I just fixed my HVAC system.
Condition , with the temperature slider in any position other than fully cold it would blow hot air. this indicates that the sensor string is broken . There are 2 sensors one under the front fender and the other one is on the dash in the little round grate next to the central air outlet. The most likley part to check first is the the one under the front fender, it is also the easiest. to do this pull the headlight up on the driver side remove the headlight cover remove the screws holding the headlight bracket to the headlight support remove the headlight bulb connection. You will now have access to the temp sensor that is attached to the front side of the fender block off panel. find the connection disconnect the black connector remove the sensor from the flexible tube 2 hose clamps or if your good try squeezing the plastic edge together on the inserted sensor it should pull out of the metal tube with an OHM meter check for resistance while holding the sensor in your hand if its good you should see the resistance changing. If this is good then get the other end of the connector that is part of the cars harness note the position of the wires going into the connector mine had 2 yellow wires, one with a green stripe note that the backside of the connector has A and B positions mine had the green stripe on the left side of the line up mark made on the shaft. Gently pull the wire out of the rear of the connector they should both pull out , with a small diameter nail gently push into the rear of the connector and the barrel will push out the front remove both barrels clean the wires and the connectors put on some solder flux and solder the barrels onto the wires reinsert the wires thru the rear of the connector using the small nail they should reseat , clean the pins on the sensor, and reconnect make sure that you watch the rear of the car side of the connector to make sure the wires dont push back out . Install the seansor into the metal housing secure the harness so it wont get hit with the headlight.
Reinstall the headlight, go for a ride and enjoy your HVAC system working as it should. I am not sure why the wires are not connected any better than the way they are , but it will only take one time of slight pulling on the wire when the block off panel is removed to disengage these wires, and the resutl will be a broken HVAC system I have some picture but cant load them if anyone will post them for me i will E mail them to you, Thanks

Here is a link to replace the thermistor in your external temp sensor
https://rennlist.com/forums/928-foru...tor-value.html

 

Here's the pics - I'm uploading for Mrmerlin.

 

Thanks Perx, a picture is worth a thousands words, this harness will be found on 88 S4`s and possibliy earlier years, the wires really do just pull out of the barrels and once they do they wont go back in without a fight, Stan

 

True...one reason.

This is what i just did last week. Part of my annual "summer HVAC" stuff. I wont be needing heat here in vegas for the next 5-6 months so i completely block of the air supply from the heater core. The area is shown in red on the pic. The metal you see back in there is the heater core. No matter what the condition of your heater valve you WILL get hot water passing in that core over time....and no matter how good your HVAC some of that hot air will mix with the cold air from the AC system. I need all the cold air i can here in Vegas.
Although its not illustrated i simply place a strip of aluminum "speed tape" ( duct tape will work as well) over the opening.
Not an ounce of hot air ever comes out of my HVAC any more. Short of by- passing the heater core with some hose, Ive found this to be the easiest for me. I get INSTANT cold air after start up no matter how long the car has been sitting.
YMMV.


Your looking straight into the center vent with the grill removed.

 

Nice tool. Details?

 

That's a surgical sponge forceps. Sometimes called a Forester (sp?) clamp. It has ridged but non-cutting grip surfaces and lots of uses. In surgery sure, but also as a handy grabber. Don't know where you'd get one on the street.

 

the holdg tool, its some kind of tool used in the operating room to clamp off blood vessles (I was told) since i am not a DR I dont know for sure but they are about 12 inches from handle to the clamping end and they have a ratchet type connection by the finger holes

 

A local swap-meet tool vendor offers them, along with plenty of other interesting and handy medical stuff, between $4 and $7 an item. I usually negotiate a three-for-ten-dollars deal on a few dozen pieces at a time. Dental hooks, forceps (what you have), hemostats (used to clamp off blood vessels), plus all kinds of other interesting scissors, hooks, clamps. Just too handy for doing car fixin'.

 

Haemostats are invaluable everywhere - used frequently when you run out of hands or fingers to do something - car stuff, modelling stuff, soldering etc.
jp

 

Tony
I see your comb flap is "down" i.e no vacuum......so this directs more airflow to the side vents (or footwell-defroster if open)....... Mine is always like this sense it doesn't work....but really doesn't seem to do much anyway...so I just leave it alone!

I like the idea of duct tape to increase cooling...can you do it just by pulling the main vent...it looks like you can?

Stan
That temp sender does look easier than the one in the dash.....pulling the console is a PITA

 

The comb flap is designed to assist in the mixing of the hot and cold air within the air box, so that you get a more constant temperature, depending on selection of the slider, however it should be fully open when required maximum cooling. The WSM HVAC test procedure fully describes the operation of the comb flap for the various slider selection for air distribution.

When I cleaned the inlet surface of my evaporator and got the comb flap operating correctly it made a great difference to the air flow.

To assist in cooling in the summer I installed a spherical shut off valve into my heat supply circuit, just after the "Y" piece and the heater core inlet. This stops the engine jacket cooling water from flowing through the heater core, however, when the engine is shut down the vacuum operated heater valve fully opens on loss of vacuum which allows the hot jacket water to 'thermocycle' to the highest point, the heater core, so on start up I get some hot air initially. The duct tape sounds a more simple solution.

I don't believe that the fitting of the shut off valve has a depremental effect on the heater core due to corrosion at the air/water interface with in the core, as the core will semi drain back into the engine jacket water system when the engine is shut down any way, as the jacket water cools and finds it own level with the system creating an air/water interface.

The "duct tap" looks to be a good simple option and will ensure full circulation of treated jacket water through the heater core when the engine is running.

I will give the duct tape a try and see the effect with the shut off valve open.

Tails 1990 928S4 Auto

 

I had a '73 Firebird and the heater core went.. Had a local shop install a new one.. took them quite sometime if I remember right. Anyway, what the guy told me after was interesting.. He suggested that in order to prolong the life of the new heater core, that once a month I should crack open the header and let it flush through. Never heard that before and not sure I believe that coolant sitting in the core will cause any undo wear, but still that always sits in the back of my mind.

 

Lol that was the first thing I was wondering when I saw that picture. I can't count how many times a tool like that would have come in handy.

 

Why not just wire tie the heater bypass valve open This shuts all the coolant from entering the heater core, this comes from experience in Florida. One note if vacuum is lost by shutting off the car and the Heater Bypass is not tied off hot coolant flows making for a warm start up. If you try connecting the bypass valve to a direct vacuum source every time you get on it you will be reminded by a blast of hot air.

 

Makes a good roach clip, too.

So I've heard...