As many will know the auxiliary fan does not come on with the aircon compressor until the ambient temperature is over 28c and several other conditions are met.
I would like to link the two .
My initial idea is to add a relay with + from a battery and the signal wire to the aircon compressor +, with a diode to prevent back feed. I learned the diode is necessary say if the fan goes on from engine temperature you do not want the compressor on all the time (trust me, as I had to once cut off the aircon belt)
My theory is that if the fan is off from the ECU , any signal to the ECU from the 12v in the fan circuit will be intercepted at the relay R4 so the ECU will not know. If the ECU switches the fan on it should not matter as it will be 12v anyway.
Can any better electricians than me see flaws in this scheme? if so I would appreciate feedback and suggestions as to how to make it work.
I have done this before on a 77 RRC where I added a manual switch but it did not have any computers! LOL
Regards Philip A
I just had a look at RAVE and the BCU provides an earth path to both compressor and fan so the relay would go to earth from a switch from the aircon compressor. I think that makes it simpler. A diode would still be needed to prevent back feed.
My suggestion for a relay to make your fan run with the compressor:
Identify the wire on the fan that changes state when it is switched on by the ECU, then cut that wire. Install a changeover relay so that in its off state the path through the relay is as per normal. Energise the relay from the compressor clutch so that the relay provides a new path to turn on the fan.
EG if the standard fan has permanent power and has an earth switched path, the new relay's common terminal would go to the fan, the NC terminal to the OEM relay and the NO terminal to earth. For a positive switched fan you would need a fused battery supply to the NO terminal on the new relay. Compressor clutch on = fan on, clutch off = whatever the ECU decides.
No diode required unless you want an over ride switch although a SPDT switch could do the same without a diode.
And is the condenser fan ambient dependent, or is it a reverse acting HP control or perhaps a temp sensor in the discharge or liquid lines that controls the fan?
Back in the day when building commercial/industrial refrigeration systems I'd use a reverse acting HP control to switch the condenser fan on and off to maintain the condensing pressure/temperature I was after (40*C), these days we use dedicated HP fan controllers that infinitely vary the fan speed to maintain the desired condensing pressure. (medium-big refrigeration systems used to be entirely custom made from individual components)
What I'm trying to say is that you can overcool the A/C system in low ambient conditions, we erroneously call it 'overcondensing' where the condensing pressure drops below optimum conditions and results in TX valve hunting, where the TXv doesn't modulate correctly and may, in extreme cases, result in liquid slugging of the compressor.
You need only one diode as the compressor to not come on when there is cooling fan request on high coolant temp input(105*C) the earth back to ECU from compressor request is irrelevant but if you are very prudent you can put a diode to pin 4 too but IMO is not necessary as i have a switch to earth on that wire withouit any diode as to switch on the fan when ever i want and nothing bad happenned... here's the squeme, work with electrical library connector view to identify exactly the wires and you can splice in with the diode near the ECU black plug between pins 4 - 29
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