i replaced heater core ,now truck wont crank
Let's preface this response with this statement: As stated in my Profile, I'm a Parts Professional, NOT a Tech. I have some thoughts and will offer them here. I'm just offering some thoughts so if they don't meet your satisfaction, please don't hammer my chops for not being perfect.
The heater core, battery and starter are all on the same side of the truck. My first guess is that you have knocked a wire loose while changing the heater core. My second thought is that there was a ground wire going to a bolt on the heater core housing that may have reconnected in the wrong place. Most heater core housings are fiberglass which will not conduct electricity. Double check to see that, if there was a ground wire going to a bolt, it was placed back on the proper bolt.
There are a couple of tests you can run to see if the problem was caused by replacing the heater core or just a coincidence.
Test the battery to make sure it has 12 volts. If it doesn't, charge it.
Take a test light down to the starter to see if it is getting voltage when the key is turned to the Start position. If it doesn't, keep testing.
Your starter solenoid has 4 terminals. With the key on in the Run position, take a screw driver and jump from the large threaded stud that has the Battery Cable on it from the Battery to the little threaded stud closest to it. You'll have to pull the little wire back a bit to expose the stud. The starter should crank if the Battery has proper voltage and there are no problems with the wiring. If the engine cranks, the solenoid is bad.
If that doesn't work, take a jumper cable and connect one end to the + side of the Battery and just "strike" the + side of the starter to see if it turns. IF it doesn't, the starter is bad.
William, that is about all I can think of. You can run the Tests first if you want or look for a wiring problem first. Either is up to you. I'm sure, within 4-5 hours or less, you can figure this out yourself.