Off the truck's engine, the starter is not under load (i.e. the electric motor doesn't have to perform any work, and just spins freely).

On the truck's engine, the started needs to have sufficient power to rotate the engine components that are under compression. That's hard work!

http://www.made-in-china.com/image/2f0j00nYTEoCazuusGM/Komatsu-6D95-6D105-Engine-Starter.jpg

From the source...

"If it goes, *click-click-click-click*, and so on, it may just be a low battery.

A low battery that makes the starter *click* a few times doesn't have enough *juice* in it to turn the starter over to turn the flywheel over to turn the crankshaft over to make the engine start...phew!

A simple way to tell if the battery is at fault is to turn the headlights on and turn the ignition switch to start.

If the lights go out, then the battery is probably at fault.

Take your battery to a place that sells batteries and have it checked.

If it is gone to where old dead batteries go, then you will have to purchase a new one.

You may have other problems...like the alternator; the tension on the alternator belt... several things could make the battery be low...did you leave the parking lights on when you parked, to go shopping?"
Source(s):
http://ezinearticles.com/?So,-Your-Starter-Is-Just-Clicking&id=2833

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