UPDATED:
Hi Boony,
You will have to bear with me - I ran into some health issues and I can't type as fast as I used to.
First, when you lock the car with the remote, do all 4 of the turn signal / hazard lamps flash? The BCM uses the same circuit as the hazard switch to control them.
If the BCM can control the lamps, then I am betting on the 2 cm-wide ribbon cable between the AC control board and the AC amplifier board. I know you've had to be in there. When I first started working on the AC head units on these cars, it gave me some fits. You need to do as the manual states - remove the window, release the socket clamps, re-seat the ribbon at both ends, close the clamps and install the window with a bit of RTV (so you can get it off next time). Trying to push the ribbon cable in without removing the window and fully releasing / engaging the clamps won't always work. It could also be that the A10 connector from the harness to the AC amplifier board is not fully seated, but I think you would also have other problems. You need to re-seat it when you take out the AC head unit anyway.
If it is not those, then check the turn / hazard flasher control wire to make sure that transistor is not blown. The flasher has separate controls for the turn signals and the 4-ways. When you have the head unit out, I THINK you should be able to check this on the A10 connector, pin 8 red wire, The wiring diagram doesn't show whether you ground the wire or apply voltage. I need to go to my shop and get one of my old boards to check. I will let you know, but it may take me a couple of days.

By the way, note to all - if that ribbon cable is not seated correctly, it can give symptoms similar to the broken temperature control ribbon cable - the blend door motor will hunt.

Boony,
Did you get the AC going or are you suffering through the heat with the window open?

 

@CR - You're worth your weight in a gold coloured Highlander. It was the 2 cm ribbon cable between the AC control board and the AC amplifier board. It was also a good tip to confirm whether all 4 of the turn signal / hazard lamps flashed at the same time (they did). A rather obvious starting point which I didn't consider.

On removing the ribbon I found a couple of wires had been crushed to the side and the plastic tape is coming apart (due to age I guess). I've smoothed everything but will need to get a replacement ribbon in due course. As to why the failure occurred when I was fiddling with the wires under the bonnet is a complete mystery. I hadn't been near the head unit for a few weeks.

I still have that problem with my AC switching off under 2000RPM after 10 minutes. Fortunately it's been a very mild summer here. Before Christmas I discovered a small leak from the weld between the condensor and dryer which obviously has been occurring for a long time from the staining. Strangely no mechanic ever mentioned the gas being low. Anyway I will have the condensor/dryer unit replaced along with the pressure switch. That was a suggestion you had as to a possible reason for my AC issue and as the part isn't expensive I ordered one to have ready just in case.

From my very limited experience I don't think the leak could be the reason for my AC issue but is being tackled as part of an elimination process. I think you're right about it being an insufficient current.

 

Hi Boony,
Hmmmm. My recommendations were based on my understanding that an AC specialist had put gauges on it already, and the pressures were good, but it was still having the problem. Low pressure is sufficient cause for this problem, especially if it is an aftermarket compressor. Point is you may not need a new switch. The pressure switch may indeed have been what caused the AC to stop, but it might have been because the pressures actually were too low.
The AC specialist should have checked static pressure, high and low pressure at about 2000 RPM, and compression ratio. (If the AC specialist gives you a quizzical look when you talk about compression ratio, time to find a new specialist). And given that the leaking spot is a known area of weakness in these older Toyotas, I am not sure about the tech's experience.
Since the system will be open, it is a good time to install the pressure switch, if it is OEM. If it is aftermarket, I would check to see how much the AC shop would charge you to install the switch after if the problem still persists after the condenser is replaced. I hope it will be a good deal since they misdiagnosed / missed the leak. There is just so much junk in the aftermarket right now that it may be a reduction in reliability to install a non-oem.

I find in my area that often techs do know how to do the diagnostics, they just don't because it takes too much time. I like what ScannerDanner says - it usually either takes 20 minutes to find the problem, or 4 hours. And most tech's don't want to do 4 hours of diagnostics. I had a friend who was charged over $600 for a P0171/174 diagnosis and a MAF which didn't fix the problem. I spent 10 minutes with it. It was clearly a vacuum lean due to a cracked plastic vacuum fitting, a known failure point on that engine. The shop was not even apologetic about it.

Good luck!

 

Thanks CR. After the AC people I took the car to Toyota twice (they didn't charge me the the second time). On both occasions they advised they put the gauges on and also emptied and fully regassed the system. The first time they believed the issue was the AC amplifier (which I swapped out) and the second time they suggested - not confidentally - that the slip sensor may be the cause. No fault codes came up in their testing.

Toyota did advise there was an intermittant issue appearing with the evaporator temperature sensor. Annoyingly they claimed the whole dash had to come out to replace the part. This isn't correct and I sent them a photo of access (again thank you CR). I later replaced it myself for a few dollars but it didn't resolve the issue anyway. Noted in regard to pressure switches. The one I bought is non-OEM and cost A$25 versus about A$150 for an OEM one.

Despite the "take out the dash" scenario I remain reasonably confident in the tests performed by Toyota and the AC tech guy. It is odd that Toyota didn't find the condensor leak when I did only two months later and wasn't even looking for such an issue. Of course it may have been a more recent leak ...

My thoughts are the following; I may have given some misdirection to Toyota when I told them of the tests I completed. I did specify that my testing was amateur at best and to be taken with a pinch of salt but perhaps they didn't... Also in all the tests performed by various techs and Toyota they may have focused on one part of the system based on the issue and my misdirection and not considered the system as a whole. Added is that this matter has been ongoing for over a year and 3 separate shops have looked into the problem... albeit each one perhaps focusing on just one area though I expect Toyota would check the whole system.

Certainly something is switching the compressor off. I may have to bite the bullet and have a genuine compressor installed as suggested by the AC tech guy (in case the 2 x non-OEM installed just don't cut the requirements of the system).

Otherwise how much does is a plane ticket and holiday in Australia cost? It would certainly be cheaper to get you over here than the approximate $4,000 being proposed by Toyota to replace all suspect parts and labour!

If/when I get to the bottom of the problem I'll certainly post the outcome on the forum for future reference. Thank you very much for all your help. Hope you are getting better.

 

The reference of the actual ribbon cable between the head board and AC amplifier is printed as SUMITOMO-Y AWM 2896 80C VW-1. That reference comes up easy with an internet search but with different numbers of pins...

I presume a pin means each little wire? I counted 18 wires so therefore I need an 18 pin ribbon?

Thank you