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Discussion Starter · #1 ·
I have a 2008 FJ with 117k miles, and the AC light started blinking 2 weeks ago. I changed the relay first, with no luck. Since then I’ve checked pressures with manifold gauges, but can’t get a stabilized reading because the clutch won’t stay engaged for more 5-10 seconds. Any ideas what to do next? I don’t think it’s a leak/low refrigerant because it’s ice cold for those 5-10 seconds it does engage and work!
Thanks everyone!
 

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The AC compressor has an integral tachometer ("lock sensor") that measures compressor shaft speed. If that value doesn't correlate with the engine crankshaft speed, the AC Amplifier circuit disables the compressor's magnetic clutch to prevent a seized AC compressor (or a badly slipping belt) from destroying the serpentine belt and disabling the engine.

If you've already replaced the AC relay, it's possible that a slipping belt is the cause of your problem, as that will cause a mismatch between the compressor speed and engine crankshaft speed.

Check the condition of your serpentine belt and belt tensioner. A badly worn and glazed belt may allow slight slippage, and a bad belt tensioner seems to be common after ~150K miles.


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Sparky sounds correct. Compressor kicks on starts pumping it down , low pressure kicks it off. Small can of gas R-134 is cheap and will keep it running for the time until it leaks down again while you look for the leak and repair. This should work as long as the system has never went negative.
If the static refrigerant pressure in the system is so low that the AC pressure switch is open, the compressor clutch will never be allowed to engage.

Note that the refrigerant pressure switch has both low pressure and high pressure cutoff points to protect from both excessively low and excessively high pressure.
 

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Static pressure can be sufficient for pressure switch to be made, then when the compressor is called for it comes on, the metering device is what will cause the pressure differential.
No, that's not the way the FJ's AC system works.

The pressure switch is located on the high side of the compressor, so if the refrigerant static pressure is too low and the switch is open, the compressor's magnetic clutch is totally prevented from engaging, even for an instant.

The system will not allow the compressor to run until enough refrigerant is added to allow the switch to stay closed when the compressor is not running.
The metering device doesn't have any effect at this point because it cannot generate any pressure differential if the compressor is never allowed to turn on.

The pressure at the pressure switch can only increase when the compressor is allowed to run.

The Toyota pressure switch is a little unusual in that it actually has two sensing mechanisms, one that detects system pressure that is too low, and one that detects when system pressure is too high. In both cases an open pressure switch prevents the compressor from turning on.

Later Toyota AC systems used an actual pressure sensor that provided a DC output voltage proportional to system pressure. That allowed the AC Amplifier to provide a diagnostic code output that indicated specifically if system pressure was too low or too high.

From the factory service manual:

Font Number Screenshot


Switch opens at pressures below 28 SPI and above 455 PSI

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With your assistance and knowledge of this system we might as well just roll the window down and live without it.
What does that mean?
If it was in response to FJTests inputs, he's by far the most knowledgable helper on this website, and does more research to make sure his answers are spot on as anyone I've ever encountered. If the delivery seems harsh or direct its just because facts can be that way.

Norm
 

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With your assistance and knowledge of this system we might as well just roll the window down and live without it.
Or, after gaining an understanding of how the system actually works, one could verify that the pressure switch was open, add sufficient refrigerant to allow the compressor to turn on and build some high-side pressure, find where the refrigerant was leaking from and fix it, fully recharge the system, and enjoy ice cold air conditioning for another 14 years.
 

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Glad you agree with my first post.
Nope ...

Relative to the OP's problem, it's not low refrigerant pressure, because if the pressure switch was open, the compressor would be prevented from starting.

OP states that the compressor starts and runs for a few seconds, then drops out.
If the compressor starts, the AC relay is good.
If the compressor starts, the AC pressure switch is closed.

The remaining probable causes of the compressor dropping out after several seconds are:
1. The AC amplifier has detected a mismatch between compressor speed and engine speed, and if that mismatch persists for more than a few seconds, the amplifier disengages the compressor clutch.
2. There is some damage to the wiring harness (CAN bus, etc.) between compressor tach and AC amplifier so the amplifier is not receiving the compressor's tach signal, or receiving the engine speed signal from the engine ECM.

Let's see what happens when the OP verifies there is no damage to the compressor wiring harness and replaces the belt and/or the belt tensioner ... hopefully the OP will update this thread when he gets it fixed.
 

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If the pressures are just low enough to open the switch when system is activated there is a problem. When the system is dormant the pressures inside the system will equalize and can rise and close the switch. “but can’t get a stabilized reading because the clutch won’t stay engaged for more 5-10 seconds.” You activate the system, compressor starts pumping, because switch is closed, because pressure has equalized, pressures start to drop, switch opens.
 

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If the pressures are just low enough to open the switch when system is activated there is a problem. When the system is dormant the pressures inside the system will equalize and can rise and close the switch. “but can’t get a stabilized reading because the clutch won’t stay engaged for more 5-10 seconds.” You activate the system, compressor starts pumping, because switch is closed, because pressure has equalized, pressures start to drop, switch opens.
Nope.

The pressure switch is on the OUTLET (high side) of the compressor. The pressure on the high side of the compressor can only INCREASE from whatever the static pressure is as soon as the compressor starts pumping.

If the pressure switch was on the INLET (low side) of the compressor, it would be possible for the static pressure to be high enough to allow the compressor to start, but within a few seconds as the compressor started to pump the pressure would drop enough to allow the switch to open and then the mag clutch would be commanded to disengage. This is the situation you describe in post #13.

The scenario you describe in post #13 would be true IF the pressure switch was on the INLET side of the compressor, but that's not where it is ... go look at your FJ, the pressure switch is mounted on the condenser outlet plumbing, obviously the HIGH SIDE of the system.
 

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Discussion Starter · #16 ·
First off, I'd like to thank everyone for giving me some input here! At this point, I'll check all of the above. I have not replaced the belt yet, but I know most people seem to think that's the most "unusual" fix for any of these symptoms. I thought about arcing the mag clt relay to force the compressor on, but I also don't want to damage it. Forcing it on would allow me to also get stabilized pressure readings other than just static readings. Thoughts?

I have however inspected all of the wiring and have not found an issue with that yet. I can feel that the ground wire is also secured atop the compressor. Every single time I turn the AC button on and off to reset and restart the system, I can hear the clutch engage and the air get cooler. But then of course, it flashes every time shortly after.
 

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Also, are the low and high pressure switches the same plug that is near the grill (near the expansion valve)? I tried jumping those but not sure I did it correctly.
Yes, later models have a single pressure switch with two sets of contacts inside. Does your system actually have two separate pressure switches?
That's not the expansion valve, that's just an aluminum block that provides a mounting point for the high-side test port and the pressure switch. The expansion valve (or expansion orifice) is located at the inlet to the evaporator.
 

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Discussion Starter · #20 ·
Yes, later models have a single pressure switch with two sets of contacts inside. Does your system actually have two separate pressure switches?
That's not the expansion valve, that's just an aluminum block that provides a mounting point for the high-side test port and the pressure switch. The expansion valve (or expansion orifice) is located at the inlet to the evaporator.
No it has the one switch on that block and that's it.
 
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