My '88 F250 4x4 Diesel Won't turn over, Makes a "click" noise when key is turned. Amps are showing low (around 9-10) on the interior gauge... Batteries are at full charge. I've replaced the positive & negative leads, the regulator, the solonoid, and the starter. What to do next?

Test the battery with a volt meter because 9-10 volts isn't enough exspecialy with the glow plugs sucking juice. get new batteries. im sure that is the problem

Probably one or both or your batteries are low or shot, check both of them. Hope this helps-Jim If water in them is low, refill with distilled water, and then charge them. If it still doesn't start, you may need a replacement.