Ignition probs with TFI

[rebeljeep]

I’ve tried reading many different topics/searches but still am not confident in what I have.

Weekend Rig: 84 J10 V8, initially had stock carb and ignition, ran good/ok.

Swapped carb/intake to 4 brl. Holley and Intake and Juicebox TFI upgrade, both upgrades at same time.
After swap, truck ran good then one day kept shutting off. Eventually found broken wire (had mechanic find it so can’t confirm which one). Drove it a bit more, wire to coil breaks. Had break repaired (different mechanic, I just moved). I know it’s less than helpful not knowing which wire color/size.

Drove it last weekend about 5 miles shuts off a bunch of times driving. Let it cool on side of road, thinking it was the coil. Ended up moving coil harness and it fired up. Tightened up the pins and drove it home 3 miles.

Tested the coil harness today, unplugged
11.9V Key On RUN
7.3 Starting

Then took voltage at back of coil running: ~1.0V

Battery voltage was about 12.07V
And 10.5V while starting

1.0V seems weird,but maybe that isn’t. I dont know.

Not sure if it is a coincidence of bad events with the break and it’s good now to keep breaking it in or I have wiring issue or a crap coil.

Thanks

[dodgerammit]

If you are having issues with broken wiring, I'd suspect that first. Check condition of the rest of wiring. Repair a brittle wire, it'll break again. Need to run new when repairing any wiring issue with brittle wiring.

[rebeljeep]

Alright I’ll check the wiring further.
I didn’t know maybe if the coil was pulling too much current. After posting, I let the truck run idle for about 20 min, seems like the module on the fender is getting pretty warm.

[dodgerammit]

The module could very well be the culprit. Is it original? I seen juicebox. Not sure what you are meaning by that. I run an MSD streetfire with my TFI.

[tgreese]

Juicebox is the TFI parts upgrade; just somebody's made-up name for the usual collection of Duraspark parts you can get at the parts store. Conventional high-energy coil, conventional triggered inductive discharge.

Running voltages don't tell you much. The ignition is not like the rest of the car running on DC electricity; it's instead pulsed AC. Your multimeter can't keep up with the changes and is giving you some time-average of the changes. With expensive multimeters, it's the RMS (root-mean-square) value, but a cheap meter could be showing anything.

Unless you ripped it out, the Duraspark ignition already has a ballast resistor (nichrome wire) that protects the module from too much current. Even with the hot coil, you should be fine unless you changed the wiring. Look on the wiring diagram; it's between the ignition switch and the coil.

On second thought, maybe not with an aftermarket module. Possible the inductive kick from the hot coil will damage an aftermarket module. Don't know. If you want a hot spark setup, the MSD 6A capacitive discharge module can replace the Duraspark module very neatly.

[PossumJr]

Are you sure about your your start and run voltages, because those are backwards. I'm not familiar with what's going on with a '84, but generally with a Duraspark II ignition, cranking sends the full battery voltage to the coil, running voltage will be reduced, just like with older points ignition. I don't know the difference in primary current between a stock style coil and a later Ford E-coil, but if you're getting full voltage on the coil during run then it's the equivalent of a missing resistance wire and it might be possible your box is getting fried. With stock ignition a missing resistance wire can supposedly cause module failure and doing it with TFI is even worse. I'm not sure how things would have to be wired for this to happen though or if it's within reason but it's a thought. I recently switched to a Streetfire CDI box and definitely recommend that route in the future if your module actually did fail.

[rebeljeep]

Thank you for the replies!
Aftermarket module.
I looked up the wiring diagram, I didn’t remove any known resistor wiring with the TFI, just spliced in the new connector where the old coil was. Will check voltage again, I see in the troubleshooting section of manual to use ground, not the “-“ on the connector. Not sure if it will change readings.
Thank you for the explanation of the AC voltage.
I’ll look into the MSD 6A.

[tgreese]

Thank you for the replies!
Aftermarket module.
I looked up the wiring diagram, I didn’t remove any known resistor wiring with the TFI, just spliced in the new connector where the old coil was. Will check voltage again, I see in the troubleshooting section of manual to use ground, not the “-“ on the connector. Not sure if it will change readings.
Thank you for the explanation of the AC voltage.
I’ll look into the MSD 6A.

I should be more accurate. It's not AC; instead it's pulsed (switched) DC. It would have to go negative to be AC. And the voltage can go higher than the battery voltage due to the switched inductance in the coil. Suffice that it's really different from the constant voltage (DC) that the battery supplies.

[PossumJr]

It's also worth noting that as tgreese says, voltage to the coil is pulsed, so DC measurement only goes so far, but the coil will still have a constant voltage supplied to the (+) terminal that you can measure referenced to chassis ground (battery neg.). This is at least useful for making sure correct voltage is going to it when it should be. The negative terminal of the coil is where the switching is done to create pulsed DC to fire the coil.

[rebeljeep]

Hey guys, sorry for the delay, life has been very busy and I wanted to do my homework and testing before i checked back here and reread your posts again. Thank you for the ignition explanation as well. I’m still elementary in those areas lol.
Yes, run voltage is too high.
I suspect a missing resistance wire. I haven’t been able to find exactly where it’s missing but all the tests according to 1984 service manual I have indicate a missing resistance wire. The voltage doesn’t step down like it should.
Save the resistance wire, looks like all the other Underwood wiring is intact and in decent shape. Gonna look up and purchase the correct resistance wire and then go from there with an aftermarket module.

I’ll post my progress next free chance I get

Thanks

[tgreese]

Well, this is a good time to test with your multimeter. The voltages at the coil will be hard to interpret, but you can easily test the resistance going to the coil. Look at the wiring diagram in the TSM - the resistance wire is there - not a lot of resistance, but you should be able to measure it. Probably easier just to look at it. It's a single-strand nichrome wire, unlike the rest of the wiring that is multi-strand copper. Easy to identify by inspection - you can observe a lot by looking, as the sage says. Anyway, The resistance wire forms a voltage divider with the primary in the coil, so if there is a short or open in the coil primary, the voltage will change. As I said, the voltages will be hard to interpret with the simple electrical instruments available to most owners.

Have you followed the flow chart in the TSM?

Really, the resistance wire is super simple and not many ways for it to fail. However, coils do fail, and they can fail all at once with no warning. BTDT. IMO every Jeeper should carry a spare coil. I'd just buy a replacement, install it and see if anything changes. If not, toss it in your parts bag to carry with you. The other most likely fault are the Duraspark ignition parts, including the module and the pickup in the distributor. You should also get some contact cleaner and clean all the contacts of the connectors associated with the ignition - connectors are another common source of problems.

[68glad]

Thank you for the replies!
Aftermarket module.

What brand? I thought all aftermarket modules were good w/12v or in fact required it. I'm still leaning toward bad module tho. Had 2 used msd fail before. Got a crane and never looked back.