jesse7flying

Recently did an intake refresh (for the 3rd time) trying to track down an inconsistent idle and replaced the ISV, which has fixed the problem. Was enjoying my new ride when suddenly, my A/C stopped working for no particular reason. On another drive, it suddenly started working again, for no particular reason. I've been cleaning grounds as I find them and nothing has looked particularly bad, but it doesn't hurt to hit it with some DeOxit and a brass brush. Anyway, I was going to a local 928 breakfast, (DFW area, great bunch of guys) and it was actually quite cold that morning for the first time in six months. I hit the heater to full hot and got a great big nothing. I know usually guys have problems with the heat being on all the time, but this is the opposite. I replaced the heater valve when I did the intake refresh. I also pulled a vacuum on the line going through the firewall to the a/c manifold under the center console and the vacuum held. I tested the heater valve before I installed it, so I know it worked. I would appreciate any direction on what to check next or what could be going on. It would be nice to have heat again as it gets colder, but if nothing else, just to keep the windshield clear. Thx

StratfordShark

You're correct that normally the problems are full heat off the full cold microswitch position, resulting from a break somewhere in the 'sensor string' connecting internal and external temp sensor and the slider on the vac controller.

I'd do the easy checks first starting with the heater flap control motor which governs mixing of heated and cool air from outside. With ignition on and in a quiet place put the temp lever to full cold — do you hear the motor moving to full cold position then stopping? Move the slider to full hot — do you hear the motor moving to hot position then stopping? Note if it's a LHD car the control motor sits somewhere to right and just above where your right knee is in footwell.

jesse7flying

Thanks for the tip Shark. I turned the key on, moved the lever to full cold and heard the motor whirring and coming to a stop. I slid the lever to full hot and I could hear the motor, but it would not stop. Bad limit switch? What do you think?
Jesse

Mrmerlin

probably the external temp sensor connector is corroded .

solution cut it out and solder the wires together its in the LF fender.

Fix your HVAC system

StratfordShark

That’s strange since at the extremes of travel the motor arm can’t move so must stop at those extremes.

Get into the footwell and look up and to the right. You’ll see the setting motor and its arm which operates the flap in heater box via a long metal rod. You can reach up to operate temp slider while you watch the motor. What’s it doing?

giddyupp

this is a bit of a long shot - but is it possible you have an air block. Did you drain all the coolant when you did the intake - if so did you put the temp slider all the way onto hot when you refilled and ran the engine ?

Mrmerlin

Giddyup FWIW the 928 has a self bleeding coolant system,
fill it with the heater control on hot,
then run the engine till the fans come on with the coolant cap off.
put the cap on then drive the car for about 5 miles ,
park the car and let it cool off,
then check the level,
open the cap and top it off if necessary.

Jason89s4

I have NEVER seen this, but since it was something you did mess with, is it possible the heater-core valve is stuck shut?. Its normal operation is valve is open until vacuum is applied. So the most common problem is that you get heat (coolant to heater core) if not operating properly. You may have an odd opposite problem ? . stranger things have happened (on 928s in particular). And I can count on two hands the new parts that were faulty.
-Jason

Petza914

Next year before summer, I'm replacing my vacuum heater control valve with a switched electric one. I bought one that's normally closed and opens with 12v applied (tested it with a battery and hose). In the summer, this will prevent the heat soak into the core from the valve opening when the car is off (same as when we wire tie it shut in the hot months) and when I need heat, I'll just push the button switch to open the valve allowing coolant to flow.

SwayBar

That is interesting - where did you get the switch?

Petza914

This
1/2" NPT Brass Electric Solenoid...
and these
2/6/12 Pack Hose Barb Fitting,...

jesse7flying

Giddyup--I used a vacuum bleeder when I refilled the coolant system after emptying the radiator and the block. Coolant temp is good at normal operating temperature, so no, I don't think I have an issue there.
Jason--I checked the new heater valve I installed before I put it in and it worked normally. I have sourced a metal bodied valve and I may put it in to replace the OEM one on the advice of a smarter person.

Still need to check the sensor in the wheel well and the motor in the center console. Hopefully I'll have some news today.
Thx for the advice guys,
.
Jesse

Speedtoys

Even at it's best, it doesnt not fully seal, I would not expect air to be trapped there for more than a few minutes..

76FJ55

A slightly more complex installation, but less manual interaction on the user's side, would be to add the electrical normally closed valve to the other line from the heater core and leave the factory one in place. then connect the electrical valve to a hot in run source (fuel pump or similar).
this would keep the system closed when there's no vacuum source, as you mention, but would open the valve when the engine is running. in this way, you can then use your heater controls as normal during operation as the temperature varies and you you want heat or not. the factory HVAC would be in normal mode where the temp lever controls the flow without the need to manually toggle the electrical valve open and closed to control the heater core flow..

Petza914

Good thought. I have a secondary fuse panel on my 928s for the additional electrics I've added and one side is constant 12v and the other switched, so won't be tough to run a switched circuit with its own fuse to the front. If I leave the button presses it will open whenever the ignition is switched on or I can release the button so the valve remains closed.