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2004 Sienna AC Only discharging Ambient Temp Air

2004 Sienna XLE LTD with ~108K miles. Coming out of a cold snap into warm weather in Austin,TX and no cold air being produced by the ac. Unable to see if the compressor clutch is cycling in and out due to location below alternator.I can spin the outside of the compressor with the engine off I have been told the low pressure limit switch could keep the compressor from engaging if the system is low on refrigerant or the switch itself is going out. In looking under the hood and on the left side of the compartment I found this switch(?).IS this the low pressure switch I have been reading about.Some say the the switch only has two terminals while others say four.This one has four. Also,I read that a possible problem on older /higher mileage vans is a MC/MCG relay located inside the fuse block inside the engine compartment.It can be tested by removing the relay for the horn and putting it in the slot for the ac clutch.Is this correct?

Any replies are appreciated and thanks in advance

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The AC system may just be low on refrigerant.

Before you start throwing parts at the problem, take it to a shop and have them connect a set of manifold gauges to determine the state of charge of the system.



Sorry for not mentioning in my initial post but I did try to add refrigerant using AC Pro but was unable to get the system to take any.I guess it is quite possible the low pressure switch has the compressor locked out to the point it will not allow it to cycle due to low refrigerant? As for the clutch relay possibly being bad,I was simply going to temporarily replace it with the relay for the horn to see if the compressor will cycle.I have read the relays are identical and if so then it would be a easy test to do and will cost me nothing but a little time. I was hoping to get clarification they relays are the same type before doing so to make sure I do not cause any electrical issues.

Under what conditions did you attempt to recharge the AC system?


Engine at normal operating temperature,ac set to lowest temperature with fan on high and windows rolled down.Refrigerant connected to low side port

That’s the incorrect procedure when recharging an AC system.

Take it to a shop before you damage the AC system, or injure yourself.



The very first thing you need to is visually verify if the ac compressor is engaging or not

Use a mirror, put the van on jackstands, whatever it takes

What if the ac compressor is not coming on because of rat damage to the wiring, for example?

If you put the van on jackstands, you might see something obvious, such an obviously leaking ac component.

Not hard to see clutch with flashlight. Should hear it cycle in quiet garage.

So I removed the MCG relay and temporarily switched positions with it and the horn relay. Tried the horn and it was dead then started the engine and turned the ac to low.Hope i am not jinxing myself but the ac blows plenty cold air.On my way to the local Oreilly’s or Auto Zone to get a replacement relay shortly after posting this.

Thanks to everyone who offered their advice and thoughts.

as you found out- the relays are the same.

Hopefully you didn’t damage your a/c system by overcharging it when it didn’t need it.

I don’t think the system took any because the compressor didn’t kick in

Just some food for thought but a failed relay is often a symptom of another problem rather than the actual cause of it.
That could mean something in the circuit has been drawing an excessive number of amps. I don’t have a schematic for this car but quite often the condenser cooling fan can be dragging and causing this.