AC compressor will not shut off

I've got at 2007 Toyota Tundra limited. Having trouble with the A/C system. After driving for about 45 minutes, the air conditioning blows very weak. When I open up the hood, the low pressure line is completely iced over(1/4" of ice all around it). I've had the expansion valve and the evaporator core replaced($1300). I've had the system serviced twice. and I've continued to have the same problem. I have no blinking AC light. After doing some testing today, i've learned that the AC compressor clutch will not disengage. It is not stuck because I can disconnect the compressor from power and the pulley will spin freely while the clutch sits idle. I can provide power to the compressor and the clutch will re-engage. So the problem is that the compressor is constantly operating, whether the A/C is turned on or off. It's giving me worse gas mileage and I can't imagine it is good for the life of the compressor. Something is failing to disconnect the power from the compressor. I thought it might be a low pressure switch somewhere that is failed, but 07' Tundras don't seem to have a low pressure switch. Any help is appreciated.

 

Well, you have a little diagnosing to do!

AC magnetic clutch relay is controlled by pin 2 on the relay being pulled to ground by the AC amplifier. I would guess if you pull the relay that the compressor will disengage, but double check that!

Next with the relay out, use a test light hooked up to +12V and probe pin 2 on the relay socket to check that the relay is being commanded to be on constantly (obviously you'd want the "AC" button to be off on the dash). These tests will eliminate the relay as being stuck closed, which is unlikely but possible.

Assuming the control wire is commanding the AC compressor to be on all the time, the next step would be to pull your dash apart enough to unplug the AC amplifier, then recheck to see if the control wire for the clutch relay is still seeing a ground reference. This may mean turning your key to the second position with no AC amp connected, which may throw some errors that need to be cleared later. If pin 2 is still grounded with the AC amp unplugged, you have a wire that has rubbed through somewhere. The wiring diagram lists connector AJ3 as being between the AC amp and the relay box, so you'd want to look up where that is and unplug it to split the system in two, then see which half is giving you the problem.

If unplugging the AC amplifier removes the constant ground, that would be enough diagnosing to throw a new AC amplifier at it. Just keep in mind that you need to do the above debugging before getting here!

 

I found the mag clutch relay. That ended up being the issue. Whoever removed it last did severe damage to the relay and it must have failed closed.

 

That is probably the easiest and least expensive possible problem you could have had, that's great news!

 

A day late but I was going to say this definably sounds like an issue with the clutch relay. The AC amp should send a signal to open/close the relay. This then connectors or disconnects the clutch.