SES light oddity

Hi guys!

Just finishing up an LM7 swap into a 92 Jeep Comanche which is slated to be raced in the King of the Hammers offroad competition this coming February.

Anyways, the engine starts and runs just fine but we've encountered a bit of a strange issue... When I turn the ignition off, the check engine light turns on and remains on for approx 30 - 40 seconds then just turns off. My scan tool is unable to connect to the PCM while the light is on and the ignition is off, so the PCM is powering down when it is supposed to. I've triple checked my power and ground feeds, everything is good. The PCM has no codes either active or in history either.

Any ideas why the SES light would be coming on after shutting the engine down?