Airconditioner Help - please

[Rovernaut]

Hi all, My aircon on my Aus Spec Disco 3.9 V8 has an intermittant fault. Briefly it stops working for a little while, then comes back on. Some times I lose all power momnetarily to the fan also, which a sec or two will come back on.
1. There does not appear to be any 'Icing up of the condensor'.
I thought it might be the TX valve on the evaporator,. but, this is what I discovered so far.
A multi meter test on the connector to the compressor clutch shows .06 - .09 volts when it is not working. When it works there is 12-13.5 volts.
I direct fed battery power to the compressor clutch and it engages. Also I get good clod air in the cabin.
2.Question, if the TX valve was faulty, why would I get cold air? Wouldn't it block the circulation of the gas preventing cooling?
There are also an Evaporator Temperature switch to sense temp to exterior fins- to detect' icing' and a Dual pressure switch to monitor gas pressure on the High pressure line.
3. May be one of these is causing my problem, but how do I test them?
4. Then there are relays under the dash to control Compressor clutch.
Any advise would be appreciated. Especially if some one has had the same problem and fixed it.
Finally, it I was to by pass the dash on / off switch and temparily wire in a new on /off switch direct to the compressor clutch., could I damage the system with over pressure of it running constantly?
Sorry for the long post.

 

[dltcharb]

Check your fan switch

 

[Rovernaut]

I replaced the A/C logic relay and the Clutch relay with a couple servicable ones and it seems to be working ok now (finger crossed.)AUS spec. Maybe the relays were not holding power and cutting out with load?
( NAS Discos didn't have a logic relay, they had just the clutch relay and the ECM)
I will have to take it for a longer run now to see if the problem has vanished.

 

[Joey]

When you say the A/C stops working, do you mean the fan inside the Cabin? If so, then it would be the fan switch in the dash, these are known for issues. It will either require a good cleaning or replacement depending on you.

Quick test is when it stops, move the fan setting from 4 to 3 and back to 4 again..... works for me.

If you mean the A/C pump stops working, then sorry I can't help.

 

[Rovernaut]

Joey, I think Mine problem was electrical within the relays MFI- V8 Aus SPec.
Here is a run down how A/C works.
The NAS spec SFI-V8s discos , have a clutch relay but do not have the Logic relay , they bypass this and use the ECM instead.

here's the circuit operation .

COMPRESSOR CONTROLS K2
CIRCUIT OPERATION 1
CIRCUIT OPERATION
Compressor Control (MFI?V8)
With the Ignition Switch (X134) in position II, battery
voltage is applied to the Ignition Load Relay (K127).
The relay is energised, applying battery voltage to
the Compressor Clutch Relay (K108) and the A/C
Logic Relay (K170). When the Front Fan Speed
Switch (X247) is turned to positions I, II, III, or IV
and the Front A/C Switch (X225) is turned on,
ground is provided to terminal 86 of the A/C Logic
Relay (K170) through the closed contacts of the A/C
Dual Pressure Switch (X102) and the Front A/C
Evaporator Temperature Switch (X101).
The relay is energised, applying battery voltage to
the Engine Control Module (ECM) (Z132), which
signals the ECM that compressor operation has
been requested. The ECM then provides ground to
the coil of the Compressor Clutch Relay (K108). The
relay is energised, applying battery voltage to the
Compressor Clutch (K107). Compressor Clutch
operation is now controlled by the status of the A/C
Dual Pressure Switch (X102) and the Front A/C
Evaporator Temperature Switch (X101).
The Front A/C Evaporator Temperature Switch
(X101) is fitted in the airflow out of the front
evaporator to sense the temperaure of the exterior
fins. Should ice begin to form due to low
temperature, the switch will open. The A/C Dual
Pressure Switch (X102) monitors refrigerant
pressure in the high pressure line. Should pressure
become too high or too low, the switch will open.
When one or both of these switches open, the
ground supply to terminal 86 of the A/C Logic Relay
(K170) will be interrupted. The relay de?energises,
removing battery voltage from the Engine Control
Module (ECM) (Z132), which signals the ECM that
compressor operation should be turned off. The
ECM removes the ground supply to the coil of the
Compressor Clutch Relay (K108). The relay then
de?energises, removing battery voltage from the
Compressor Clutch (K107), thereby disengaging the
Compressor.