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Ford Expert, Shop Foreman, Master Technician
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Experience:  Ford Master Certified in, Advanced Engine Performance, Advanced Electronics,
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93 mustang: trouble code 15 pcm rom fault..bucking..normal driving

Customer Question

I have a 93 mustang that is giving trouble code 15 pcm rom fault. with the key in off position. Its a new computer in the car but has been bucking very bad under normal driving. the car also has a chip. dont know if computer was bad or maybe chip
Submitted: 8 years ago.
Category: Ford
Expert:  Ford Expert replied 8 years ago.

did you take the chip out and drive it?

What mods?

Is the replacement computer new or used?

Customer: replied 8 years ago.
trick flow heads e cam trick flow intake,full legnth headers,410 gear.the car ran the same with chip out. It was a new A9L computer that I put in. I also changed the fuel pump to a 255lph. the problem is the car ran great until i put away last year. it ran consistent 12.30's at 111mph. then this year car went 12.99 102 to 103mph. after that the car would buck real bad at 2700 in third 4th and 5th gear. when driving normal. the chip was also set up to run without o2 sensors. alot of friends, and mechanics,but im ready to give up, pretty muchlost the whole seaon b/c of this problem
Expert:  Ford Expert replied 8 years ago.

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Sounds like a fun ride.


Read all.


Code 15 indicates that the pcm has experienced a power interrupt in its KAM (Keep Alive Memory). You need to disconnect the pcm and test between pins 1 and 40 then pin 1 and 60, while grasping and wiggling the harness from the pcm to the dash ignition switch area. If you see a power fluctuation then repair the harness. You also need to be sure that NOWHERE is the eec harness close to any ignition component ie: coil, coil wire, secondary ignition wires etc. Some of the time I found no voltage to the KAM circuit and some of the time I replaced the pcm. So make sure to check the voltage. If both computers run the same with the chip out and voltage checks out ok, you may to look at a secondary problem. The bucking @ 2700rpm's indicates to me a secondary ignition breakdown. This is a typical scenario for ignition failure, high gear and low rpm's. This is exactly how I diagnosis ignition concerns today!, when road testing a car. You could have a concern with induction in the way the secondary ignition wires are routed. On the 15426378 firing order make sure #7 & 8 ignitioin wires are NOT paralell they must cross each other or be seperated from each other. Run another wire between them at the cap. More than once has someone brought a car in with buck and a miss with the plugs wires bundles together with a zip tie! A definite no-no. Remember code 15 could only be a loss of voltage to the pcm. Pin (1) should have voltage at all times, and pin(s) 40 & 60 are the ground side. Pin 1 is only the KAM circuit and a lot of guys disconnect the computer and/or battery when going the track to reset the fuel and ignition tables to get quicker et's anyway generatng a code 15. If the code is just a loss of voltage on pin 1 then I don't think this is your concern for running bad. If the voltage is good then I believe the pcm could be the cause of concern, because it indicates an internal problem with the pcm which may be affecting more than one circuit. Good luck Happy Thanksgiving. I will be here to answer any further concerns you may have. I race as well autocross, 02 GT 5spd bassini full exhaust crazy horse performance tune, sucks when it is not running right. This problem can be fixed just may take some time.

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Customer: replied 8 years ago.
I appreciete all the info. just to add one thing.code 15 is reading with scanner on koeo. when we switch it to koer it says communication loss. also when i changed battery it seems like it would create a 15. happy thanksgiving and im very thankful for your help
Expert:  Ford Expert replied 8 years ago.

With the code in koeo the pinpoint test says to replace the pcm without doing any testing...

Thank you for the accept.