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Thread: Thermo fan bypass

  1. #1
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    Thermo fan bypass

    Here's a few photo's showing how to wire up some bypass switches to your thermo fans. As noted by many people the thermo fans come on way too late on the P38 and by the time they come on you are already running very hot. If you know your temp is climbing you're better off switching the fans on early.

    The fans have two speeds - I'll call them low and high speeds. High is quite loud. I turn mine on when I'm doing low range hill climbs and crawling in traffic in middle of hot summer with aircon etc on.

    Wiring is easy - there's two connectors located near the lower left corner of the radiator. Both connectors are connected to part of the aircon system. Simply tap into both these (or one or the other if you only want one speed switch). Personally I ran a piece of 5 core trailer wire through the firewall to the area as it's tidier and easier to work with - 2 wires to each connector. I go through the firewall where Ron has suggested in another post (below the radiator expansion tank)


    I forget which switch is low speed and which is high...
    Photo's were taken with bumper off.

    2 wires go to each connector. The connector on the right (photo's right) only has two wires so that's easy. Yellow and brown wires are my switch wires to this switch.
    Bypass connectors.jpg

    Connector on (photo's) left has four wires - connect to the black and the black/blue wires. Red and green wires are my switch wires to this switch.
    Bypass connectors - Closeup left.jpg

    Switchs mounted inside the ashtray - ashtray removed and bent bit of aluminium in place. Yellow switch is low speed, red is high speed. Knob is subwoofer control. HEVAC waiting for replacement to arrive ...
    Bypass switches.jpg

    That's about it.

    edit: One thing to note is that the 'low' speed won't activate unless the aircon is on. High speed works regardless.

  2. #2
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    Quote Originally Posted by Junosi View Post
    Here's a few photo's showing how to wire up some bypass switches to your thermo fans. As noted by many people the thermo fans come on way too late on the P38 and by the time they come on you are already running very hot. If you know your temp is climbing you're better off switching the fans on early.

    The fans have two speeds - I'll call them low and high speeds. High is quite loud. I turn mine on when I'm doing low range hill climbs and crawling in traffic in middle of hot summer with aircon etc on.

    Wiring is easy - there's two connectors located near the lower left corner of the radiator. Both connectors are connected to part of the aircon system. Simply tap into both these (or one or the other if you only want one speed switch). Personally I ran a piece of 5 core trailer wire through the firewall to the area as it's tidier and easier to work with - 2 wires to each connector. I go through the firewall where Ron has suggested in another post (below the radiator expansion tank)


    I forget which switch is low speed and which is high...
    Photo's were taken with bumper off.

    2 wires go to each connector. The connector on the right (photo's right) only has two wires so that's easy. Yellow and brown wires are my switch wires to this switch.
    Bypass connectors.jpg

    Connector on (photo's) left has four wires - connect to the black and the black/blue wires. Red and green wires are my switch wires to this switch.
    Bypass connectors - Closeup left.jpg

    Switchs mounted inside the ashtray - ashtray removed and bent bit of aluminium in place. Yellow switch is low speed, red is high speed. Knob is subwoofer control. HEVAC waiting for replacement to arrive ...
    Bypass switches.jpg

    That's about it.

    edit: One thing to note is that the 'low' speed won't activate unless the aircon is on. High speed works regardless.

    For several years I had a V6 Calibra (a b-e-a-u-t-i-f-u-l car ) and the thermo fan arrangement was such that when the car was parked on a hot day, the fans kept running to reduce the effects of heat sink...your wiring gives me an idea ...thank you!!!

  3. #3
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    Thanks for the How to, looks very simple. Just wondering if I just make a permanent connection across the 2 wire switch so the fans come on with the aircon will the fans be on low or high speed.

  4. #4
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    Quote Originally Posted by wayneg View Post
    will the fans be on low or high speed.
    If you put the short on the correct switch it will be low speed.
    Last edited by Junosi; 4th November 2010 at 02:31 PM. Reason: Misread

  5. #5
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    D.one in 5 mins, easy peasy..... Driving the Holland Track starting tomorrow so extra cooling is there if required

  6. #6
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    Just looking at this post, does anyone know which of the wires here are for the high speed of the fans? I’m tinkering at the moment and want to play around with manually switching the fans on high speed, but I don’t have access to the wiring diagram. I don’t want to just go cutting wires for the sake of it unless I absolutely have to.

    From reading this post, it appears that the fan power supply is normally closed (that is, when the circuit is live the fans are told not to run), but when the earth/negative is removed the fans will be triggered to run.

    If you’re super keen to get an idea of why I’m doing it, check out this video series:

    Thermofan Installation Series - YouTube

    Cheers
    Keithy

    2002 P38 Range Rover HSE

    Sequential LPG - Redarc Charger - TPMS - Ashcroft Locker
    Wheel Carrier - Bullbar & Spotlights - 285/75/16 BFG KM3’s
    On Board Solar - Stainless Snorkel - 2” Suspension Lift

  7. #7
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    I would have to look that up, don't know from the top of my head. However, I remember your thermofan project. Did you ever revisit your original post on here? I updated it a while back with some power experiments: Thermofan over Viscous?

    Wonder what you are thinking

    Cheers,
    -P

  8. #8
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    Cheers mate, I did a bit of research playing around with the factory A/C thermo fans yesterday afternoon and worked out that they’re negative switched, and normally open (so applying a negative/earth activates the relays).

    Watch this space!

    Ill go and re-visit the post now! I completely forgot about it!

    Cheers
    Keithy

    2002 P38 Range Rover HSE

    Sequential LPG - Redarc Charger - TPMS - Ashcroft Locker
    Wheel Carrier - Bullbar & Spotlights - 285/75/16 BFG KM3’s
    On Board Solar - Stainless Snorkel - 2” Suspension Lift

  9. #9
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    It’s easier to explain the outcome via video than over typing, so here it is!

    Operation extra-cool with a TWIST - Can I enlist a helper to back up the Range Rover thermofan? - YouTube

    Cheers
    Keithy

    2002 P38 Range Rover HSE

    Sequential LPG - Redarc Charger - TPMS - Ashcroft Locker
    Wheel Carrier - Bullbar & Spotlights - 285/75/16 BFG KM3’s
    On Board Solar - Stainless Snorkel - 2” Suspension Lift

  10. #10
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    Keeping a P38 cool

    Quote Originally Posted by Keithy P38 View Post
    It’s easier to explain the outcome via video than over typing, so here it is!

    Operation extra-cool with a TWIST - Can I enlist a helper to back up the Range Rover thermofan? - YouTube

    Cheers
    Keithy

    Hi Keithy, I have looked at your videos and I am having a bit of the same problem. Although mine is a bit of my own doing. I have rebuilt my motor and made it a 5lt with 9.8 to 1 compression ratio. So it generates a fair bit more heat than a standard motor. Also I have fitted aftermarket headers which dump a bit more heat into the engine bay. I have also fitted a Aussie Desert Cooler all alloy Radiator which holds 25 percent more coolant. 37c here the other day and I was out playing in the traffic and the motor got up to 94c. I have the condenser fans wired up to a switch so I turned them on and the temperature stopped rising. Once the traffic got moving the temp went down to the normal 85c. I am going to stay with the engine driven fan, but I will add a bit more viscous fluid to it to get it back to original performance. I am interested in that gauge you put in your dash. What brand is it as I would like to make the thermos come on automatically. I cringed when I saw you drill that hole in your dash. I made and fitted a gauge pod on top of my dash that holds 5 gauges and there are no holes in the dash. A lot of thought went in to that. I will let you know it the extra viscous makes any difference. I am also looking a a bonnet scoop with a rear opening to let some of the hot air out. My background is the last 40 years I have worked in the automotive air conditioning industry, so I have some experience in trying to get air flow and AC to work on vehicles that were not designed for this climate. I did spend a lot of time up your way out at Century mine where the temperature in summer averages 52c and down in the pit can get up to 65c. Air cons work really hard up there.
    Regards, Alan Temperley

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