She runs till she warms up, then nothing.

After fixing / replacing lots of things as described in this thread, I went driving. After about an hour she simply stopped running and I was a bit stranded, luckily near home.

I checked the fuel, plenty going into the throttle body. Nice squirts every time I rotated the throttle.

Spark was obviously a problem as it would catch but not run. A fellow Jeeper (Robert) came by and we did some quick troubleshooting (ok, he knew lots more than I did so I just turned the key when he said "go").

He confirmed my plug wires were right and we set the gap just a bit wider, but not much (used the 2:1 rule plug gapoint gap) so the gap is now 0.018 on the points and plugs are 0.035. Spec is 016/035. No joy.

Cap and rotor were new that morning. Condenser and points were new from day prior. We bypassed the ballast resister (it was very hot) and still no joy.

We also tried testing coil output by simply turning the engine over while holding the coil output near to a ground. Very weak spark, yellow.

After it was towed home I slept on it and this am I ran the following volt/ohm checks per the FSM section H-4 Primary Circuit.

Of note, she starts and runs just fine this am, but I will NOT leave the driveway!

a: voltage at coil primary terminals while cranking: 4.57v
b. Confirm points are closed: yes
c. Voltage from battery terminal on starter solenoid to to coil primary -- assume this is +: 7.0v
d. Voltage from solenoid terminal to batter terminal on ignition switch: 1.0v
e. Voltage from batter terminal to ignition terminal of switch: n/a
f. Voltage from ignition terminal of switch to coil terminal: 5.1v
g. Voltage from Distributer primary terminal on coil (-) to coil terminal on distributer: 5.1v
h. Voltage from coil terminal on distributer to distributer body: 5.5v
i. Same as h. with points open: 12.4v
j. Same as h. with condenser disconnected: 5.7v to 12.4v jump when condenser disconneced
k. Voltage from distributor body to battery negative: 0.019v
l. Voltage across battery ground cable: 0.000v
m. Voltage drop between distributer body and engine block: 0.015v
Bonus -- Ballast cold = 0.75 ohms, 7.1v
Bonus -- Ballast hot = 7.5 Mega Ohms, 1.2v

Items f, and possibly g & h, tell me I need to replace the wire between ignition switch and coil (that's where the ballast is, so that may be the issue as the wire was replaced just a few weeks ago); and the wire from ignition to starter solenoid (it's an original with a rusty terminal on solenoid).

Item j tells me I may need a new condenser. It is a new one, just replaced earlier this week.

Items k, l & m tell me I need to remove the distributer and clean it to ensure a better ground to the engine block.

Questions:
1) Is the ballast the source of my issues? I've got a Prestolite distributer (figure H-8 of the FSM). The ballast is original and I recently cleaned the terminals. The FSM says the only "check" is continuity -- cold = 0.75 ohms, hot = 7.5 megohms.

2) While condensers are cheap, what is likelihood of a new condenser being "bad"?

3) Is that very small voltage from distributer body to ground significant? The FSM says it should be near zero...how near to zero should it be?

Again, this morning she is running, but not going anywhere soon.

Regards,
Tim

 

Starter switch isn't loose in any way, it is original. Ballast is not bypassed nor was it for any of the FSM tests.

Regards,
Tim

 

It starts when cold, runs for quite awhile.

Regards,
Tim

 

You might want to try swapping out a different coil. Sometimes a coil will be fine until it has been on for a while. As it gets hotter during operation, it can short itself out. It will be fine for 15 or 20 minutes, then just quits. Let it cool down for 1/2 hour, and it works fine again. I have had this happen with other vehicle through the years.
-Donny

 

Yes, 5v. This is what really confuses me. Yesterday I started it up, it warmed up and ran fine for about 45 minutes, on the road. Then after a sudden loss of power it backfired, sort of ran for another mile (I coasted most of that) until it just quit. Would not restart.

Then this morning while doing the above test a it turned over. Reliably starts and runs cold. Once warm not so much. Just rough so I turn it off.

Could a faulty ignition switch cause this? Vibration ultimately causing the failure in the switch then turn off the engine? It feels solid, wires are all tight and clean, connections not corroded. Key is solid in the tumbler.

Regards,
Tim

 

Coil is new, replaced earlier this week. I guess I could reinstall and retest.

Regards,
Tim

 

Switching in all that all that new stuff at once creates a host of complex possibilities. "New" is no guarantee. Many times a new item will fail rapidly, whereas an old one is "proven."

You now need to very slowly, carefully, and systematically work through the whole system one item at a time.

 

Yes, all tests were with key on run. I think the voltmeter was bypassing the ballast, but I'll recheck tomorrow. And "rough" yesterday was "not at all".

Regards,
Tim

 

coil is definitely a good possibility. I had a new condenser to run about 30 min. and then die, so don't rule it out.

 

Checked (f) again tonight; too hot to do any more I almost melted as it was.

Voltmeter measures voltage between ignition at switch and + side of primary field on coil. Switch set to run, voltage jumps from zero to 7.7v with the switch then smoothly drops to 5.3v steady after about 30 seconds. Technically this does not bypass the ballast as it should remain in the circuit to provide the proper voltage to the coil.

FSM says the voltage should be less than 0.05v. If greater, clean and tighten connections and try again. I can confirm all connections are tight and the wire is fresh with new connectors.

A difference that great tells me something else is going on. Or I'm measuring the wrong thing. Best I can understand the ignition wire is the center wire of the ignition switch, purple on my jeep. That wire goes to the firewall junction, changes color there to a light green, then goes to the starter solenoid (solenoid wire item 8 in fig H39). In my mind it SHOULD have a large voltage, not be .05v. Otherwise it wouldn't have sufficient voltage to engage the starter.

The step prior, which establishes need for this measurement, is battery terminal (+) to ignition terminal on ignition switch. That value fluttered between .09 and .1v ... Which may mean, YES I do have a bad ignition switch as a value greater than .1v indicates a failed switch.

Close enough to claim failure?

Regards,
Tim

 

New coil...I do have the old one but I don't want to replace too many things at once. And it's so hot in garage I can't work there.

Regards,
Tim

 

I believe he is supposed to be measuring voltage drop in the circuit - the ideal reading would be zero.