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The AC's electric fan is controlled by the operation of the cab switch and meeting the airflow needs of the condenser fair enough. However one of the side effects is that when working hard at low speed with a trailer switching the AC on pushes air over the transmission cooler and helps keep the temperature around 110c but at the expense of running the AC pump with the associated losses in power and fuel consumption.
So before I get busy with the multi meter and install a relay and switch for manual control of just the fan is there anything I should be aware of or is it just a case of power and go.
Temeraire (2018 quasar grey automatic)
One of the last 200ish of the gen3s, probably.
ADOS Attention Deficit Ooooh Shiny!
I have only one concern about this, and it's the possibility of accidentally leaving the fan running after the engine has been switched off - it should be possible to rig a second relay with the contacts in parallel with the existing relay and power that relay from an ignition source so switching the ignition off switches the relay off.
Years ago I did something similar on a transverse engined Simca wagon that had an electric fan, in slow moving traffic the coolant temperature would rise until the fan cycled on, the temperature would then fall until the fan cycled off and this would repeat - it was the way the system was designed to work, but I found it irritating.
I installed a "bypass" switch that turned the fan on manually and would flip that switch when I encountered slow moving traffic - there's no real chance of "over cooling" things as the thermostat is there to maintain the engine coolant temperature.
If you use a relay in parallel with the existing relay you should have no problem. I'd recommend powering your switch from an ignition feed so you can't accidentally leave the fan running. Admittedly it's pretty obvious if you leave it on, the fan isn't exactly silent.
Just be careful to use a big enough relay and fan wiring. Fans can pull quite a bit of power.
I wonder if the simplest thing would be to interupt power to the AC compressor clutch, so rather than turning on the fan independently of the air conditioning, you are instead turning off the compressor? This could be as simple as 2 wires and a switch.
Hmmm - interesting approach - the a/c control relays are actually driven by the ECU, turning the a/c on causes the ECU to "bump" the idle to compensate for the additional load, disabling the compressor would theoretically remove that load causing an increase in idle speed, bypassing the fan relay would increase the electrical load without the ECU being aware of it, so that would theoretically cause a drop in idle speed - neither situation would/should cause a problem in the long term, and have a slightly high idle might be preferable to a slightly low one.
I wonder if the simplest thing would be to interupt power to the AC compressor clutch, so rather than turning on the fan independently of the air conditioning, you are instead turning off the compressor? This could be as simple as 2 wires and a switch.
So you are saying to just interrupt the feed tó the magnetic clutch, would that guarantee that the fan would run permanently on the dash switch though?
Just thinking about it then either way is going to carry a potential for the ECU to be aware that something is not right either the load on the engine is gone at idle or the fan is showing voltage when it shouldn't either of which might give a fault? I will have a look with the power probe at the weekend and see which makes it least unhappy. That said SZ viewer doesn't read HVAC systems on mine so it may just be a logic controller rather than a feedback one?
Temeraire (2018 quasar grey automatic)
One of the last 200ish of the gen3s, probably.
ADOS Attention Deficit Ooooh Shiny!
Last edit: 22 Aug 2023 03:40 by Lambert. Reason: Clarity
I'm no AC expert, but my understanding is the system isn't smart. You press the button, it turns on the compressor, the fan and, as Fordem says, tells the ECU to speed up the idle. As far as I know there is no self-diagnosis of the system.
First thing I'd try is unplugging the compressor and seeing what happens when you press the AC button.
The AC is disabled on some models when the transfer is in low definitely, possibly all 4wd modes, so likely the fan won't run in anything except 2wd high if that makes a difference to your proposed use
(The AC pressure OK switch runs to the 4WD controller)
Just test by switching on the AC and seeing if the fan stops when it is in 4WD & low.
If it suddenly breaks, go back to the last thing that you did before it broke and start looking there
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