Compression Ignition Addict
Join Date: Dec 2007
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Could be a couple of things,,, If the relay clicks the Key sw & Safety sw are OK. Next you gotta think about current getting to the Starter. That's everything from the Batteries themselves to the Starter itself, and ALL the cables & connections in between.
Here's where I'd start. Turn the headlights ON and have someone turn the key while you watch what happens to the headlights brightness.
Do they start out white and go yellow?
Do they start out yellow and go out?
Do they start out white and change little?
Next you can slide under the truck, with a hammer, and "Rap" on the starter body a few times to reseat the Bendix/Brushes, which could be hung, and an early sign it's failing.
In any case, this is the time of year (when your electrical system has to be 100% as it gets colder in order to start in the cold) you need to go over the battery cables & ends. Scrape clean, grease & tighten them all, Both sides, Both polarities. That may be all that you need now.
The batteries are tested by having them fully charged and Load Tested, individually, at a High Rate (250/300A). The Starter you can take out and have tested at the Parts Store.
Good Luck - Let us know about those headlights...