Compressor blows fuse and stays on all the time

Tiny
WILLIAM RANKEL
  • MEMBER
  • 2001 TOYOTA RAV4
  • 2.0L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 348,434 MILES
Okay, so I have been working on this issues on and off for a little over six months. The AC compressor blows the fuse on the 10 amp gauge Fuse and now the compressor remains despite the blower and AC set to off on the controls. Condenser fan also cycles on and off with the compressor remaining on all the time. (At least the condenser fan is doing what is supposed to do)

I replaced the compressor twice now (first with a junk yard one then with a new one to rule the compressor out). Wiring looks okay in the engine compartment for the harness although it is old. I have also replaced the entire heater control dash assembly which ruled out any of the switch controls including the AC button. I have ruled out the AC Amplifier as well.

I am at a loss for words. This is the worst electrical gremlin I have ever seen in a vehicle. I refuse to give though! FYI, the Toyota service manual and aftermarket manuals do not have a troubleshooting section for when the compressor never turns off.

I doubt this would be a pressure control switch issue? Could it be a pressure switch? I have never heard of a failing switch causing the compress to remain on. I have not check the pressure switch yet. The only other thing I can think of is a hypothetical relay issues but I do not think there is an actual relay for the compressor itself. I only see relays for the heater and fans unless the heater relay somehow controls the compressor as well. Even so the relays looked fine and I have swapped them with spars in the past to test without any change.

could it also be ECM? Or a grounding issues? Or a relay issue? What the heck am I missing?
Saturday, September 23rd, 2017 AT 3:53 PM

3 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,443 POSTS
It sounds like the thermistor may be bad. The compressor is triggered on and runs until it is either turned off manually or the thermistor detects that the air is cold enough to freeze the evap core. The AC amplifier turns off the compressor in response to the thermistor resistance.

The fuse blowing could be a few things. It powers the heater relay coil and then goes to the blower switch. It also powers all of the fan relay coils for the cooling and condenser.
When does the fuse blow? As soon as you replace it or only if you turn on the blower?
Or? If it is not when you turn on the blower you could pull the relays one at a time with an ammeter or current loop in place of the fuse. Then just watch for the high current as you cycle controls.

Try some testing on the relays and the thermistor. The tables for testing are below. The manual you have should cover removal.
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Tuesday, April 13th, 2021 AT 10:23 AM
Tiny
WILLIAM RANKEL
  • MEMBER
  • 17 POSTS
The fuse blows whenever it wants to blow. Sometimes right upon turning it on. Other times it will not blow the fuse for an entire day. But remember the compressor is now running all of the time regardless of whether the blower controls are on or off completely. Those controls were replaced so I have ruled them out. I have not tried the thermistor, but would that keep the compressor on regardless of the controls showing the AC and blower is set to off?
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Tuesday, April 13th, 2021 AT 10:23 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,443 POSTS
It could. The AC Amplifier is what actually controls the compressor through the junction box. Easy test would be to simply disconnect the thermistor when the clutch is locked on. It's also possible that passenger side junction block has an internal short, unplugging the thermistor, and the AC amp would isolate that if it's the issue.
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Tuesday, April 13th, 2021 AT 10:23 AM

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