91 f250 dead battery’s

[503sp01]

I’m thinking I have a draw somewhere can’t find my test light. Both batteries will be dead after a week of sitting ones less than a year one on the other is 3 years old anyplace I should start looking before I go and buy a new test light

 

[IDIBRONCO]

If you have a little time, I would suggest this first. Make sure that your batteries are fully charged. Then unhook them both and let them sit to see if one is bad and discharging itself. A bad battery will drain a good one too.

 

[bbjordan]

Could be a bad diode in the alternator. With the wires disconnected from the alternator, you can test for continuity between the output stud and the case with a Volt-Ohm Meter. One way you should have continuity, the other way you should not. Or just take the alternator in to get tested.

 

[Runningaford]

I could've read your op wrong, but it read to me like your batteries are different ages? If so, that alone I think could be your problem.

 

[jwalterus]

Aftermarket radio? If so, what brand/model?

I got rid of my Alpine, it drew a full 3 amps when the truck was off. (Also required a 10ga wire be run for the unswitched power according to the installation instructions.) But that one was far from a "base model".
I currently have a Pioneer, it draws the batteries down to the point the truck doesn't want to start over the course of 3 weeks, but my batteries are 6 & 7 years old.......