voltage A

Measure voltage from ballast resistor "input" to ground. It should be within 0.2 volts of voltage A. Call this voltage B.

Measure voltage from ballast resistor "output" to ground. It should be between 7 and 9 volts. Call this voltage D.

Measure voltage from coil + to ground. It should be within 0.2 volts of voltage D.

Measure voltage from coil - to ground. It should be 0 volts. Call this voltage E.

Measure voltage from the coil-to-points terminal on
the outside of the distributor to ground. It should be 0 volts. Call this voltage F.

Measure voltage from the coil-to-points terminal on
the inside of the distributor to ground. It should be 0 volts. Call this voltage G.

Measure voltage from the moving points arm coil to ground. It should be 0 volts. Call this voltage H.

Measure voltage from the fixed points to ground. It should be 0 volts. Call this voltage I.

Measure voltage between the distributor case to ground. It should be 0 volts. Call this voltage J.

Now open the points with an insulated tool and place a insulator of some kind between the points. I use a business card.

Measure voltage from battery + to battery - Call this voltage P.

Measure voltage from ballast resistor "input" to ground. It should be the same as voltage P. Call this voltage Q.

Measure voltage from ballast resistor "output" to ground. It should be the same as voltage P. Call this voltage R.

Measure voltage from coil + to ground. It should be the same as voltage P. Call this voltage S.

Measure voltage from coil - to ground. It should be the same as voltage P. Call this voltage T.

Measure voltage from the coil-to-points terminal on
the outside of the distributor to ground. It should be the same as voltage P. Call this voltage U.

Measure voltage from the coil-to-points terminal on
the inside of the distributor to ground. It should be the same as voltage P. Call this voltage V.

Measure voltage from the moving points arm coil to ground. It should be the same as voltage P. Call this voltage W.

Measure voltage from the fixed points to ground. It should be zero volts. Call this voltage X.

Measure voltage between the distributor case to ground. It should be zero volts. Call this voltage Y.


Report any incorrect findings.

Thanks very much for that, Bill.

This turned out to be a faulty connection with the wire going from the ballast resistor to the coil. Installed a replacement and everything is back to normal. Thanks, everyone. Just to be thorough, I went through Bill's checklist and everything looked good.

While making that sequence of tests (also known as a "series resistance test") do it in small bites, turning the switch off for a few minutes at a time in the process. Leaving the ignition switch on with the points closed for more than a minute or two at a time can result in burned points, and/or an overheated coil or ballast resistor.