Fried Check engine light
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Fried Check engine light
NEED HELP --- 1989 F150 302 Manual XLT Lariat
I pulled the EEC-IV codes KOEO with voltmeter, worked fine.
Then I went to run the KOER test and wired the STI using a poor diagram that I downloaded off the web. Long story short- I fried the CEL system by connecting the positive battery terminal directly to the STO pin 4 (no resistance from a voltmeter wired in-line). The STI was jumpered to the SIG RTN. The CEL came on and stayed on for a short time, then stopped and now does not come on at all (I checked bulb it is fine). The computer still seems to be running the self-tests as I can hear relays etc clicking and turning on and off, but there is no longer any sweeps of the analog voltmeter (nor any CEL).
DID I FRY SOMETHING, WHAT, AND HOW CAN I FIX IT?? Thanks.....
I pulled the EEC-IV codes KOEO with voltmeter, worked fine.
Then I went to run the KOER test and wired the STI using a poor diagram that I downloaded off the web. Long story short- I fried the CEL system by connecting the positive battery terminal directly to the STO pin 4 (no resistance from a voltmeter wired in-line). The STI was jumpered to the SIG RTN. The CEL came on and stayed on for a short time, then stopped and now does not come on at all (I checked bulb it is fine). The computer still seems to be running the self-tests as I can hear relays etc clicking and turning on and off, but there is no longer any sweeps of the analog voltmeter (nor any CEL).
DID I FRY SOMETHING, WHAT, AND HOW CAN I FIX IT?? Thanks.....
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
You may have fried the computer since the STO is switched to ground to energize the light. With 12V on the line and no resistance from a light bulb, it likely overloaded the circuit and burned that part of the computer board.
Fleet Owner
Joined: Aug 2005
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From: Easton,Ks
If Q10 is just open the computer should work OK.
If Q10 input is shorted to another leg then you would have other problems.
The Ford dealer should be able to check for codes with their test meter.
Image of Q10:
/
If Q10 input is shorted to another leg then you would have other problems.
The Ford dealer should be able to check for codes with their test meter.
Image of Q10:
/
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From: vernon hills IL
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I pulled the ECM, and couldn't find any obvious signs of damage. I tested many of the transistors with a multimeter, and all that I tested were fine. Does anyone know which of the transistors (possibly Q10) on the 1987-91 302ci is the one that grounds the STO for the check engine light so that I can test and possibly replace it?