94' Cobra Dies with AC is on

RioRed95Cobra

Active Member
Apr 7, 2017
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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.

Most, if not all stuff these days for these cars, is made in china/taiwan/etc.. not much is being made good quality anymore.. sadly. If you want something good, you gotta pay.. i'm looking at injector dynamics 1000cc injectors for my next stroker build.. lol... $960. Good lord.
 
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Jul 2, 2019
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TIFFIN, OHIO
So the plot thickens. And without dredging through the thread again, recap what wiring has been changed

All interior wiring behind the dash. Full dash out, replaced with used wiring loom, re-install. Does not include the ECM wiring. It is part of the engine compartment loom.

Added miscellaneous grounds direct to the battery throughout the engine bay; all the standard spots, and a few extra on the engine.

Added direct battery ground to CCRM. Removed/replaced all three constant hot leads to the CCRM with direct lines fused at battery. Also added direct ground to the ignition module.

Added a direct ground to the ECM case ground pin.
 
Jul 2, 2019
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TIFFIN, OHIO
Man that sounds confusing.
what was the reason for all the ccrm rewire and extra grounds?

Attempts to find a random short / making sure the CCRM has the power it needs uninterrupted. Weird things happen when circuits are not properly grounded.

The whole car has been spooky ever since we owned it. The old dash clock used to count backwards. It also has had the strange tendency to run fine and then all of a sudden crap out. Finally trying to find the real reason she randomly turns off. I can run all day now, with the AC off. I want solid reliability and hope someday to find the reasoning behind its crappy performance. Or on the other hand it might just drive me crazy.
 
Jul 2, 2019
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New ignition module....Same old story. Disappointed at best.

Since that failed attempt I replaced the mass air flow meter with an aftermarket stock replacement. I also checked / reset the timing to 10° BTDC base timing. It was pretty damn close in the previous setting. Now it appears to run longer and better than in my previous attempts, but I am still not 100% conclusive. It might just be a smoke in mirrors show.

It will not support heavy electrical loads at idle. I can operate the AC, full blower fan, and highbeam headlights without much issue (ran for 15 minutes no issues). However when i turn on the radio on top of those it slowly chugs down to an ugly stall. Also I attempted the same scenario as previous tests; max ac, headlights and rear window defrost....This scenario makes the idle run time much short and she craps out. During all of the testing I monitored the voltage at the battery. Once the voltage reached ~12.5v the car would start chugging, running on a couple cylinders until it eventually stalls out. It is like it loads the engine up / changes the timing once the voltage gets to 12.5.

Curious why my alternator will not maintain 13-14 volts at idle? It has 14 plus volts during the initial warm up period, but once the accessories turn on I can watch it tick down below 13v.

Contemplating trying a new one wire hook up alternator.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Go to the 3g.
Much better system upgrade than a one wire alt.
BTW, have you cleaned and checked your gounds?
Was the battery load tested? (Not a autozone test)
 
Jul 2, 2019
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Go to the 3g.
Much better system upgrade than a one wire alt.
BTW, have you cleaned and checked your gounds?
Was the battery load tested? (Not a autozone test)

I like the upgrade guide....Unfortunately it already has a 3G alternator (at least I think so). It is a 94' mustang cobra. I tried replacing the alternator, and I am currently running a stock 94 mustang unit. The car is a cobra and the factory original had an under drive pulley. Technically should get more power with the replacement at idle.

In the previous attempts I replaced the main ground cable and the main starter power cable. The battery has not been tested, but it is a new replacement from last year. I could swap the battery out and test with a used one in the garage.

Curious if the load sense wire isn't reading the load correctly? Shouldn't the computer tell the alternator to be on when the ac is on or does it just sense battery voltage?
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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I'm not sure about alternator dynamics but it sure sounds like a bad battery or wire to me.
Yea, on that 3g stuff I forgot what year it is. Only got 2 brain cells left, CRS you know.
i do know that the one wire alternator is not the way to go on these modern cars.
do you run under drive pulleys?
 
Jul 2, 2019
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No the factory cobra pulley (the original alternator) appears to be smaller than the stock GT version. So i am actually running the alternator faster with the stock GT replacement alternator. I bought a DC clamp amp meter today. When it arrives I will test the amperage coming from the alternator charge wire.

I feel as if something is going goofy with the charging system from a wiring standpoint. However, I am not savvy enough to properly diagnose all the wiring. I tried it before as a temporary test, but may install a more permanent heavy gauge ground directly to the alternator mounting bolt.
 
Jul 2, 2019
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Do you know anyone with another computer? This whole thing sounds so odd.

I have went way past crazy with this car. It is my Unicorn.

Yes, I went down that road several steps ago. I have a spare stock GT computer in the trunk i swapped out in previous iterations. I could give it another try. It only takes about 15 minutes to swap.

The last time I tried it the car would still noticeably miss around 2000 rpm with the AC on (same results both computers). However, it has made a weird evolution since then. Now it runs fine with the ac on, but completely dies off when the voltage drops below 13V. Typically after heavy electrical loads at idle maybe 10-15 minutes depending on how many accessories I turn on.
 

RioRed95Cobra

Active Member
Apr 7, 2017
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I have went way past crazy with this car. It is my Unicorn.

Yes, I went down that road several steps ago. I have a spare stock GT computer in the trunk i swapped out in previous iterations. I could give it another try. It only takes about 15 minutes to swap.

The last time I tried it the car would still noticeably miss around 2000 rpm with the AC on (same results both computers). However, it has made a weird evolution since then. Now it runs fine with the ac on, but completely dies off when the voltage drops below 13V. Typically after heavy electrical loads at idle maybe 10-15 minutes depending on how many accessories I turn on.

I probably would have pushed my car over a cliff by now. Lol.

At this point i'd seriously suggest spending the money to take it to a dealership and have it hooked up to a real scanner. Damn man.
 
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Jul 2, 2019
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It's the labor of love I suppose.....

Unfortunately, I haven't had much luck with professional shops in the past. They usually poke around replace something trivial and it magically runs more so out of coincidence than corrective action. I pay $400 and it starts getting weird again. Happened with my, dare I say, Dodge Daytona....AKA Turbo-Turd
 

jozsefsz

Mustang Master
Aug 11, 2013
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Something noteworthy, your car won't change the timing with low voltage, but it will change the amount of fuel (via the Injector Offset vs Battery Voltage table). So if the voltage drops too much, it can start to run poorly.

If you haven't tried a new battery, I'd try a new battery, after triple-checking all the cables and grounds which you mention you've already done. The one-wire alternator might introduce other problems as because as I mention, the computer adjusts parameters based on sensed voltage. Because of this, it could be a problem with the harness somewhere as well.

If it were me I'd probably install a Quarterhorse so I could at least see what the heck the computer was doing (sensed voltage, etc.) which is pretty much like having a decent OBDII scanner on our old cars. If it kept up much longer, I'd take the whole thing carb'd (while I'm usually not a fan of that, I'd make an exception for a cursed vehicle).
 
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Jul 2, 2019
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I think I am finally headed down the right road......I hope this proves to be reality and not another trick.

Today I received my clamp ac/dc fluke current meter. I clamped the alternator charge wire and the main battery cable to the fuse box in the engine compartment. I also left my multi meter on the battery to sense the voltage. Here are the results with the replacement alternator.

Idle with no accessories on:
charge wire 15-20 amps / main fuse panel wire -8amps ( - means charging battery, ie flowing from fuse box to battery)

Idle with ac/headlights on:
charge wire 50-65 amps / main fuse panel wire -5amps ( - means charging battery, ie flowing from fuse box to battery)

Idle with ac/headlights on/rear defrost:
charge wire 50-65 amps / main fuse panel wire + 0-2mps discharging (the alternator isn't keeping up)
ultimate dies in short order, the voltage ticks down until about 12.5 or so at the battery maybe 12.2 dead

---Swap back to original alternator---

Idle with no accessories on:
charge wire 15-20 amps / main fuse panel wire -8amps ( - means charging battery, ie flowing from fuse box to battery)

Idle with ac/headlights on:
charge wire 70 amps / main fuse panel wire -8amps ( - means charging battery, ie flowing from fuse box to battery)

Idle with ac/headlights on/rear defrost/radio on turned up:
charge wire 80-82 amps / main fuse panel wire + 0-1amps discharging (the alternator isn't keeping up)
ultimate dies in short order, the voltage ticks down until about 12.5 or so at the battery maybe 12.2 dead

What I noticed today. The pulley size difference was actually opposite of what i thought and a slightly larger difference than I originally thought. The replacement stock GT alternator came with a larger pulley. I originally assumed the larger pulley would make it spin faster....Duh it was opposite. The pulley on the replacement alternator is approximately Ø2.5" and the stock cobra pulley is Ø2.125". The 3/8" in diameter difference accounts for about a 15% difference in rpm. So what I am experiencing is a new alternator that is spinning about 15% slower than the original.

Just doing the basically math 15% of 65 amp ~ 10 amps. Adding 10 amps to the new alternators max amp output 65+10=75 amps. This still doesn't look like I have enough charge amperage on idle? Judging from the test with the original alternator I need at least 85-88 amps on idle running all accessories. I feel like I have been duped with the replacement alternator. It was one of the first things I replaced and I think the replacement is a pile of junk.

Next step....Try to swap the original pulley to the new alternator. If it still doesn't put out the amperage of the original I want to melt down in a forge. I also ordered a new replacement alternator. In the world of :poo:ty parts, why does it always have to be the one you least expect???
 
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Jul 2, 2019
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Longest story ever....Told in short hand format.

It was always the alternator. D'OH....The original went bad and the replacement was a marginally functional piece of crap. It would not maintain voltage and caused all the other systems to function improperly. It figures that the original culprit was one of the first items I replaced. I did not realize this until purchased a Fluke clamp on DC amp meter. This lead the way to the solution. Replaced the alternator with another replacement version and all is well again!!!

:)
 
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Jul 2, 2019
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And the trauma continues....She got towed today after driving through a little rain.........I have a moisture related short. The weird part is it reacts identical to the way it did before the alternator swap. Thinking about a new regulator plug end and maybe a new AC clutch. Any ideas on what would short bad enough to cause charging issues when wet? I am going to spot check in the driveway by target spraying different components with water.

:confused:
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
Start it up, monitor voltage while turning stuff on, ac, headlights and windshield wipers, stereo, note volts for each addition until you have everything on.