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Discussion Starter #21
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.
 

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Discussion Starter #22
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.

Hopefully in the home stretch for diagnosing. I will confess that electrical terminology is not my forte, so before doing anything too destructive to my ECU, I need a bit more help in clarifying my observations to date. Using my voltmeter and discovery to date.....

1. Shorting the control side of the relay (30/87) engages the AC clutch, compressor, fan = nice cold air.

2. All the inputs to the ECU confirmed functional

3. Constant 12v at connection 30 on the relay

4. Relay connections 85 and 86 become a closed circuit when engine running (14V approx.)

5. Circuit 85 and 86 remains closed when AC button pushed.

So - here’s my question. When I push the AC button, all things working as they should, my understanding is that the electromagnet (low current) is energized (+12V) and causes the AC clutch circuit to close. But the circuit is always closed with the car running. Or… Does the electromagnet switch instead activate when the low voltage circuit opens (85/86 becomes an open circuit).

Sorry if my electrical terms and descriptive are a bit lame - hope you understand my descriptive.
 

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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.
 

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Discussion Starter #24
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.
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.

To answer your questions, I have power at relay connectors 30 (always) and 85 (car started). 85 and 86 being primary (activates electromagnet); 30 and 87 being secondary (clutch). When car running and AC button depressed, relay electromagnet not closing (finger on relay, no click).

I agree I need to bite the bullet and take the car in to the garage and have them run another diagnostic. Make sure pressure is in range (I understand low pressure cut off is somewhere around 50 psi, but not sure).

I am at the point where it is unlikely I will sort this out to a fix without more sophisticated equipment and better understanding (I think that's becoming more obvious each day! Ha!!) . At best, I hope to be able to more intelligently converse with the mechanic and have sufficient evidence of the issue not being attributable to a faulty ECU as first course of remedy.

And, I do like to learn....
 

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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.
 

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Discussion Starter #26
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.
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....
 

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Discussion Starter #27
Update: My error - relay 86 is hot so connected pin 85 to battery negative as you directed. No continuity. So went to DME for next step in checking that continuity occurs when pin 13 connected to battery negative while AC switched on. But uncertain because wire at pin 13 (if I am reading wiring and location diagram correctly) is not BRN/BLK/YEL. Here's a photo of what I am looking at. Wire looks to be RED/VIO perhaps? Can you help clarify for me?
 

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Apologies for the slow reply, been a bit busy. I was dubious of the wiring diagram when I saw that code, I've never seen a single core cable with 3 colours in a car. Are you able to check the colours on the wiring diagram by lifting the fuse box and seeing what goes to that position? It may not be possible to check this way because of the internal conductors in the fuse box but worth a quick check. I will check on mine too as it should be the same as yours.

Also I just wanted to check that you are testing the earth side of the control side of the relay is earthed with the AC selected? Your last post reads like you may be testing without selecting the AC on.
 

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Discussion Starter #29
This is interesting. And yes... I did check with the AC selected. Took the car in to my mechanic today. He was also intrigued, as the different sources he had on computer for wiring diagrams showed the 3 colors and pin 13. He finally landed on a diagram that showed the brown/black/yellow wire from pin 93 on the short DME connector. He assured me that this was it, and that pin 13 was the throttle body power pin. Anyway - tested the wire (from pin 93 - it was black/brown/yellow) and no continuity. Which still leaves me wondering if that was the right wire - if not, one wouldn't expect there to be continuity. So - his diagnosis ended as a faulty DME. At this point, I may as well run a ground wire through a toggle switch from the relay (85) and, as I should need AC, just turn it on and off using that as a workaround. Sure do hate giving up!
 

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Discussion Starter #30
Quick note. May explain my confusion around pin 13. I just found an old discussion that clarifies pins 82 to 121 are represented as "B" 1 to 40. Explains perhaps why I see the Brn/Blk/Yel wire at pin 93/13 and why pin 13 (red/vio) is the electronic throttle body power. Make sense?
 

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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.

Another poster on here posted the following in another thread:-

 

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Discussion Starter #32
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.

Another poster on here posted the following in another thread:-

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.
 
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