Permanent Fix to summer heater control valve heat soak
 

Have been planning this fix for a while and had some time today so figured it was time with summer temps arround the corner.

For the summers, I have been removing my airbox and wire tying the heater control valve shut. The reason we do this is because after driving the car in the summer when the system loses vacuum and the valve opens, hot coolant heats up the heater core and it takes a while to overcome this once the car is running again and the AC activated. Wire tying the valve closed prevents this heat soak event so the AC gets colder faster.

My solution to this was to replace the vacuum actuated heater control valve with a electric solenoid valve that is closed when no power is supplied to the solenoid and wire it to a switch in the interior. This way it will always be closed unless I need heat, in which case, I'll activate the switch which will open the valve and let the hot coolant run through the heater core.

Not a particularly difficult job just had to figure out how to fit the solenoid valve in the space without interfering with the airbox, install and wire the valve, get a wire pair from the heater valve area into the passenger compartment, wire up a switch and route the wires.

https://cimg1.ibsrv.net/gimg/rennlis...cb4acb4196.jpg
Here's the solenoid valve with the proper sized hose barbs installed and I shortened the barbs so it would fit in the factory location. Note flow direction arrows.


https://cimg9.ibsrv.net/gimg/rennlis...636f144130.jpg
Drilled a small hole in the cowl area and fished an insulated wire pair through to the interior.


https://cimg3.ibsrv.net/gimg/rennlis...0853559263.jpg
Black wire is the one from the front. I have a secondary fuse.box installed in this car so I don't have to corrupt the factory wiring for my accessories. One side is constant power (remote door locks and hatch release, etc and one side for switched power like Homelink dimming rear view mirror, inside powered stereo antenna, etc). I had an open switched circuit which is how I supplied power for this


https://cimg5.ibsrv.net/gimg/rennlis...224d394e4e.jpg
Also had to shorten the coolant barb off the back of the engine but there's no need for that to be 4\" long like it is, so it's about half that size now and the valve fits perfectly. It's as far back as possible, canted to be low for the airbox and doesn't touch any of the metal lines in that area.


https://cimg9.ibsrv.net/gimg/rennlis...4f583886b8.jpg
Mounted the light up push button switch in the trim panel under the pod over to the right side.


https://cimg2.ibsrv.net/gimg/rennlis...c721a80fe8.jpg
Wired things with a 3 wire connector so when I need to disassemble things to work on the pod or ignition switch, etc I can still remove the trim piece and set it aside.


https://cimg4.ibsrv.net/gimg/rennlis...d2d49fdcfd.jpg
Here's the switch and panel remounted


https://cimg5.ibsrv.net/gimg/rennlis...7e1c71d087.jpg
Parcel shelf reinstalled


https://cimg7.ibsrv.net/gimg/rennlis...8aa350ff75.jpg
Wiring of the solenoid complete


https://cimg8.ibsrv.net/gimg/rennlis...757ba2a64a.jpg
Passenger shelf reinstalled with secondary fuse box mounted to the underside.


https://cimg9.ibsrv.net/gimg/rennlis...0d761eacba.jpg
Switch lights up with a red ring when activated.


https://cimg0.ibsrv.net/gimg/rennlis...fc63422a27.jpg
Shows location of solenoid relative to the lower airbox - perfect fit.

This is the solenoid I used from Amazon - 1/2" NPT Brass Electric Solenoid... https://www.amazon.com/dp/B01N0Q4QYI?ref=ppx_pop_mob_ap_share

Here's my solution. As soon as the temperatures climb enough in spring here in Madrid, the valve is shut. Takes 20 seconds and no tools are required.
https://cimg7.ibsrv.net/gimg/rennlis...eb16ae13dc.jpg
https://cimg9.ibsrv.net/gimg/rennlis...047a2bfdfa.jpg

That was my original plan but seems like I'd do this then we'd get a cold spell and I'd want heat again, so now I have full control of that valve from the cockpit.

I have some string attached to the arm. Pull from the driver side opens...pax side closes...or is it the other way around?

Oddly... I was just thinking about all this today heading into spring. I like the the solenoid idea. It would work on the 944 also right?

So..why cant you intercept the signal that is sent to the solenoid that operates the heater valve on the HVAC solenoid manifold and have that drive this electric one you installed?

The current one is open when at rest until it's pulled shut by vacuum. The purpose of this change is to use a valve that's always closed until you want it open for heat and to prevent hot coolant from passing through it when parked after the vacuum from the last drive relaxes and allows the valve to open again.

Tying into the HVAC controller could send a signal to the valve but it would have to work opposite the way it's designed to since the OEM valve gives you heat unless the slider is on cold and then the vacuum pulls it shut. I didn't want to get into the HVAC system when I could just manually open the valve with a switch when I wanted heat.

I have thought about this before as well.
I was thinking of just adding a second vacuum operated to the return line from the heater core, then just connect it to any vacuum source from the engine (the spider at the back of the valley would be convenient). this way when you shut the engine down it would loose vacuum and the valve would close, then when you start the engine the vac would return opening the valve and letting the stock system take over control of the heater flow. Simple, cheap and no input required from the driver.

This is interesting

https://www.oldairproducts.com/produ...ve-kit-50-1555

https://cimg1.ibsrv.net/gimg/rennlis...dc96207760.png

So you use their temp control knob to control the amount of hot coolant allowed to flow in and out of the heater core? That could be a good solution for anyone that has a problem with their HVAC head unit where the temp stuff doesn't work. Makes for a slightly more complicated plumbing install and still requires some electrical work, but it's neat what's out there.

That might work also, but how long does it take to lose enough vacuum for the valve to relax and close? If the vacuum system is healthy, couldn't that take a while or would it be instantaneous. If it takes a while it will still allow for hot coolant to pass into the core or maybe this one would close at the same time the normal HVAC opens so a little flow for just a bit but not enough to matter.

The electric one I've installed would close immediately even if used in the open position for heat and when used in the closed position for cold, never opens.

I don't know the exact answer to that question, and it is definitely a via question, but would guess that it would close fast enough to eliminate any significant flow into the heater core. as you state it will be transition at the same time as the factory heater valve. there is no check valve or restriction going to the vacuum spider, so it should follow manifold vacuum fairly directly and go to atmospheric when the engine is shut down. Also when the engine shuts off the water pump will no longer be creating the pressure differential trying to push water through the system, so the only force would be natural convection encouraging water flow and since the inlet and outlet are at approximately the same height, this force should be relatively low requiring significant time to exchange the water in the heater core with that in the heads.

Agree that the vacuum solution could have a delay and allow some hot coolant into the core.
Also note that the multi vacuum connector ("spider"?) is not the correct place for the vacuum source, as it is a variable supply.
I'd suggest over by the Brake booster where the Black/Blue check valve is.

Annie's 1987 S4 had an electric one installed by the PO and it worked great, however new user training was an issue, "oh when you want heat, you need to flip this additional switch" ;)
The more automatic the electric install is the better IMO.

Dave K

The source by the booster may be a slightly more stable, but they will all have a variable vac signal as they follow manifold pressure. If you do pick up by the booster you would need to make sure to get the signal before the check valve or the now flow control valve would not close when the engine was shut down.

I had initially was just thinking of the vacuum operated valves at it seem incredibly cheap (<$15) and fairly easy to incorporate. I agree that the electric control valve may allow more control options, depending on a persons desire to automate or maintain more manual control of the system. TTO automate it one fairly easy solution may be to trigger a relay by they fuel pump fuse. the fuel pulp is controlled by the LH and is only supplied power when the engine is running, so connected to that source the electrical control valve would only allow water flow when the engine is running and the factory valve should have control at that point anyway. of course a manual switch is quite a simple solution.

I actually like manual control of things. In one of my 928s I even wired a switch to the cooling fans instead of using the temp sensor. When I'd get stopped in traffic or pull into the drive thru lane, I'd press the button and turn on the fans which would keep the temp carry stable instead of it having to get hot to kick on the fans and then take a while to cool back down again. Also worked to improve the AC when driving at medium speeds by improving airflow across the condenser. Now that I'm trying this, I should have run a 2nd wire into the engine bay while I was in there this weekend. I have a other spot on the other side of the trim piece for a 2nd matching switch - maybe I'll use a blue lighted one over there to remember which is which, but the one by the HVAC control is for the AC and the one under the temp gauge for the fans.

I don't really need to worry about next owner education - my son is fully versed on what I've done and how it all works as long as his memory holds out for 30 more years or so :)

I don't think variable vacuum would matter for something like this as almost any vacuum will actuate these valves. That concept of the manifold won't work on my supercharged car though as it would keep forcing the valve open under boost :)

Yes, that's kind of why I quit pursuing it too, once I put the twins crew on my 86.5. 😆