ChuckD

I have never seen this on anything else. Why was it done...is there just poor airflow to the alternator...even though it is located down under the car? Is it because there was a shield on the bottom and it needed more air?

What does the sensor within the ductwork do? What does the car do with the information from this sensor? I am assuming it is a temp sensor. Since it essentially measures the temp of incoming air from in front of radiator, does that make it the equivalent of the IAT on my 1986 Corvette?

westija

The sensor measures outside air temperature for the A/C to work. If you do a search it will explain in details. Not unusual to be a reason for poor A/C (dirty contacts) according to what I've been reading here

I don't know about the need for a hose, and am puzzled by that as well.

depami

The alternator doesn't regulate voltage well when hot.

dprantl

Yes, the alternator needs a source of cool air. Not only is there a cover right under it, but the exhaust manifold is right above.

Dan
'91 928GT S/C 475hp/460lb.ft

j.kenzie@sbcglobal.net

I think it's actually a very clever design. The fan on the alternator pulls in fresh air to cool itself. The continuous supply of fresh air reflects the outside air temp so the a/c can be controlled electronically. The alternator is very close to the exhaust manifold and live in a hot, sometimes wet environment. The ductwork isolates the alternator somewhat while bringing in cool air. My ductwork and sensor were missing when I got my car. The a/c controller became an on/off switch. I obtained a temp sensor, which is hard to find, and put it in my brake duct. My aftermarket alternator does not use a duct system (and I think it takes a beating, requiring a rebuild after 4-1/2 years of use.)
Anyway it's a pretty nice design.
Dave

depami

Most vehicles have the alternator mounted higher up. The 928 alternator is at the bottom so the shrould not only collects/directs air but is also a shield to keep water and road crud out.

MFranke

The alternator shroud and hose was ripped off for years on my '88 with no adverse consequence to charging ability or AC functionality. I got a new cowl and hose to replace anyway.

Alan

Its definitely a function of how hot it gets... winter - doesn't matter anywhere, in many places it doesn't matter in the summer either...

Alan

ChuckD

The bottom engine cover is long gone on my car. The AC belt isn't even on the car and the fan for the HVAC doesn't work either. I am sure it's a major problem and I have no immediate plans to tackle the AC. Is it OK to not use the duct work for the time-being?? I know you can re-do it inexpensively, I guess. I saw that write-up as one of the first threads I read all the way through when I bought the car.

77tony

title = self explanatory. t

j.kenzie@sbcglobal.net

Yep.

depami

If running without the duct the inlet of the shroud acts like a scoop and wants to pick up sand, stones, water, etc. If running on absolutely clean and dry road there is probably no issue. Otherwise you might be subjecting your alternator to contaminate that will likely shorten its life.

If the temp sensor is not functioning properly your HVAC control will be all messed up even without running AC. The fan speed will not function properly and likely on run on high and when at full defrost and then blow full hot to boot. Kinda sucks when caught in rain in stop and go traffic on a 90+ degree day. The windshield fogs and the only option is full hot when you are already sweating like a pig because the air doesn’t work and you can’t open the windows or sunroof because it’s raining. . .

martinss

Is there a test procedure somewhere for the temperature sensor in the duct?

westija

From some here:

"....Climate Control
Temperature Regulation Failure

I just completed changing out the vacuum controlled heater valve. Now, the only place I get cool air is if the temp. control is on 65. Anything above that and it gets real hot real fast. Does that sound like a bad dashboard temperature sensor?

Here's my take on the binary temperature problem, based on a sample size of one:
There is a servomotor that controls your heat, located above the driver's right knee. You must remove the parcel shelf to get at it. It has a little arm which controls the temperature mixer flap. If the arm is all the way up, you get max heat; all the way down, no heat. Normal position with the inside temperature stable is roughly horizontal. So the first step is to find out what that little arm is doing. With the parcel shelf out, you can feel its position while sitting in the driver's seat. If it never stops anywhere but all the way up or all the way down, you have isolated the problem somewhat.
The key input to the servomotor is pin 4 of the upper plug (there are two). It connects to the outside temp sensor (in the air duct to the alternator), which connects to the inside temp sensor (behind the slotted fitting in the dash next to the glove box), which connects to a rheostat on the temperature control slider. According to the manual, the resistance between pins 4 and 12 should be about 3.7 KOhms with the slider at 18, and about 4.7 KOhms with the slider at 30. I think that assumes approximately room temperature for the sensors.
Now, if there is any break in the chain from outside temp sensor to inside temp sensor to temperature control slider, you will get the binary temperature problem. So check that reading, if you're electrically inclined. If it's wildly different, you've found the problem. If not, maybe you'll be lucky and the connector just needed reseating."

mdkelly1

Still had a "marginal" charging issue after cleaning all of the electrical ground points and replacing the battery ground strap. Decided to have the alternator rebuilt and replaced the cooling duct which was pretty mangled.

Charging system now works as it should, even with full-time A/C fans running in the Florida heat. I'm a believer in the need for the cooling duct in a hot environment