paalw

Hi

When I bought my car (92 GTS) it would only blow cold air. I did some searching and it is suggested to check the setting motor movement, and I found that there were no movement or sound when hot/cold lever was moved.

I have since then removed the dash(to have it re-leathered), and that is where I am at now. Is there a probable cause that is more likely to be the problem with the setting motor/HVAC? The motor itself? Any tips on where to go from here to find the problem?

Here is a pic of todays status.

StratfordShark

When you say no movement of setting motor are you going by sound, or have you stuck head down there to observe

It's possible for transistors to fail in the comparator/driver circuit inside the setting motor. It's pretty simple to remove and open up.

I believe there are four NPN silicon transistors. How handy with electronics are you? You can desolder each and test it (a transistor behaves as two diodes connected back to back, so conducts one way only between base and collector, base and emitter and infinite resistance collector-emitter or just replace all four and hope that's the problem!).

But do simple stuff first. Is rod connected to revolving motor arm? Are both edge connectors plugged into setting motor?

Schocki

Most fail and go full hot. The transistors can be bought cheaply and if you know a little about soldering, it is no problem.

paalw

Yes I witnessed it with my own eyes, no movement Linkage looked good from what I could see from that angle.

paalw

You are saying that if the setting motor itself fails, it is most likely to fail in the "hot" position? Mine is probably in "cold" posistion, based on only cold air in cabin.

Thanks for the transistor tips. Any way to test motor after it is removed from the car?

Schocki

Replace the transistors first and hook up your HVAC controller before you put the dashboard back in. Makes sure that you connect all electrical users like Airbag and instrument pod first. Ignition on and see...

jej3

Make sure someone didn't permanently cap the heater valve. It is in the engine compartment and mine is permanently closed (with a petcock) because I live in Florida and don't ever drive in the cold

G8RB8

Replace all the diaphragms while you have the dash out.

paalw

Hi, thanks I have visually inspected it, it looks like the original valve. But anyway if the heater valve was stuck, would that make the setting motor not move?

paalw

Hi. What is that, why should I, and is there a writeup?

Schocki

Replace the valve no matter what and also replace the short hose from the cylinder head to the valve. The same valve is sold by Mercedes Benz and is a lot cheaper than Porsche. The temperature flap motor also serves as a switch to open and close the heater valve. If it is stuck in the cold position, the heater valve would never open.

There is an excellent write-up from Dwayne about this http://dwaynesgarage.norcal928.org/1...0Procedure.htm . The diaphragms leak over time because they are made of rubber.
The comb flap and footwell diaphragms are available aftermarket made out of silicone. The other two are defrost and recirculation and are only available OEM.

It looks like you never worked on the HVAC system in your 928. The design was never changed significantly and remember the car went in to production in 1977. It uses a combination of vacuum and electricity to move various flaps and valves.

I would highly recommend to give Roger a call 817.430.2688 he can give you advice and get you the correct parts quickly and for a very fair price!

paalw

You got that right None of my previous cars have had any critical need for repairs on this system. Some have had some smaller issues, but not like this, I finally have to dig into it...Well at least now I have some things to work with. Thanks for sharing.

Tom in Austin

Is the 'setting motor' what operates the 'comb flap'? IIRC, the comb flap is what allows heated air passing over the heater core to mix with cooled air passing over the evaporator, thus influencing the final output temperature.

The point about the heater control valve is when A/C is on, there is no need for any hot coolant to be admitted to the heater core, so the valve closes. Of course, once the engine's off the vacuum goes away and the valve opens again, allowing some amount of heat to conduct throughout the core and unhelpfully warming it before you return to the car and start up the A/C again.

As discussed in prior posts, Porsche chose to design the system biased for heat so if either the heater control valve or the setting motor/comb flag are not working correctly you get heat!

polecat702

Most owners bitch that all they have is hot air coming out the vents. I guess some people are never happy.

paalw

Ok, finally had time to move a step further here.

I connected all the electricals, turned on ignition, and moved the temp lever back and forth. Definately no movement in the setting motor.

So I removed the setting motor and opened it up. This how the motor was in position when removed. I guess it isn't possible to see if the transistors are bad or not, but anyway here is a couple of pics of the print board. I have ordered new transistors and will have them soldered on, then test again before putting the dash back.