Does a constant blown fuse always indicate a short?

Question

My wife was driving our 96 Ford Windstar recently and she said right after she hit a bump in the road the ac stopped working and the instrument panel lights went out. I found the fuse from the wiring diagram and replaced it bringing the instrument panel back since it's on that circuit. However, every time the ac is turned on, the fuse blows, otherwise it's fine. I did pull the fan belt and the compressor clutch turns freely, so it's not a mechanical issue.

My question is, does this always mean it is a short or can a bad switch or relay cause this?

Also, what is the best way to track this with a multimeter and ciruit tester or should I invest in another tool for this kind of work?

Aftermath:

I was able to manually trace the circuit and figured out that there was a cable bundle leading to the a/c cycling switch where the problem was, based on directions from Ben's answer. So I tied it up snug and out of the way using a plastic tie which solved the issue. For now I'm just doing the jerry rigging.

I did pick up a Centech Cable Tracker from Harbor Freight which was rated very high for tracking shorts inside cable bundles, including computer cables, vehicle cables, and phone lines. It was $25.00 USD, so it was worth the small investment.

You do need to rig up something for a fuse socket connection or buy the CT6100 from Amazon so you have something to attach to the fuse via the tester's aligator clips.

Answer 1

If the fuse is blowing as soon as you perform an action it's a short to ground.

Multimeter won't do much good in this situation. It'll come down to wiring diagrams and visual inspection of the harness and connectors on the circuit. Unless you have a fancy tool that will find the short for you.

It looks like fuse 21 powers a bunch of stuff, but since the fuse only blows when you turn the a/c on that narrows it down quite a bit. If you look at an a/c wiring diagram you'll see fuse 21 only powers the blower motors, blend door motor, the constant control relay module and the a/c pressure switches.

Try unplugging the low side a/c pressure switch as it's the easiest to access and see if the fuse still blows. You may want to use a circuit breaker while testing. This will tell you if the problem exists south of the switch as I assume the wiring harness will run along the subframe and up to the CCRM.

If the fuse stops blowing then it's time to follow the harness to the high pressure switch and from there to the CCM and PCM. Check for rub through, corrosion, obvious breaks in the wire etc...