Constantly blowing hot air

[mixybro4]

I'm in the process of restoring my 1980 928 and there is no way to stop the hot air from coming in, even with the fan off and temp set to the coldest setting. I closed all the vents but hot air just pours through the defrosters. I don't know if the compressor is good or not, not sure what kind of refrigerant this takes or where to put it in exactly. Any help at all would be appreciated.

[AirtekHVAC]

Check the heater valve, near the firewall, in the center of the motor. This is a known failure point.

Also, car takes r-12 refrigerant, unless someone has converted it, if so, 134a. Goes in the fitting on the larger line coming off the AC compressor. Once you confirm your heater valve is functioning, I recommend going to an AC shop, and let them diagnose....

Good luck.

[Alan in AZ]

You likely have 2 issues:

AC system not working & heating is on unrestricted hot.

First solve the heat issue.

The hot water valve defaults to open (hot water flows to the heater core) without vacuum to the actuator - this can be caused by lack of source vacuum (T off the brake booster vacuum lines) or electrical issues from the head unit which controls the vacuum or it could be a failure of the valve or the valve actuator. The hot water valve is under the air filter box (remove) - you can test the actuator with a vacuum pump or just wire it closed for now to reduce heat.

You may also have issues with the temperature control loop but it will be hard to say until you have the water valve working properly.

The AC will not operate if the refrigerant charge is low. Check the pressure switch on the dryer (pass side in front of the radiator). It needs to be conducting - else it means the refrigerant charge is low. You could try temporarily bypassing it BUT only to check that the compressor clutch will engage when the switch is bypassed. Once that is proven reconnect the switch and get the refrigerant vacuumed out, the system pressure checked for leaks and the dryer changed out before you consider recharging it.

Alan

[mixybro4]

I zip tied the water valve closed.

[mixybro4]

The hot water valve holds a tiny amount of vaccuum. It slowly drains away until about 15mmHG. The rubber line going into the car does not hold any vaccuum.

[mixybro4]

The rubber line going into the car from the cross connector by the brake booster holds vacuum. The one that branches off into two under the MC does not hold vacuum. The line that runs into the main hose for brake booster vacuum does not hold vacuum.

[Rupesy]

You should have at least one check valve in your HVAC system under the dash and maybe two. You can find some diagrams with a quick search.

[mixybro4]

That's still really hot air coming through the vents. Should the valve be tied so it can't move up or should it be tied so it's always up?

[JK McDonald]

Hey Mixy, More than likely you will find that you have a couple of problems to sort out in order to bring your A/C back to life. There can be a combination of electrical and vacuum controls that must be checked for proper operation.

Like others have already mentioned, many times the first and simplest step to take to eliminate the hot air from coming through the dash vents is to zip tie the heater valve to the closed (raised) position. If this doesn't solve the HOT AIR problem your heater valve my be leaking substantially internally or bypassing. I personally manually close/zip tie my heater valve each spring even though my A/C works fine because it will sometimes slowly creep open when the car is parked. Since the heater core is stacked right behind the A/C evaporator up inside the dash, the A/C must first dissipate all the hot water that seeped into the heater core before you once again will feel the cold air through all the vents. I've found that when my car is parked out in the Texas sun every little bit helps to cool it down faster. In the fall the heater control valve is returned to normal operation.

By the way the A/C systems in both my 928's were initially dead when I bought them. After testing the integrity of the system and changing out the "O Rings" , I replaced the drier and recharged them with Freeze-12. Although I had initially expected to find some major issues, I was able to reuse the same compressors, expansion valves, etc... and I've been very satisfied with the results. If there had been the need for a substantial tear down, compressor, condenser, evaporator, expansion valve, etc.....replacement, I would have gone through the "Full Monty" 134 conversion .....

Here is a very useful site that will help answer and explain many of the troubleshooting issues you may run into.

928 Tips

Good Luck, Michael

[Alan in AZ]

Like I said - the default position with no vacuum is water valve open - so you must move it to the closed position - then tie it in place. As noted sometimes they can leak even in the closed position.

Can you post an annotated picture of what you have tested and results - descriptions are hard to follow.

Alan

[mixybro4]

Pictures tomorrow. Is this canister in the driverside fender supposed to do something? If appears nothing is plugged into it.

[mixybro4]

The hot water valve must be leaking through, would you guys recommend getting a new one or is there a way to fix the old one? I imagine there is no Freon in the system, car sat for 15 years. I never put it on the cold setting, incase it would've activated the compressor clutch, I am not exactly sure how it works.

[Alan in AZ]

Replace the valve - if it leaks it can't be repaired. The AC compressor won't come on if there is insufficient freon charge (there is a pressure switch). Just truy it and monitor the compressor clutch to see if it engages. or test he pressure switch near the dryer - pass side front of radiator.

If it doesn't conduct between the 2 terminals than you have insufficient Freon. In this case you should assume a leak and change out the dryer & have the system vacuumed down to evacuate & test - and only if good refilled with R12.

Alan

[Danglerb]

The valves are cheap, like $20, replace the hoses at the same time.

[mixybro4]

Cheapest I found one so far was $45. Do you guys have a recommended website? Or does Roger sell them?

[stepson]

yes. Roger sells them

[Danglerb]

Originally Posted by mixybro4

Cheapest I found one so far was $45. Do you guys have a recommended website? Or does Roger sell them?

Is that a serious question?

Pelican has them for $35, google is your friend for options.

IIRC they are a common part to many cars like the Audi, so many cheap versions, but the hoses will more than make up for any reduction in costs.