Starts, but won't run!

[erics421]

So I posted a while back about the ignition system - not being able to get the starter to engage. Well, I got everything worked out (some bad splices had given out). So, at the end of the day, I have replaced everything releated to the ignition system (new coil, battery, solenoid, switch, module, grounds, cables- you get the idea). The only things that were not changed are the plugs, plug wires, and distributor, and none of them are that old.
ANYWAY, the starter engages just as it is supposed to, checks of the voltage all look good, BUT she won't keep running. As soon as I let up on the key and disengage the starter, she sputters out. I replaced the coil thinking that was the problem, but no difference. Also put in fresh fuel since it had been a while. So, what obvious thing am I missing???

Thanks, eric

 

[Torxhead]

If you have the "S" and the "I" terminals wired backwards on the starter relay it will give those symptoms. The terminals should be identified if you didn't buy a cheap relay, the "S" terminal gets the light blue wire from the ignition switch and the "I" terminal gets the red w/white tracer wire from coil with the motorcraft ignition.

 

[Old Dog]

It will also do this if the resister wire is burned out.

 

[erics421]

Fortunately for my pride (what little I have), I have the connections correct on the solenoid. Unfortunate that the answer looks to be more complex.
I did flip the wires on the S and I terminals just for kicks. This results in the starter engaging when the key is in the on position. This kind of makes sense as I know there should be current on the red wire when the switch is on. If that's not correct, maybe I have a starting point. Can someone confirm that this sounds right?

 

[Torxhead]

It will also do this if the resister wire is burned out.

I don't think the resistor will burn out as easily as it could break due to fatigue. It is the skinny dark brown wire coming off the alternator and is about 40 inches long terninating at the drivers side firewall near the junction box. It should have around 15 ohms resistance along it length. I'm thinking it could even be in the ignition switch, where it would shut off the run circut. Hopefully it is not a wire break between the starter switch and the coil. That could be checked by turning ignition key to the on position and you should get around 12v to the coil positive.

 

[ijeeper]

If know the AMC 304s have the ignition wired through the tach. If for some reason you have removed the tach, it will start but won't run. The run position bypasses a lot of accessories. When the key goes to the run position, the accessories go hot. So, any under dash re-wiring could be the cause. The key start position also bypasses the ballast resistor. So, if it's missing, not hooked up, or broken, the Jeep will start but not run.

 

[erics421]

Thanks Torxhead. I checked the voltage to the coil, and everything is good there. I need to check the resistance - not sure of the best way to do that. I am on the second new switch, and everything seems to check out on the meter. There's power to the right places in both the on and start positions.
I am curious about the wiring behind the dash as mentioned by ijeeper. I don't have a tach (and never had one), but now I wonder if I need to at least check the wiring. Any thoughts??

 

[Torxhead]

To check the resistance wire, all you really need to know is that it is hooked up at each end that I described. It is connected to the ignition switch and will have power in the switch run position, so check and see if you can get about 6 volts from the alternator side of that wire. On your vehicle the tach wire is one of the dark green wires coming off the coil, one is for the tach, and doesn't need to be connected to anything, and the other goes to the ignition control module. The next time you try to start your vehicle and if it dies when you let off the key, check that coil voltage again.

 

[erics421]

So, I checked the voltage on the coil. The red wire has the 12+volts, but the green only has 2V. This is consistent when I measure it before starting and checking it right after it stalls out. Checked that I am getting spark when it is cranking (even though I knew I was since the engine was running with the starter engaged) and confirmed spark. Although I have replaced the module, could there be something wrong with it? Should I change that out again?

 

[Torxhead]

I think your problem is somewhere in the secondary ignition system, not related the the icm, which can be tested at a good auto parts store. Since you do have coil voltage with ignition on has me stumped.

 

[BusaDave9]

Since you do have coil voltage with ignition on has me stumped.

Yes, that is the confusing part.
You are checking that voltage with the key in the RUN position? Not Start?


We all seem to have the impression that the engine starts up fine but when you turn the key to run it's like it turns off. That would point to a bad key switch not giving 12 volts to the coil when turned to RUN.
Since thats not the issue I'm thinking it's running so poorly it can't stay running.
Are we sure we have the spark plug wires on right? The firing order is 153624

 

[BusaDave9]

Follow this thread: http://www.jeep-cj.com/forums/f99/engine-wont-start-troubleshooting-guide-19494/

We know the starting system is good but you need to check for spark and fuel.
Let us know what you find.

 

[BusaDave9]

This also sounds like a carburation problem.
I wonder if the choke is working.

 

[erics421]

Thanks all for the many suggestions. I did check the spark, and found no issues there. Rechecked all the grounds, and voltages at the connections. As you all gathered, the engine would start up but shut down once the key moved from start to run. Double checked the voltage on the switch in all key positions, and all were right where they were supposed to be. So, I went ahead a replaced the ICM. Fired up and has been running great ever since. I'm not sure why the ICM would have been damaged (other than most seem to think they are junk). The resin was still intact unlike the first ICM I replaced. Maybe someday I'll get around to reading all the info on replacing the ICM with a more robust system.

Now that we're up and running, I have a few minor tweaks (slow radiator leak, and a few gauges), and then I'll have to connect with the locals and get back to wheelin'!

Thanks again everyone!!!

 

[Torxhead]

Glad to hear you got that problem solved Eric. I have never heard of an icm causing that problem before. From what I have picked up the oem Motorcraft icm units were the best but have long since been out of production. There are a lot of cheap Chinese versions out on the market that have a poor history. N.A.P.A.s Echlin version is supposed to be the best, but pricey.