ebfox

I have read about a dozen threads regarding 964 heater/AC/servo problems, but my 92 C2 seems to be unique. During warmer weather, I am getting a great deal of heat out of the center of the dash (under the radio). My left, center, and right vents are blowing air which is close to the temp setting on the control **** (I have verified that all servos appear to be operating when I modulate the temp control **** and the upper and lower slides). I just found that my rear heat fan fuse is blown (I put a new fuse in and it immediately blew too); I have ordered a new heater fan, but will this affect the heat I am getting under the dash? The heat is more on the passenger side (US car) but in warm weather the lower dash become very hot to the touch.
The AC will barely cool the car off in hot weather; I don't need any extra engine heat dumped in the interior. For what it's worth, my 78m 911SC exhibited a similar problem. I always thought there was a valve coming off the heat exchangers but if prior posts are to be believed this is not the case. Any help appreciated!

1991 C2 Targa

Jeff Curtis

Like it or not, one of your servo motors is NOT functioning properly. I believe, from memory, the center servo is for defrost, the ones on the right and left of center are for passenger/driver dashboard vents, corresponding sides and the far left and right servos are for your floorboards.

Again, this is all from memory, as I went through this B.S. as well. You can make a valid attempt at disassembling the offending servo and cleaning the contacts...or just do what I did after the failed attempt...buy new!

They are about $85 from the dealership and fairly easy to replace. If you would like a few pics, let me know via email.

ebfox

OK, I will recheck the servos. Part of what has me stumped is that I didn't think there was any heat/AC outlet in the lower center of the dash where the heat is coming from. The center and side dash vents are blowing conditioned air; the defrost is cool, also the vents at the sides of the footwell are also cool. It makes me think I have a leak in the plumbing.
Car must be going through mentalpause; having hot flashes.

91 C2 Targa

Jeff Curtis

Hmmmmm, sounds like you might want to check the operation of your mixing chambers. These are the "bulges" in the plastic units up front, on either side of where your servos are located.

Just another suggestion...good luck!

Adrian

Dear Ebfox,
If you switch defrost on the air coming out of the vents towards the windshield should be warm with aircon. The aircon will switch on with defrost depending upon air temp. The fact that your rear blower fan is kaput this will affect the defrost function and I believe your defrost valve is in the defrost position. You need to fix the fan. The heating system cannot function without the rear heater fan circuit being intact. I actually doubt you have any additional problems but it is not worth troubleshooting a system which has a major failure already not repaired.
Summary to answer your question:
YES the failed rear blower fan circuit has a negative major impact on the operation of the heating system. Defrost is a heating function. The aircon not operating corectly is quite common anyway. This needs to be looked at separately but you need to get the rear blower fan fixed first.
I am assuming your blown fuse was in the engine bay. The fuse in the front panel (central electric) is not for the rear blower fan but the entire front system. 2 blower fans, aircon and heating control etc.
Ciao,
Adrian
911C4

ebfox

Yo Adrian,

The new heater fan is on order so I will know in a few days. Yes, the blown fuse is the 30 amp in the engine bay. I replaced it and the replacement blew as soon as I turned the key. All fuses in the front are good.

Thanks!

91 C2 Targa

Randall G.

Hey Eb,

If the hot air isn't coming out of a vent, I would suspect a leak in the hoses that connect the heat exchangers to the HVAC unit. One on either side of the cabin.

Did you see the wonderful pics of the HVAC unit Roly posted a few days ago:

<a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic;f=4;t=004164" target="_blank">Pics of HVAC Unit</a>

In the pics, the round ducts are for the heater hose, supplied by the heat exchangers.

You're right, there aren't any electronically controlled valves for the heat back in the engine compartment. When there's no demand for heat, spring-loaded valves dump the excess heat into the rear wheel-wells.

As Herbie points out in the thread linked above, the hoses always have hot air inside of them. Even when the check valves are open, I would imagine there's hot air in the hoses. The hoses (and ducting) are pressurized, which in turn opens the check valves to exhaust the unneeded hot air to the rear wheel-wells.

If you have hot air coming out the vents, I would then suspect a mixing chamber servo. Hot air on the left = left mixing chamber servo. Hot air on the right = right mixing chamber servo.

How does your interior fan work? With the fuse blown for the rear blower: 1) the interior fan will cut out when the temperature control **** is set to ~21C or greater; 2) Defrost will cut out shortly after being selected.

Best of luck!

Adrian

Dear Ebfox,
Get that fan installed and the fuse not blowing before you do anymore. Nothing will work properly until the rear blower fan circuit is repaired.
I cannot agree with the statements made in the post above by when heat is not required. If the rear blower fan is not running there is little or no pressure in the heater system, which means technically nothing will get to the vents up front. The static air around the heat exchangers will be hot but there will be almost no flow and what movement of static air will be stopped at the mixing chamber. You can actually check the air pressure. Pull off the vent line in the plumbing. The white one. Do this with the fan running and then with fan not running. See the difference. The valves in the rear can only dump hot air to atmosphere when the air pressure is sufficient to force them open. They are differential pressure valves. When the rear blower fan does not operate these valves cannot be open. The pressure required to open them is rear blower at high speed when the engine is running and heating is selected and the rear blower fan running at low speed with engine turned off.
Oh one last thing. The 964 RS does not have the rear blower fan fitted. The engine fan provides a little more pressure to the airflow when the fan is physically removed and replaced with a pipe. If the fan is fitted and not turning there is a great big blockage to the air pressure created by the engine driven fan in the system.
Ciao,
Adrian
911C4

ebfox

I have had a productive day today. Fixed my original problem and found a new one.

First the fix; I removed the glove box and carpet on the passenger side; the cardboard tube which connects the rear heat exchangers to the front air distribution system is shredded in three places. I ran the car for about 10 minutes and heat boils out of the holes in the tube. With the carpet down the heat is directed up under the dash and comes out under the radio. Note that this is without the rear fan fixed; the tube is either pressurized or somehow there is enough negative pressure in the cabin that hot air flow is induced. The replacement tube is $35 so not too bad.

The lesson here is that if you are getting heat out of places other than the distribution outlets, check these tubes.

Now the bad news; my AC compressor is not pulling in. I am going to start a new thread on troubleshooting this.

Thanks to all who replied. This website is a great idea.

E. B. Fox
91 C2 Targa

Randall G.

Glad to hear you found the source of your hot air leak. Not surprised that you had a decent amount of pressure in the hose. My old '66 911 blew plenty of hot air just hooked up to the engine fan, albeit without a non-operating fan in the path. As you know, the engine fan is still blowing air into the heat exchangers, even when the fan isn't running. Problem is/was with my '66, the amount of heat was directly proportional to engine speed.