rough idle and won't rev/ tach jumping around

fasttruck93l

Registered User
Where do these cars pick up a tach signal from. I think that's my problem. I was guessing the Ignition Control Module from reading other posts on some old threads. I now know that that's not the problem and still have the same issues. It seems to be intermittent. Runs badly more than good right now. This all started last week. Started it up for work and when I came out to leave, It was idling rough and wouldn't rev up. I drove my wife's car. She drove mine a few hours later and didn't have a bit of problem. It did it again last week on friday and only straightens up once in a while now and the tack seems to follow suite. When its running right the tach works good and when it's running rough, the tach isn't working properly. Just need some experienced advice.
 
Did you check the harness at the cam sensor and replace the cam sensor?

How are the engine grounds, including the cable from the pass. side motor mount over to the body?

Did you check for codes?
 
most folks say the cam sensor will be the problem. When i had a tach that intermittently jumped around. it ended up being the DIS module. I replaced the Cam sensor first and it ended up being a DIS problem. I have seen other people's pictures of the harness going down to the crank sensor getting damaged by the supercharger belt.
 
I will check wiring harnesses. I just changed the DIS. If I cant find a problem there I might try a cam sensor. Got a 215 code before. That's when it was idling rough and carrying on. When it smooths out the CEL goes back out. That's why I thought it was the ICM. Are the ICM and DIS the same thing?
 
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No. 'IRCM' is the box behind the pass. side headlignt with the relays for A/C~fan and fuel injectors. DIS sends signal to the coil.

What year is your SC?

Do you get a flashing CEL when cranking? How many cranks does it take to fire?
 
ICM vs DIS

ICM is the ford nomenclature. It stands for Ignition Control Module. People call it the "DIS module", which is incorrect. DIS actually stands for Distributorless Ignition System.
 
According to some Ford wiring diagrams, the tach feed leads thru the coil wiring. Do you have a spare coil to test with? Does that code re appear after clearing codes?
 
According to some Ford wiring diagrams, the tach feed leads thru the coil wiring. Do you have a spare coil to test with? Does that code re appear after clearing codes?

I used one off of a 96 ranger that I know is good. Didn't seem to make a difference. Yes the code comes back when the idle gets rough again.
 
Let me mention grounds yet again and suggest adding one for testing from the coil mount to the battery negative terminal. Check the resistance from the DIS mounting screws to the negative terminal.

And just for good measure, lay eyes on the multi-wire ignition switch down on right side of the steering column, inside the dash (not the lock cylinder switch).
 
when you replaced the DIS, excuse me... ICM.... did you clean up the mounting surface and use some fresh heat sink compound?
 
when you replaced the DIS, excuse me... ICM.... did you clean up the mounting surface and use some fresh heat sink compound?

Yep. Made sure to do that. Checked grounds. All seemed good. Pulled off and cleaned behind for good measure. I unhooked the crank sensor and tried to start the car tonight. It wouldn't even fire without it. I plugged it back in and it started. I put a timing light on each plug wire while the car was running and found cyl 6 not sparking at all. I found intermittent sparking on 4 and 5. I was getting pretty consistent spark on 1 and 3 and no spark on 2. Found a burnt spot on the inside of my coil pack where the wire wasn't in properly and had melted a spot out of the coil pack on cyl 6. Im assuming that's why I'm not getting fire on #6. I think my coil pack is faulty. I'm going to try a used one if I can find one. I tried one off of a 4.0l ranger and it's wired different. It didn't fire the car properly anyways.
 
Installed my new coil pack and it started very nicely and idled very smooth. After a moment or two of idling. Same story as before. My problem is very intermittent now and car runs smooth vs. rough about 50/50. I'm at a loss. Don't know what to do now.
 
Not sure if you have an EGR.
If you do then, check the EGR Transducer vacuum line.
I had a problem like this years ago, the engine would run smooth one second , then rough as hell the next, tach was all over the place.
The cause was that little black hose was cracked, I replaced the hose and all was good!
That is a very tight place to work, I recommend removing the windshield cowl, that will open it up a little more for you to work.
Hope this solves your problem!
 
I hate to say it but if you confuse a rough idle with a dead misfire perhaps you should have someone more knowledgeable diagnose the issue

other than throwing parts at it, have you tried improving the grounds as has been mentioned?

From what you describe it sounds like the external TFI module "DIS" / "ICM" is going north, however, the EXACT same symptoms can be caused by a faulty ignition switch as well which I have seen was mentioned

All 3 of those issues are very common problems on the supercoupes now due to their age.

In fact, out of all 9 of my supercoupes, only my 93,94, and 95 have the factory ignition switch, ive had to replace them in the other 6 already, same applies to improving grounds and the DIS module
 
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