l have a 94 gmc 1500 clubcab 4x4 6.5 turbo, k here the problem if l drive for about 40 min engine sevice light will come on but if l put it N it goes off put it back in D it goes back on and if the truck sits for a couple hours good for another 40mins k l have tried to find errors but the computer reports no errors!

What are you using to pull the codes from the computer? are you blink coding it or using a scaner? Does the truck act up when the light comes on shift weird, miss shifts, Skip shifts, torque convertor lockup...?

Do the diagnostic checklist so you can help us help you. I would also update your signature and include all your mods!

no mods and it was but on a scanner at a garage and i waws not there so l don't know what they used but l due notice amp gauge dropps just a bit before this problem starts

Does the Tach quit when the problem starts?

no tac is fine and it only does it if you drive around for about 40 mins could the altanter be getting tired ?

Do the paperclip test... see OBD-I computers in FAQ's... once you've gotten the codes by the blinking service engine light, let us know what comes back. If only code 12, let us know.

Any other strange things going on when SES light comes on? Did you check your battery cables and connections? Known troublespot.

The reason I asked if the tach worked is because I know on the 6.2's the tach signal is drawn from the Alt. and I belive that the 6.5 is the same way. normally if an alternator fails it will show a large drop in the volt gauge. Keep us posted with any qustions and any progress.

The reason I asked if the tach worked is because I know on the 6.2's the tach signal is drawn from the Alt. and I belive that the 6.5 is the same way. normally if an alternator fails it will show a large drop in the volt gauge. Keep us posted with any qustions and any progress.

Same for 6.5, Tach comes from Alternator. Test it at any local part store, should be free. Also check ALL your battery cables, positive and negative.