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Gary, Australia Car Mechanic
Category: Australia Car
Satisfied Customers: 1004
Experience:  Senior Technician
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My issue is regarding AH Astra aircon not working, have

Customer Question

Customer: Hi, my name is ***** ***** issue is regarding AH Astra aircon not working, have replaced compressor, as thermal fuse was U/S, A/C system still inop, can make system work by supplying power to terminal 87 of relay
JA: Thanks. Can you give me any more details about your issue?
Customer: yes, F20 fuse has power, but relay has no power getting to it what so ever, or earth either at terminal 85
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Customer: yes should be fine, any idea how long? only i'm at work (on nightshift)
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Submitted: 1 year ago.
Category: Australia Car
Expert:  Gary replied 1 year ago.

Hi there,

Have you recharged with the correct weight of gas?

The pressure switch will not allow the compressor to operate if not within set parameters.

The switch, or wiring to the ECM could be faulty.

You need to look at ECM compressor data for further analysis.


Customer: replied 1 year ago.
Yes, i have put exactly 500g of gas in the system as manufacturers specs.. Do you have any wiring schematics you could send me,to suit the vehicle
Vin W0L0AHL4885127170
Expert:  Gary replied 1 year ago.

Hi Rich,

I can't access the wiring like I used to when I worked for Holden, but that may change shortly.

You say both sides of the relay coil have nothing, as you would understand, one should be there while the engine is running, then the other appears after a switch request occurs.

Supplying 12v to 87 proves the output side of the relay wiring is fine.

We need to find which is truly missing, 86 or 85.

Has the fuse box (UEC, underhood electrical centre) been out at all?

Can you please check the 10mm bolts that hold it in?

The A/C relay would have ignition 12v to say 86, and the ECM provides ground to 85 after it receives a switch request from the HVAC facia panel.


Customer: replied 1 year ago.
Hi, no it hasnt they were the first things i checked, there is no power to any terminals on the relay 30 or 86 with the engine running, obviously can make a/c system work when supply power to 87, but have to earth out 85 to make fan speed increase or hi pressure increases too far.. Will try a second hand hvac unit (switch control assembly) when i get it later, it has me stumped, it is a very good friend of mine's car, and there short on money, the car was sitting for nearly 4months while they went away on an extended holiday, thought at first maybe ants in relay, but no ants present at all in vehicle, and obviously no power to relay, only fuse F20, aircon works great and gets very cold if I hot wire system, but obviously it doesn't 'cycle' as its by passing pressure switch..
Expert:  Gary replied 1 year ago.

Can you check tbe pressure sensor wiring, 5v ground and signal?

One will have 5v, one should have 0 ohms to ground, and the 3rd back probed will conected to the sensor should be about 2.5 volts.

I had one car that had no ground, and hence no a/c.

The sensor ground wire was broken from a car accident.

The sensor signal goes high volts, about 4.5 volts, and disables compressor operation due to the ECMs interpretation of high system pressure when it clearly isn't.

A Tech2 would help you with a data list.


Customer: replied 1 year ago.
Hi yes i have checked all the pressure switch terminals, all are within spec, problem still there, i'm back on dayshift now which means I cannot spend anymore time on vehicle till i'm on RnR again as i work 12 hour days 7 days a week on a 4week on 1 week off roster.. Will be quite some time before I can spend anymore time diagnosing it..
Expert:  Gary replied 1 year ago.

Hi Rich,

I have Astras where the ECM is the faulty component.

I hope my suggestions to you are of some help.

Please post another question when you ready, and I'll keep my eyes open for it.



Customer: replied 1 year ago.
It is starting to look like ECM, but without a wiring diagram or tech 2 it is very hard to pin point, i tried a new HVAC (a/c switch assembly) not sure if i already mentioned it or not, and still no joy,Is the ECM an easy swap over? I realise physically its easy, but will it need to be coded and personalised to the vehicle with Tech2?Any idea of replacement cost of an ECM for this model astra?Once i get more time to soend on it, i will be in touch,Thanks again
Expert:  Gary replied 1 year ago.

Hi Rich,

An ECM swap is easy for a Holden tech.

The ECMs need to be reset, which unlinks them from the immobilizer.

Then you swap them, and Tech2 relink to immobilizer.