Burnt Capacitors On A9l Board

Discussion in 'Fox 5.0 Mustang Tech' started by 90stang5speed, Jul 17, 2013.


  1. 90stang5speed

    90stang5speed Member

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    there are three 47uf 16v capacitors on the circuit board. 2 of them each have 1 leg burnt off. im not an electronics guru by no means so for those guys here that are here are some questions/background. Ive been having a low rpm miss for a while and couldnt pull codes. I checked pin 40, 46, & 60. they all had 0.2 ohms. I decided to move on thinking the A9L was ok. then things went nuts. what i mean is the eng started running so rich it had black smoke like a diesel from the exhaust and I actually had to turn the fuel pump off to get it to run sitting in the drive way. I sawped out the A9L ECM with the original A9P and it ran but the miss is still there. Found i bent all the exhaust valves so thats another issue. Are the caps a common failure with the A9L? if so, were is a good place to find these caps? is there any diference between an A9L & A9P wiring harness that may have been what fried the A9L or are they interchangable? If i replace the capacitors is there a way to function check the eec since the car is going to be down for a while?
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  2. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    Electrolytic caps are prone to failure over long periods of time. A voltage spike or just the passage of time can cause the problem with the caps you are seeing.

    The main computer wiring harness is the same, the O2 sensor harnesses are different. Basically you use a auto trans harness with an auto trans, and a 5 Speed harness with a 5 Speed trans. In a 5 speed car an AL9 or an A9P will work with no problems, I have done it.

    You can use higher voltage (25v or 35 volt) caps but the uf rating has to stay the same.

    WWW.digikey.com, best prices and good delivery. Use the caps with highest temp and longest life specs.

    The computer will give an 11 on the power up test if all is working properly. Look for codes, 15, 19 when you dump the codes. Code 15 will pop up if you have a custom chip on the computer, so disconnect the battery, remove the custom chip, power back up and dump the codes. If' its gone the computer is OK.
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  3. 90stang5speed

    90stang5speed Member

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    thanks for the info on the caps. just so we are clear about harnesses and what i did. i took the engine harness from my red 90 fox that was a factory auto and put it in the black car that is a factory 5 speed the O2 harness is the factor 5 speed car plugged into the factory auto harness. I cut the power wire to the heaters and hooked that up to a seperate power suply so that when I have the power turned on to the eec for tuning purposes im not heating up the O2 sensors. what is diferent about the O2 harness that might cuse a problem switching from an a9p to a9l if any
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  4. jrichker

    jrichker StangNet's favorite TOOL SN Certified Technician Founding Member

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    O2 Sensor harness interchange and modification

    Originally Posted by 302EFI


    Revised 16-Oct-2011 to add O2 sensor harness warnings
    The wires for the 02's and low oil did not change throughout the years, they are all in the same place.
    The main ones you need to worry about are (on the harness end (ECU) that plugs into the 02 plug) is:
    \- 1. Lightblue / yellow
    - 2. White / Purple
    - 3. Purple / Yellow
    The White/Purple & Purple/Yellow gets looped for a automatic ECU
    The Purple/Yellow & Lightblue/Yellow for a manual ECU

    See http://forums.corral.net/forums/gen...manual-auto-differences-year-differences.html for more O2 sensor wiring harness info

    Basic premise to use with transmission swaps:
    Only run a 5 speed trans O2 harness with an A9L. Do not run an Auto O2 sensor harness with an A9L. Doing so will damage the computer’s internal signal ground.
    Only run an Auto trans O2 sensor harness with an A9P in a car that has an Auto trans. Using a 5 speed trans O2 sensor harness with an Auto trans will cause no crank problems.
    See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix to the computer internal signal ground.

    90 model year harness only works with 90 model cars without inspection/rework.
    The 4 cylinder O2 harness uses 4 wire O2 sensors. It probably won’t work correctly without modifying it.
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  5. 90stang5speed

    90stang5speed Member

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    thanks again for the info
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