There is a 3 way pressure switch which has to have enough pressure at the switch for the sytem to operate. Too high or too low system pressure opens the switch, and the PCM won't turn on the AC.

 

thx for the quick reply. I'm in London now so will check that later.
Hope it's that and not the ecm.

 

If you have a schematic for the system you can do this techique I use to trouble shoot electrical problems before grabbing a wrench. Get 4 highlighters and designate them accordingly.
Red = constant power (+)
Yellow = controlled power (+)
Blue = controlled ground (-)
Green = constant ground (-)

Highlight in the following order:
Red, Green, Yellow, Blue...

With this method you can see a schematic in a whole new perspective. You know what component recieves what kind of power weather it be (+) or (-) and you know when it should or should not get the power in the case of relays and switches etc. Now you know what you are looking for when you use a test light or digital multimeter.

 

thanks milkbubble.
Interesting tip and a neat way to visualise sources and sinks on a circuit.
(as long as you can colour well and keep your tongue in your mouth :-)

I'm ok with electronics it's the ac gas that worries me. Just wish I had a simple way to test and repressure myself without the Honda washing machine.

 

I don't know about the UK, but here in the US you can buy a 4 or 5-valve manifold with 2 gauges/hoses for low and high pressure connections (before and after compressor). Plus a common third hose (no gauge) for the refrigerant can. When you add refrigerant, it goes from the can to the LOW side, BTW. They are not uncommon at good auto parts stores, serious hardware stores, etc.

And yes, you might have to jump out the hi/low pressure switch to get the compressor engaged so that you can take the pressure readings.

 

I can not stress how much of a bad idea it is to try and charge a AC system without a the proper equipment and training. You need to understand how the system works before even THINKING you can work on it.

It's also illegal to charge a known leaking system, and not to recover R134A.

A manifold gauge set isn't going to get the job done unless you also have a vacuum pump that can pull 30 in Hg for at least 30 minutes.

Charging a system with moisture or air inside is asking for even more problems. With a properly working system there is no need to jump the pressurte switch to take any readings. It's also a 3 way pressure switch, so you better now which wire is which before shorting the switch connector.

Some shops will refuse to work on a AC system that has been charged by the DIY'er. There is too much risk of contaminating a 4K AC recovery recharging station from someone cross contaminating refrigerant.

Without the proper training and equipment Servicing your own AC system is a BAD idea.