Fuse #3 blown - diagnosis help please

Ok, I have a 2003 Prius and the fuse #3 (10A, AC) blows when the AC gets turned on (or when the defrost is turned on). The fan works when AC is OFF and does a good job; no noises like a bad bearing or failing fan motor. What else is on that circuit that would cause fuse #3 to blow? Thanks in advance!

 

It's the fuse panel inside the driver side door. 10A fuse, top row, farthest right.
If you go to FuseCheck.com for the NHW11 build,
the "Passenger compartment fuse box diagram", it is fuse #3 (out of 20 in the diagram).
The designation is "HTR" (Heater?) and the description says "Air Conditioning System".
When the fuse blows, there is no power to the fan motor, and all the indicator lights on the climate control panel go out.
Thanks in advance!

 

<bump> found the culprit. The fuse which blows is designated HTR. It is one of the red 10A jobs, in the driver side fuse panel (just inside the door, on the side of the dash), top tow, #3, furthest to the right. It supplies power to the climate control system, the fan, and the AC Clutch Solenoid. Apparently the solenoid developed a short. The AC Clutch Solenoid controls the clutch which couples the engine to the AC compressor.

To disconnect the AC Clutch Solenoid, remove passenger tire, remove one plastic panel, loosen the serpentine belt, and be prepared to wiggle and curse a lot because it ain't obvious how to release the connector catch.

So the AC Clutch Solenoid is disconnected, and the Climate Control panel stays active and the fan is powered and running no matter what setting the Climate Control is on (so at least air is blown across the windshield, even it it isn't dried by the AC, if needed when it rains). No more blown fuses.

The solution is to replace the solenoid; I also found that the AC clutch itself was badly worn, so I am having Toyota replace the clutch and solenoid all in one go. The current estimate is $250 in parts plus whatever labor for changing the clutch and solenoid (getting the clutch assembly off the compressor shaft was more than I cared to wrestle with). Now waiting 3-5 days for the part to arrive at the dealership, then will schedule the replacement service. I'll post the full damage report when it's over.

The procedure is on pages AC38-AC42 of my 2002 Prius Repair Manual RM883U2 (that's volume 2). The manual calls to take the compressor off the vehicle. I observe that the clutch assembly is pretty accessible from the passenger side tire bay while on the car.

 

<conclusion> The fuse-blowing '03 is back from Toyota, and everything appears hunky-dory. Fuse #3 (red, 10A, HTR) is happy. It was indeed the compressor solenoid, and the magnetic clutch assembly was pretty grotty... not unexpected given 20 years in the salty Northeast. The total bill came to about $950, of which $250 was parts and the rest was Toyota labor. The manual calls for removing the AC compressor from the car before doing the mechanical work, which means capturing all the CFC from the AC system, disconnecting the compressor, doing the work, reconnecting the compressor (new seals, too) and purge/refill with CFC. So I won't scowl (too much) at Toyota for the amount of labor and the total labor charge.

Looking at the electrical wiring diagram, it is NOT obvious that the AC magnetic clutch solenoid gets its power from HTR Fuse #3. The electrical diagram shows it gets its power from the AC amplifier module, and the connection between the HTR Fuse #3 and the AC magnetic clutch solenoid is... well, tenuous.

So here's another thing-to-check for keepers of elderly Gen I Prii: periodically inspect the magnetic clutch assembly and clean out any buildups of rust, dirt etc as best you can every couple of years in order to keep bad things from happening to the solenoid. It's about a 90 minute job, recommended for a summer or autumn day, as the procedure entails removing the passenger front tire and the inner fender liners in order to get to that side of the engine where the AC compressor, magnetic clutch, and water pump reside. The adjuster for the drive belt for water pump and AC clutch may need some lubrication in order to get it to move... this is a good time to exercise that adjuster and examine the condition of the drive belt (this car had that drive belt replaced a couple years ago).

 

Please, do scowl for me. Does my soul good to have a scowling buddy...
I have replaced the clutch (on another prius), and that does NOT require removing the AC compressor.

This time, it was the magnetic coil that failed (I was really surprised, too) (plus, the magnetic clutch itself was really grotty - I can post pix if you're REALLY interested) and the solenoid is somewhat deeper in the innards of the clutch mechanism. I spent an hour trying to get the solenoid out in my driveway before I consulted the service manual, and decided that getting it done right was worth the Toyota labor charge. Plus I was required to order the Toyota part.

Looking at the solenoid afterwards, it's apparent that dirt and (probably) salt over the years worked its way into the solenoid area, and finally abraded through the insulation to cause the malfunction. As I said earlier, the fuse blowing problem went away when I finally found and unplugged the solenoid...

I'm a pretty fanatical DIY-er; I have no hesitation to drop the entire exhaust system to patch a leak, or to tear apart the HV battery as needed and replace the battery wiring harness, but this was a task I decided to farm out and pay the coin. I must be getting old...

 

My adventure ended (or I chose to end it) after I tried to remove the pulley/bearing... no room (when compressor on the car) to get a tool to pull that assembly off. If you ever get a chance to repeat my experience, I'd be interested in your reaction, even with a bearing puller.

At which point I decided to let someone else solve the problem. It's one thing when it's a southern car that has never seen a northern winter... it's quite another to deal with rust on a northern car.

Even the manual says to use a hammer... :=) Thanks for the commentary and encouragement.