I have a 91b2600l 4x4 automatic transmission. Seems to be in limp mode. I shift it manually. It has second, first,and reverse. No derive or overdrive. Overdrive lock doesn't work . Occasionally it will work fine,but if I turn the key switch off it goes back to that mode. The hold light in the dash doesn't light up if it is in this mode.. Is it possible to scan thi ecm on this truck?