94' Cobra Dies with AC is on

Jul 2, 2019
33
6
18
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TIFFIN, OHIO
Hello! I am have been battling a multi year problem. My sisters 94' Mustang Cobra does not run reliably. The vehicle will run great all day long at medium to light acceleration. During those times it will occasionally encounter a random miss through the range of acceleration (barely noticeable, not often). I have since targeted the weird condition down to a unique pattern of which I do not understand. Please see if you can make any sense of these conditions. I am baffled and nothing makes sense anymore.

The car will run all day long until the AC or defrost is turned on. When the AC is switched to the on position the car will miss badly / stumble and fall on its face around 2000-2500rpm. When the AC is off this condition is not present. I am basing these tests on a free rev in the garage. When the AC is on, and the headlights are on it will barely chug down the road. Hit WOT throttle and all miss fire conditions disappear.....Weird right?

My first thought was this has to be an electrical issue.

Step 1: Replace alternator in case the internal regulator is malfunctioning and or its shorting out under high loads. Same results.

Step 2: Disconnect AC clutch just in case it is locking up or malfunctioning. Same results.

Step 3: Install a complete new tune up kit; wires, plugs, cap, rotor, fuel filter. Same results.

Step 4: Install new IAC and TPS (just because nothing else makes sense). Same results.

Step 5: Install new fuel pump (complete new sending unit and pump). Mainly because someone swore it was the pump. Same results.

Step 6: Install new MSD pro distributor (at least it looks cool) and MSD blaster coil. Same results.

Step 7: Install a new Ignition control module. Same results.

Step 8: Install a new power control module. Same results

Step 9: Add extra grounds throughout the entire engine; pcm, ecm case ground firewall, intake. Same results

Step. 10: New main ground cable / positive cables (including starter). Same results

Step 11: Check for shorts on alternator charge cable. remove and use a different battery cable as test. Same results

Step 12: Run new constant power hot leads to PCM in case the originals are shorted, also ran a new ground 10 GA. Same results

Step 13: Replace all of the interior (behind the dash) wiring with a complete used set. Was hoping for a short. Same results.

Step 14: Replace ignition switch. Same results

Step 15: Replace computer with a used unit. Same results

Step 16: Test grounds at main computer plug for continuity. All good...Same results.

Step 17: Test battery voltage at coil, icm with ac off / ac on key on engine off. Results within 0.4-0.5 volts of battery voltage. Same results

Step 18: Reinstall old alternator. Just in case i got a dud. Same results.

Step 19: Replace all fuel injectors in case they are plugged/sticking. Same results (this was done several steps ago, didn't want to re-label everything)

Step 20: Ordered a new ICM. Going to test soon just in case i got a malfunctioning unit previously.....Results to come...


If you made it this far. The weirdest part is the car will run fine with the alternator charge wire disconnected. Leave it disconnected and it will run fine with the ac on or off. Reconnect the charge wire and it misses badly when the AC turned on. WEIRD.......

If you have had a similar problem please respond. I ran out of ideas several steps ago and I am starting to question my mechanical ability...Nothing makes sense with this car, but the radio works great! :/
 
Last edited:
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Jul 2, 2019
33
6
18
36
TIFFIN, OHIO
Update. The new ignition module arrived today. Installed the unit. Tested with the AC on/off and much to my surprise no miss. She runs perfect alright time for a test drive......Drive across town with the AC on, five miles and two shutdowns to get out stops, no problems.....Da daaa daa I return to the car at the second stop after approximately 20 minutes sitting in the parking lot and it does not run. It starts but quickly bogs out only running on a couple cylinders and knocking until it stalls out. No throttle response. Dead....Same as previous results with AC on. Turn AC off same response. Thought i was going to need a tow. No tools nothing on hand. In a last ditch effort i uplug the MAF meter to trick the computer into limp home mode. This allowed it to run well enough to make it home.

Step 20: Swapped out ignition module....Ultimately same results

Any logical ideas?
 

2000xp8

SN Certified Technician
Aug 8, 2003
6,500
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NJ
From what i've encountered in the past, is that if you unplug the maf and the car runs, the maf is the issue.
It is not a 100% sure thing, but twice in the past i've had it happen, so...

I didn't see where you said you checked the codes.
The guys that are going to be able to help you are absolutely to ask you to do so, that's where i would go next.
 
Jul 2, 2019
33
6
18
36
TIFFIN, OHIO
Thank You! I actually attempted a code scan yesterday, but I couldn't get my reader to work correctly and it wasn't running good enough after I made it home. Strange part is i tried working on the car again after it was home. I could recreate the missing/barely running scenario from earlier (by turning on the AC), but the simple unplugging of the MAF did nothing. After it was unplugged it would not smooth out or really run at all. Gave up for the day.

In regards to the codes on this vintage of computers. The computer will lose all the data once the battery is disconnected correct? I never have a malfunction indicate lamp illuminated. It only comes on when I unplug something. Any idea of the failure modes of a bad MAF? I thought the computer was smart enough to tell when its values were out of range. I have tried operating it previously with the MAF unplugged just to test and it never gave me consistent results. Was hoping to hook up my code reader and read the live data voltage of the MAF while operating, but I don't think my unit supports this car (at least it fails to connect using the obd2 port instide). Happy Independence Day!
 

trebor3170

Member
May 31, 2019
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2
13
61
Salisbury, MD
94 cobra is OBD1, not OBD2. The OBD2 connector under the dash is used on V6 models only. You need an OBD1 scanner, and use the connector on the rear of the passenger side inner fender.
 
Jul 2, 2019
33
6
18
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TIFFIN, OHIO
I see....I wonder why the OBD2 plug is included on the 5.0 versions? I found the OBD1 in the engine compartment. Mine is actually on the drivers side directly behind the battery. First attempt to retrieve codes was a bust. I did not plug in the small grey connector (signal input wire). Have to give it another attempt.

Any ideas if the OBD1 will allow live data to be viewed or is that just an OBD2?
 

trebor3170

Member
May 31, 2019
18
2
13
61
Salisbury, MD
The connector on the drivers side behind the battery is for testing the ABS, yes it looks just like the one for checking codes. The connector for pulling codes is on the passenger side behind the strut tower, dig around, it has a cover that is normally attached to the inner fender, that may have come unattached, and the thing will just be hanging in there somewhere. I made the same mistake on my 95. I don't know if OBD1 will allow live data, if it does, my reader doesn't have the ability
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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No live data like obdll, well, unless you count the koer codes.
here is where you will find the correct procedure for checking for codes.
 
Jul 2, 2019
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18
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TIFFIN, OHIO
Ah Trebor3170! You are a wizard my friend. I had no idea I was attempting to test the ABS unit. I pulled the codes and got 552 & 111. I assume 552 is a miscellaneous emission purge valve, but not 100%. At any rate I don't think it should be causing the poor / none running conditions I am experiencing.

Today I tried a few other tests. I hooked up a volt meter to the battery and monitored the voltage of the vehicle at idle. I was getting 14.3 - 14.5 volts. Then proceeded to test with the accessories turned on. Here are the results: turn on AC max, max fan, headlights and rear window defrost. I can systematically watch the voltage begin to drain as soon as the accessories are turned on. The voltage slowly drops until it gets below 13V. Once the voltage reads 12.8 the engine begins to chug and only starts running on a few cylinders until it ultimately dies. Turn it off, charge the battery back up and it will repeat the same results. Any ideas why the battery would not maintain voltage?

I repeated the test, with the engine dialed up to 2k rpm. The voltage would maintain around 13.5 volts and operated fine until i dropped to idle.

Things i have changed after the initial voltage test: I swapped the alternator to a different / new unit (previously I put the original back on as test, now it has the replacement unit installed again), replaced the main power cable from the battery to the engine compartment fuse box (heavier gauge cable). Thinking about trying a new alternator feed wire. I feel as if I tried this before without luck, but it is reacting differently now with the new ignition module. Let the chaos resume...
 

2000xp8

SN Certified Technician
Aug 8, 2003
6,500
818
194
NJ
Not sure what ignition module you are using but you can only use a real Ford unit, the chain store garbage is unpredictable and unreliable.
 
Jul 2, 2019
33
6
18
36
TIFFIN, OHIO
The old unit that was originally replaced before this whole fiasco, when the car first died several years back was a: Delphi DS10056 Ignition Control Module... The delphi module was the one i was running up until a few days ago. I bought a replacement aftermarket unit last year just as a test, but it was DOA. Thought that couldn't be the issue i just replaced this thing.

Just tried a different replacement a few days ago and it actually runs with the ac on, but dies once i get below 13v. The new unit is WVE/AIRTEX/WELLS 6H1086...Do you think it really could be 3 ignition modules in a row that are junk? Or maybe my car is eating them???
 
Jul 2, 2019
33
6
18
36
TIFFIN, OHIO
Not sure what ignition module you are using but you can only use a real Ford unit, the chain store garbage is unpredictable and unreliable.
I purchased a new motorcraft icm. It will arrive Tuesday. If it fixes the problem I will be overjoyed! The last one I changed allowed me to drive the car with the AC on without missing. Maybe this is the missing link...Junko modules
 
Jul 2, 2019
33
6
18
36
TIFFIN, OHIO
Just curious, have you checked the ground on the ccrm?
I had to look up the CCRM acronym. I always called it the power control module (PCM). I ran a new ground wire from the battery to the unit. Don't remember the pin #, but i thought it was the main ground for the unit at the time. I also tried replacing this unit several attempts ago as I thought it was faulty.
 

2000xp8

SN Certified Technician
Aug 8, 2003
6,500
818
194
NJ
I purchased a new motorcraft icm. It will arrive Tuesday. If it fixes the problem I will be overjoyed! The last one I changed allowed me to drive the car with the AC on without missing. Maybe this is the missing link...Junko modules
While i'm not so sure it's going to fix anything, it will prevent it from failing in 6 months.
It's just one of those parts that should have never been knocked off.
 

RioRed95Cobra

Active Member
Apr 7, 2017
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Maybe a dumb question.. have you swapped out the actual switches on the dash? Like the actual unit that holds the 3, fan/ac/speed etc.. ?

If not.. i'd do so.. also.. msd is trash. I have a brand new billet "pro" as well, tossed in the corner of my garage because it didn't work out of the box. Got a refurb from o'reilly and it works great, but the PIP sensors go out often. Go buy one, get a lifetime warranty.. replace as needed.

Kinda sounds like part of your starting issue and erratic running. That sensor controls injector pulsing and if it's going out/bad it'll do this all over the place random as all hell. Trust me, i've replaced at least 6 of them in the past 2 years, lol.
 
Jul 2, 2019
33
6
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TIFFIN, OHIO
She always smells a little funny....Not really just normal exhaust smell.

Yes, the whole interior wiring, was swapped out. I believe it included a whole new control board for the HVAC, but would have to dig through the old wiring to be 100% sure.

I don't have much experience with MSD distributors, but it is the second replacement unit. The first attempt at correcting the issue was a rebuilt auto parts store version. It worked, but I experienced similar results.

Wonder what would make the PIP sensor go bad so frequently? I have only replaced a few in my day.....Maybe all the new parts are just become secondary throw away junk from china. My 86' 5.0 mustang engine was still running the factory original (to the best of my knowledge) until last year.

I am excited to see what this new Motorcraft ignition module does.
 

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