Alright, we have gone around and around with trying to figure out what is wrong with this truck..

Its a 1990 Dodge Dakota 4cyl.
just bought a new alternator and battery.

The truck keeps throwing a check engine light.
sometimes we can crank the truck up, check engine light does not come on and battery is charging.....alternator in charging it.

we can go anywhere from a mile to several miles down the road, the check engine light will come on and boom battery charge is low..

We went an got it tested, when the check engine light is on, the alternator is running straight off the battery, when the check engine light is off it runs fine....

any advise??

 

Check your grounds.

 

Plus 1

 

Sounds like you got a wiring problem. Just get out a multimeter and start testing stuff.