My 91 GMC 2500 Sierra with the 4L60E or 4L80E tranny is acting up. what it is doing is the Check engine light comes on and the Torque converter unlocks, then the light goes out and it locks back up. sometimes it does this off and on every few seconds... but mostly when its warm the engine light will stay on and it wont lock up.

Does any one know what this could be,??? and how to tell a 4L60 and 4L80 apart?

And By the way im new to the site.

Thanks.