I am having the following problems with my 1997 Jimmy 4.3L:

First problem: When I turn the key to the "ON" position, the diagnostic lights (brake, battery, air bag, ect.) do not come on right away. After a couple of minutes they come on and I can start the engine. This is an intermittent problem.
Second problem: While driving, whenever the Tachometer goes over 2000 rpm, the needle on the "VOLTS" drops into the red zone and the battery light comes on. When the Tach drops below 2000 rpm. the light goes out and the needle rises.
Third problem: After the engine warms up, the Transmission does not shift into Overdrive.

I just put a code reader on and the only code it gave was P0785.
I don't know if these problems are related, but they all started happening at the same time.

 

You sure it was p0785 and not p0755?

Problems 1 and 2 seem to be related. Check your drive belt, and belt adjuster. Other than that start to suspect an alternator.

 

Check your voltage at the alternator while you vary the engine speed. If it is really dropping the voltage like that, take it off and have autozone, etc. test if for you for free. Sounds like the alternator might have a bad voltage regulator?

That first problem is strange! So will the engine not crank at all until the lights finally come on? You might want to look at the ignition switch, but I really don't have a clue.