YJ will start, but won't run

[TJ Fern]

One of my buddies has a problem with his '87 and it's been so long since I did any ignition work that I can't figure out where the problem is.
It's a stock '87 6 cyl auto.
When he tries to start it it sounds and smells like combustion is happening, but as soon as you release the key it shuts off.
The coil has 10.3 vdc on the primary (yellow wire to ground) when the key is in RUN.
The coil meters 1.2 ohms across the primary and 9k on the secondary.
We put a new coil on (it metered the same) but no change. the one we took off looked original. It had an AMC tag on it.
There is good spark to the plugs while cranking.
The way it shuts off as soon as the key is released it sure acts like there is no ignition power in the RUN position.
Any ideas?

[CjRobPos85]

will it crank and just not turn over??? it could be fuel problems then

[TJ Fern]

It cranks over just fine and seems to start just like normal until you let go of the key. Then it shuts off. since it has 10.3 volts at the coil when you let go of the key, I figured the keyswitch and ballast are okay.
There is fuel in the carb and the exhaust smells like the engine started.
I don't know if '87's have anything out of the ordinary in their ignition circuit. Those were odd years for ignition electronics on many vehicles.

[wissemeier05]

-i just swapped in a 4.2 into my 90 and talkinto my jeep guru (he lets me know the symptoms of thins that go wrong and how to trail rig them....

"when the balast resistor goes bad it will run when u are cranking the engion but the moment your key goes back to the run position it will die. it norm goes bad cause it gets wet, if it bolws on the trail just remove it and cross the wires, the resistor is there so u do not burn up your coil but u can run w/out it."

-i have run w/no blast resister for about a month, i was running a strait 12v to the coil we just swppaed it on saterday along w/my motorcraft 2100 (omfg its great)

as mr.allen was telling me all about it i wrote my self a note to revisit this post.
hope this helps
:cool: dan

[Knuckelhead]

Originally posted by TJ Fern:
There is good spark to the plugs while cranking.
The way it shuts off as soon as the key is released it sure acts like there is no ignition power in the RUN position.
Any ideas? The ballast resistor is used to step down the voltage to the coil, which is 6 volt. The starting circuit bypasses the ballast resistor to give the coil 12 volts for a hotter spark during starting. When you release the key (from the starting position) the circuit is now supplying 6 volts (reduced by the ballast resistor) for normal operation. It sounds like the ballast resistor or the wiring related to it is your friend’s problem.
Bypassing the ballast resistor can burn up the coil in minutes, hours or days of operation. This makes it run hotter than it’s designed for. It’s just how lucky you are…
IMO

[TJ Fern]

I suspected the ballast resistor but couldn't remember how much voltage it dropped. Since I had 2 less volts in RUN than START I figured it wasn't the problem. If my voltage reading is correct it should still run (barring whatever the real problem is).
Is the ballast resistor on an '87 an in-line device or a seperate component? And where would it be located?

[edudley]

Your YJ has a Ford Dura Spark Igntion There is actually a start circut and a run circut in the system. There are two plugs going into the ignition control module under the washer fluid bottle. One plug has oragce and purple wires and goes to the computer and the distrubutor. The other plug recieves power from the ignition switch. One lead should be hot coming into the module when cranking and another should be hot when running. If you can find this to be true by probing the leads then replace the module. If you can not find this to be true then replace the igntion switch. I dont have a diagram handy as i am on vacation but I hope this helps. Good Luck

[wissemeier05]

Originally posted by TJ Fern:

Is the ballast resistor on an '87 an in-line device or a seperate component? And where would it be located? i will get the location next time i see him but it is a VERY flexible wire in the harness. On a 360 it runs along the passenger side of the motor and is red.
HTH


o and you can replace the wire w/an actual resister (57 chevy) uses them that is what mine is from

[edudley]

While you may need a new ballast resister this will not casue the running problem you are having. On an early YJ it is a resistance wire not an actual resister. Good Luck

[TJ Fern]

Thanks for the ideas. I'll test the voltage at the module later this week when he gets back in town. Hope the holidays were good to you.

[TJ Fern]

As usual edudley nails the problem. Put the new module on it and made sure the case was well grounded to the chassis, problem solved. I now know way more about duraspark ignitions than I ever wanted to.
Duraspark seems to be a decent design, but leave it to the Ford parts to stop a Jeep.