Thanks for the direction. I have done 1) and 4) and all good. So I have 2) and 3) to do before even beginning to suspect the ECU is faulty . Fingers crossed... Keep you posted.
My method would be:-
1) short the control side of the relay to ground (with the engine running) and confirm that the compressor clutch engages.
2) Test at the relay to see if the ECU is grounding the relay when you push the a/c button.
3) if (2) isn't working, test the wire at pin 13 of the ECU to see if it's being grounded when you push the a/c button.
4) if (3) isn't working, you're now into confirming the inputs to the ECU (pressure and temp sensors, believe you may have already done this?).
Garages have a habit of blaming the ECU when they run out of diagnostic ability. I'm always extremely skeptical when someone tells me that a garage has diagnosed there issue as a failed ECU.
I'm digesting your advice. I believe I understand the principles of the relay. I tested it this morning. Used small 12V battery charger. Clicks when +12V applied to terminal 85 and ground to 87.I'm afraid I'm struggling with your description of what you're seeing. please don't apologise, it's a tricky thing to convey over text only.
In (1) you state shorting the control side of the relay but describe shorting the load side.
In (2) you say the inputs are functional. I presume that we know that the ECU is receiving signals but do we know whether or not the signal (for pressure) is in the range that the ECU requires it to be to activate the AC? Remember in the system description that I linked to it required the pressure sensor to be within a range. Did the garage's computer say it was within that range?
The other points I think we are going to need a picture of where you are taking voltage readings.
However, to answer your actual question...
The electromagnetic clutch is energised to engage it. When it is de-energised it freewheels and when energised it locks the mechanism to allow the belt to drive the AC pump.
From the way you asked the question however I believe that you may have misunderstood how a relay works and what is meant by a 'closed' circuit.
If you remove the relay and start the car, do you have battery voltage at 2 of the relay connectors? If so, which ones? A picture would be great if possible.
Not once taken your comments as criticism. Always supportive. I actually am amazed by the direction (and patience) you have offered. I will take these next steps and keep you updated... There will yet be a celebration when this is all resolved!! Many thanks again....That's much clearer, thank you. Please don't take my comment as negative criticism, It sounds like you're giving it some real effort and conveying this kind of diagnostic route via writing alone is difficult. There are still easy tests you can do on the drive.
If you set your multimeter to continuity/resistance and check between pin 86 and battery negative (whilst turning AC on and off) what happens? This is the terminal that should be earthed by the ECU when the AC is selected on. If this is being earthed with the AC selected then you just have a faulty relay. Just because a relay clicks doesn't mean it is making a connection so people often go down the wrong diagnostic path when this happens.
If that doesn't work, leave one lead on the battery negative terminal and move the other lead to pin 13 of the ECU (the wire we previously talked about) You'll need to 'back probe' the connector or use a wire piercing test lead (costs about a fiver) so that you can connect to the wire with the ECU plugged in. You're looking for the same results as in the previous test. If there is continuity/low resistance when you switch the AC on this proves a broken wire between the relay and the ECU.
Before anyone moans about piercing wires...you also need to go back to where you used your piercing tool and fix the puncture in the insulation.
I missed on the hyperlink. Would have kept me on the right path had I not overlooked that earlier on . Sorry. Now I have my bearings. No continuity when probing that pin/wire. Going to get my mechanic to run through the sequence again to be certain all the conditions are being satisfied before attributing things to a faulty ECU. I'll let you know the outcome and hopefully still provide you with a screen shot of the diagnostics.The pin in your picture is definitely the wrong one. If you have another look at the wiring diagram I linked to it has a hyperlink next to the pin number. If you click on that it takes you to a picture of the connector. Pin 13 should be on the smaller (40 pin) connector.
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