Heater bypass valve

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Re: Heater bypass valve

Postby ReelBigFish on Fri May 19, 2023 7:55 pm

Hi RHKTriton.
Just wondering if you had any more information about how you went about your heater hose bypass into the cabin. I've had my aircon regassed but that hasn't really helped so looking at options to make the aircon perform better.

Looking at the bypass valves they all seem to be suction controlled. Did you end up using one via vacuum hose or did you come up with another solution?
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Re: Heater bypass valve

Postby RHKTriton on Sat May 20, 2023 9:55 am

Hi RBF. I cut the existing heater hoses outside the firewall, leaving appropriate lengths to allow fitting the valve.

I repurposed the vacuum switch for the manifold flappy valves, which I deactivated years ago.
Then I simply ran my own supply to the valve, with switch on the dash.

You can find a suitable vacuum switch on eBay. Saw one for BMW EGR that should be the ducks nuts.

Then you need a vacuum source, eg brake booster line or feed to VGT controller, etc.
Don't let the b'strds get you down!!
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Re: Heater bypass valve

Postby ReelBigFish on Mon May 22, 2023 8:07 pm

Cheers for the prompt response. I'm about to add a thermofan (unfortunately my ML doesn't have the factory plug) and depending if that makes enough difference or not I'll decide whether or not to bypass the heater hose
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Re: Heater bypass valve

Postby RHKTriton on Tue May 23, 2023 8:28 am

Hi RBF. I started with the add-on fan, just a suitable sized unit from SC slaved off AC clutch circuit.

This helped a fair bit.
Don't let the b'strds get you down!!
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Re: Heater bypass valve

Postby ReelBigFish on Sat May 27, 2023 9:29 pm

HI RHK,
I Just tried triggering the thermo fan from the AC Clutch but it doesn't look like the AC Clutch is being triggered. I'm pretty sure my wiring is correct, the blue and white wire on pin 4 of the A-27 connector (6 pin connector by the battery) and I couldn't see any busted fuses. I also noticed that the engine revs don't change when the AC switch in the cabin is on so i'm starting to think the AC Clutch relay isn't be triggered... perhaps the refrigerant is low.
Any suggestions?? it might be out of my league
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Re: Heater bypass valve

Postby RHKTriton on Sun May 28, 2023 8:28 pm

Starting point; ignition on, cabin fan on at least speed one, AC button on. Check if volts appear at compressor relay coil. Without consulting circuit diagram, you should see connection for other end of coil near ground.

So in summary, you've ot to prove the supply and control voltages are where expected.
Don't let the b'strds get you down!!
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Re: Heater bypass valve

Postby ReelBigFish on Sun Jun 04, 2023 6:15 pm

Turns out I have a slow refrigerant leak and the line was empty which is why the compressor wouldn't kick over. All regassed and thermo fan is working fine. I'll wait for some warmer weather to see if I do the heater hose bypass as well
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