Voltage to coil changing

[jarlt]

I have been troubleshooting my '68 F250 with a 390 trying to get it to fire up. Voltage to coil was low with key in "on" position at 5.8-6.0 V. I finally found a bad inline fuse holder that was old a broken apart from the relay to the volatage regulator on the yellow wire (152 on the diagram), so I temporarily cut it out and spliced together to check. Voltage to coil in "on" was then at about 10.0 V, then it started up and ran. I shut it off and opened up the garage door to back it out, would not fire up and coil voltage now is back at 5.9 V in "on"...Any ideas?

[68RGR]

Just guessing but I'd suspect a bad connection possibly due to corrosion. I had a problem with my dome light. During my tests both it and the cargo light "magically" started working when I checked the fuse with a test light. I determined there was a little bit of corrosion between the fuse and its contacts in the fuse block. You did much the same thing - When you "disturbed" the circuit you might have "burned" a temporary hole through the corrosion. Unplug and re-plug any connectors several times and loosen and tighten any screw-type connections to see if that will "clean up" any corrosion that might be present.

[Ozarktraveler]

If the ignition wiring is unchanged from factory new, it is designed such that when the key is in the "on" or "run" position, voltage is applied through the ignition coil thru a "resistor wire/lead" which drops the 10-12V to about 6V. When the key is turned to the "start" position, the resistor wire is bypassed giving the full 12V to the ignition coil. Not sure how you got 10V after bypassing the inline fuse. (Not saying you didn't. 68RGR is likely correct.) My point is only that it "should" run with "only" 6V to the coil with a stock ignition system, though it might take 10-12V at the coil to start it.

Check the voltage to the coil with the key in the "start" position. Disconnect the High Voltage (High Tension) lead from the coil to the distributor when checking to prevent engine startup. Keep the key in the "start" position just long enought to check.

[tqwrench]

...and

To put it another way, did someone before you change this out to a Pertronix or other aftermarket system designed to replace points & condenser? I don't have 1st hand knowledge but reading here tells me those coils are supposed to get full voltage all the time. A small wiring mod is needed with a system like that from what I've read.

If you still have points in the dizzy, you should get about 6V to the coil key on, full battery voltage in start position. This is normal.

A best practice when working with a coil like this is to ground the high tension terminal if you need to crank it with the coil still hooked up and do not want spark. The coil can be damaged if the discharge cannot find it's way to ground. Plus, it's a safety measure, if there's a gas leak near that coil while cranking, it might try to find a path to ground through that mess and kaboom.

[ChrisB]

I found 3.5v at the coil in run position. Did not check 'start' position. Guessing its getting 9 or better, engine first lights fine, but tends to foul or maybe just weak spark miss after that till warm and or engine rpm brought up to 2500-3000 to clean things up.

I see the orange/red and yellow wire coming out of ignition switch harness and come together in metal connector right at + coil terminal.

I just installed Pertronix Ignitor and 40Kv coil. Ran like crap with that 3.5v without points. I ran a jumper from bat + to coil + and it got better but not sure it was better than with points, not sure why, probably due to damp plugs that were coming in and out of steady spark during testing.

Guessing I have corrosion at either end and in between to get only 3.5v. Plan is to leave existing two coil wires in place and run a 12v non resistor 14-16 gage wire from what looks like an auxiliary unused pin on the ignition harness, right to the coil +