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Hello All,

I just wanted to post this here for anyone experiencing the same problems I had with a new 1992 5.0 5spd Mustang LX Hatchback me and my wife just bought to restore.

Issues:

No injector pulse (injectors would not fire), engine would not run except for starting fluid being sprayed in the intake, and fuel pump would continuously run with key in on position

Test performed:

Fuel pressure tested at the fuel rail test port and showed around 40 psi. (test confirmed not a fuel delivery issue from the pump.

Swapped fuel pump relay with another relay to confirm relay was good (test confirmed relay was good and not reason for fuel pump run on with key on, pump should always just prime for a few seconds and then shut off til time to start engine)

Injectors all had 12+ volts on the red wire or light on with test light grounded and probe inserted into injector harness. Injectors confirmed to be good(not shorted or clogged) by inserting the test light probe into each injector trigger wire(not the red wire) and seeing the light come on and the injector pulse and spray fuel (could hear fuel spray and also engine would crank and run for a few seconds using this method to fire injectors). These test were performed with the key on and the test light grounded.

cleaned and inspected all grounds; ground behind batter on fender, ground on firewall to back of head driver's side, and ground for injector harness on passenger side and I bolted the ground to the passenger side head but I believe it goes to a bolt on the lower intake manifold. Also the ground below the computer behind the passenger side kick panel was good. All grounds came back good.

performed ohm test on PIN 46, 40, and 20 with everything reading less than 1 ohms.

performed voltage test on PIN 1, 37, 57 all showed 12+ volts.

TFI and PIP test showed good by spraying starting fluid into the intake and car would crank and run for a few seconds. Also rotated Distributor with key on and could hear the coil firing.

check for codes and came back with a code 34 so that told me that the computer is communicating.

ECU relay tested with ECU pin tests stated earlier. key on.

TPS VREF (orange wire) tested and only showed 1.6 volts. (should be 5 volts). Test was performed using the TPS ground wire and also chassis or battery ground for confirmation. Test were performed with key on.

* Checked PIN 26 on the ECU with the ECU wiring harness connected and key on. checked PIN 26 coming out of the back of the harness with everything connected and results showed 1.6 volts and should be 5 volts. *

I unplugged TPS, replaced TPS with a spare I had, and unplugged other sensors to eliminate any suspect wire problems. Nothing changed with same issues.

Took computer apart and two capacitors were leaking so installed new capacitors and re installed computer. Nothing changed and car would still not run with fuel pump running continuously with key on. Installed higher voltage capacitors so thought maybe that was the issues so changed to lower voltage capacitors with the same microfarad ratings (2x47uF and 1x10 uF). Still the same results. No traces inside the computer were burnt and the computer looked good other than the capacitors leaking. I even checked the SIGRTN trace and it looked good with no burnt spots. Also confirmed with PIN 26 and PIN 46 ohm test that showed 10,000+ ohms(ECU disconnected). Even to confirm I ran a test lead to ground each point and no change. Check engine light was also noticeably dimmer than other warning lights like AIR BAG light next to it.

PIN 26 out of the ECU was still 1.6 volts after capacitor change so that led me to believe that the ECU was bad.

I borrowed an A9P computer from a friend, installed, and car fired up with correct fuel prime up of pump and the check engine light was brighter than with the bad ECU.

Diagnosis tested/confirmed/solved:

ECU was bad.



* Great website that gave me all the tools I needed to test, confirm, and determine was the issue with my fox body was *

It was nice to have a friend with a spare ECU to test but the result would have still been the same with the procedures provided in the forums. Like I read in one of the posts, get a multimeter and use the test procedures to solve the problem.

Thankful for this site!

Does anyone know how to repair the ECU for a PIN26 less than 5 volts issue?
 
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