Heater bypass valve

[ReelBigFish]

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?

[RHKTriton]

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.

[ReelBigFish]

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

[RHKTriton]

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.

[ReelBigFish]

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

[RHKTriton]

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.

[ReelBigFish]

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