Cannot read ECU trouble codes

1992 MAZDA B2600
80,000 MILES • 2.6L • 4 CYL • 4WD • MANUAL
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SCHMIDTROD
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This vehicle (listed above B2600i) has the 6-pin green MECS and 1-pin green TEL connector under the hood. Started, but quit immediately. Auto shop gave up. Opened the ECU and found the usual leaking electrolytic cap and a little PCB damage. I replaced the Mitsubishi ECU with a same OEM number rebuilt unit. The truck is now running but the check engine light is still on. Unable to pull codes. Grounded the TEL lead, put LED with10K resistor between B+(pin 2) and FEL (pin 1) of the MECS connector. Stays lit full time - no codes. Using a voltmeter to battery ground,
grounding TEL shifts B+ from +12 to 0, and FEL from 0 to -12 (minus 12!) relative to battery ground. I'm an EE by profession, but this seems magical.
Jun 12, 2022 at 1:56 PM
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JACOBANDNICKOLAS
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Hi,

You should be able to retrieve codes via the check engine light (malfunction indicator lamp MIL) by grounding the single-pin connector. Try this. First, allow the engine to warm up and then turn the ignition off.

Ground the single pin connector and turn the ignition key to the run position (engine off).

Watch the MIL to see if it stays off, stays on, or if it flashes.

Let me know.

Joe

See pic below.

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Jun 12, 2022 at 9:53 PM