a while back i threw a bunch of new ignition stuff at a vacuum leak, go figure... i figured it wasnt all bad since now i have good ignition stuff right? it drove great for about a week then i started to notice random missfires, no backfires or anything, it just seemed to be dropping out from time to time. at first i suspected my msd blaster II coil. i put on an accell coil and later that day i broke down in the middle of an intersection, the coil melted and some liquid was comming out of it. i put the msd back on to get home. the missfire is still there. i read somewhere that if the coil was bad the negative post would light up a test light. my meter shows 7 volts on the negative post when the truck is running and .4 when off and the key on. that doesnt seem right to me. are there better tests i can do? other things to check? the carb is spotless by the way... thanks

 

Yes there are tests you can do with a ohm meter.

This test is for factory coils, not sure if they are the same for your aftermarket system however here is the proceedure.

Ignition Coil Primary Resistance.

1: Detach the ignition coil wires.

2: Mesure the primary resistance between the BATT and TACH terminals.

3: Resistance should be between .8 to 1.6 ohms.

4: If it's not, then replace the coil, then check coil power supply voltage.

Ignition Coil Supply Voltage Check.

1: Check the coil supply voltage from the ignition switch with the switch in the run position, coil disconnected.

2: attach positive lead of voltmeter to BATT terminal, and negative lead to the distributor base.

3: Power supply should be between 6 and 8 volts. If greater or less check balast resistor.

Ballast resistor Check.

1: Turn the ignition switch off.

2: Remove the coil wires and the red and white wire connector from the ignition module.

3: attach leads of a ohm meter to the BATT terminal connector and the Red wire of the harness connector that goes to the module.

4: Resistance should be between .8 and 1.6 ohms.

5: If it's not then replace balast resistor, and recheck supply voltage.

Ignition Coil Secondary Resistance

1: Check the resistance between the BATT terminal and the center terminal of the coil.

2: Resistance should be 7,700 to 10,500 ohms.

3: If it's not within range, replace the coil.
4: if it is within range check the tach test lead from module to coil for a short to ground, or break in the wire.


Hope this helps...

 

If all of the above checks out.
You can open the hood on a dark night (no moon or streetlights) let your eyes adjust to the darkness. Watch around the plugs, wires and coil for electrical leaks. Many times you can see an external misfire between plug wires. I have seen plug wires firing down the outside of the plug instead of through it. Electronic ignition has the power to fire to any ground that gets to close.

 

Excellent posts - those are exactly what you should be doing to check the coil. The coil is all too often blamed for problems because they're so easy to replace. 81-F150's tests will make sure your electrical is set up correctly and Bart's will tell you if you have any misfires due to faulty plugs or wires.

 

As fmc said these are great posts. Very helpful info. rep points sent for these posts.