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91 Mustang missing


scrollmaster's Avatar
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04-07-04, 06:05 PM   #1  
91 Mustang missing

This is a 91 Mustang LX 2.3L I bought for my 16 yr old daughter last year. For clarification it is a 4 Cylinder with 8 plugs and 2 ignition coils. I put photos of the parts I'm describing below that can be viewed if it will help by going to...

http://community.webshots.com/album/132225574STPbyn
Click on any photo and it will give a large detailed view.

It is running VERY rough on idle and only gets worse when put in drive. It was doing this when I initially purchased the vehicle and I discovered the front ignition coil wasn't firing at all. I replaced both coils Nov 2003 and it ran beautifully until last week. Now it is doing the same thing. The front ignition coil doesn't seem to be firing at all. I can pull one of the wires on top of this coil leading to the plugs and it makes no change at all and it's not firing. If I pull one the plugs on the other coil it changes the idle greatly and it is firing well. I can even pull the large black plug in the front of the coil I just replace and it makes no difference. If I pull the corresponding large black plug from the 2nd coil it immediately dies. Thinking maybe the front coil failed for some reason I purchased one this evening and replaced it but made no change.

The large black plugs mentioned above are in a wiring harness that goes from the 2 coils to several items including a grey module (shown in photos) located behind the alternator that I'd guess to be a computer??? If it's not the ignition coil would the harness or this module be the next area to check?
Thanks,
Dennis

 
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bejay's Avatar
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04-07-04, 06:35 PM   #2  
the module may very well be the culprit but often it is due to a poor ground which can be tested with an ohmeter and backprobing the module ground wire on the lower module connector, and checking resistance to ground.
or you could just remove the module clean its mounting surface apply a fresh coat of dielectric grease and reinstall making sure all hold down screws are installed usually 3 and see if the other coil now does spark.

 
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04-07-04, 07:56 PM   #3  
Thanks for the suggestion bejay. I pulled the lower connector (grey in color) from the module and tested each of the 6 leads to the metal the module is mounted to. Nothing read absolute zero. Seems like three of them read .2 or .3 two read open and one of the end leads read 1.68. Hope I was measuring the right places. I didn't test the upper connector to the module. It is like the lower but black in color.

Would I be correct in assuming the upper & lower connections relate to the 2 coils? ie: upper module connection goes to one coil and the lower module connection the other coil.

I'll have to get some dielectric grease tomorrow and try cleaning the module. Sorry to be ignorant about this but what is best way to clean this?
Dennis
http://community.webshots.com/album/132225574STPbyn

 
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04-07-04, 09:32 PM   #4  
just remove the module and wipe off the area on the intake where the module bolts to it, and then wipe off the back of the module you may have to use some alchohol or carb cleaner on a rag to remove all the dried up grease easily and then just remount the module.
a wiring diagram would be alot of help to locate the module ground if you was going to check it and an auto ranging ohmmeter, or a service manual with a no start, no spark flow chart should also have checking the continuity of the module ground, in some cases neither coil will fire and the car will not run.
the module sends and recieves signals from the ecm for correct spark timing and you would really need a wiring diagram to find out which 4 terminals go to the coils and which connector those terminals are on they may all 4 be on the same connector i dont really recall, but the module then pulses ground to each of those 4 terminals to fire the coils at the correct timing based on inputs it recieves from the ecm. and in your case the module is only affectivly grounding 2 terminals and one coil.

 
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04-08-04, 03:28 AM   #5  
Thanks again. I'll try this as soon as I can get of work today.

 
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04-08-04, 03:44 AM   #6  
How about ignition wires? What kind of shape are they in?

 
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04-08-04, 06:54 AM   #7  
bejay, I removed cleaned and replaced the module and its back working great. Thanks a million for the idea.

Originally posted by Desi501
How about ignition wires? What kind of shape are they in?
Desi, I looked at them last night and I think it's time to replace them as well. I'm going to do that this afternoon after work.

Thanks again

 
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