Hello , im having a problem with my volts they dont seem to go higher than 12 volts, the truck sat for a week and it was dead, charged batterys and had them tested they were fine, when I turn the key on and wait for the light to go out the volts drop down to 8 volts , it sound like my heater grid is coming on, it has been in the 90's here. im stuck. help !

 

Unplug the relay for the grid heaters and see what that does.

 

Either your testing is wrong on the batteries , or maybe a bad connection .
You should have your location on your signature , you find someone is in your area .
How were the batteries tested ?
The should not be coming on at that temp , thats a sign of low volts , electrical things not working right .

 

where is the relay for grid heaters, I had the batterys checked at pep boys, could the grid heater be bad.

 

Not sure where they are on that yr , but most are on the drivers side near battery , almost look like ford starter relay/solenoids .
I would get a 2nd opinion or test on the batteries .
Again put your location up , maybe nearby .

 

how do i put my location up im on long island ny

 

On the top of every page is " User CP " , click on that , put your location in box .
That would be a long way to go .
Did you find the relays for grid heater ?

 

I dont know it said fuel diesel heater, and there was only one, I unpluged it and still did the same thing.

 

Fuel heater is not the grid heaters .
There are 2 grid heaters in the intake manifold , a couple large wires on the side of the manifold , follow them to relays .

 

The grid heater relays are behind and a little to the rear of the driver's side battery. Just follow the large wires from the intake to them.
You only need to unplug one small wire from each relay to disable them.