Art20c

Problem that I began experiencing since this fall is that no matter what lever position is chosen for temp on HVAC control unit, it always blows hot air.

Today I pull everything apart according to John Pirtle's instructions on

http://members.rennlist.com/pirtle/svc_act.html

This is what I found:

When control unit is turned off (both levers are at the left position) and all the lines disconnected from HVAC Vacuum Console, except black (which is vacuum feed), I got vacuum coming out for Orange (central flap) line and for White (heater valve) line.

When air direction lever is moved to any other position, with temp lever at 65 position thus turning climate control on, vacuum for white line disappears, there is no suction from white line solenoid. I didnt hear any "click" from that solenoid at any time, even when I turn climate control off.

It was very hot inside the car, so I guess, control unit should react to that by sending command to open vacuum line for heater control valve to close water flow. It aint happening here. I dont feel any vacuum for heater valve.
What could be a problem, is it possible to check climate control unit, could it be bad HVAC Vacuum Console, etc...?

87 S4 Auto

928ntslow

So you checked the vacuum actuated hot water valve for the heater (under the air box) and it holds vacuum?

If you did, you need to check all vac lines one by one. To be honest, I am kind of confused by some of the things you had mentioned. None of the lines will draw vacuum. You need to attach a Mighty-Vac to each line and see if they hold vacuum for about a minute or so. 5psi is the level to check. If even one line (usually DEF) does not hold vacuum, then you end up with an unsealed HVAC system and you need to plug the nipple on the solenoid or replace the actuator.

I just did this to a car that always blew hot air not matter what position of the control levers. Now it is vacuum tight and all operates properly.

BTW, all levers to the left is OFF...but there is still vacuum....and will still blow hot air if there is a leak.

Fixing the HVAC leak will also create more vacuum for the engine...which is a good thing!

Art20c

Let me clarify:
When control unit is turned off (both levers are at the left position) and all the lines disconnected from HVAC Vacuum Console, except black (which is vacuum feed), I got vacuum coming out for Orange (central flap) line and for White (heater valve) line.
I meant that the suction was from nipples on solenoids, not from lines.

So, if there is no suction from heater valve solenoid with climate control on, there will be no vacuum in white line?
How strong suction should be from heater solenoid with climate on?

928ntslow

In climate control, the solenoid will fire depending on the air temp inside and outside. Of course you should make sure that both of those are operating properly or you could be chasing your tail. When the lower lever is OFF, there should be vacuum to the white line. make sure the AC button is off too.

There should always be vacuum to the heater valve unless defeated by the control head.

Charley B

It's not clear to me what you did here. My first test would be to push the heater valve arm all the way in, then plug the vacuum port and let go of the arm, if it drops you have a bad heater valve, if it doesn't, then check the line for proper vacumm, if that's OK, then you have to move to the HVAC controls to find the propblem.

Art20c

When lower lever is off, there is good vacuum to white and orange lines.
When lower lever is in any "on " position, suction from heater valve solenoid stops, or at least it is so little, that you can not feel it.
From what I have read about HVAC system, if there is a problem with any of these two temp sensors (outside and cabin) , then "all or nothing" condition applies. I have hot air blowing all the time, whether temp control is set to low or high or anywhere in between. I didnt check them though.
Any suggestions how to check them, obviously there should be the way to measure an output from them to control head?

Alan

I think you may be chasing the wrong thing. If your AC worked and its very hot all the time - its probably the sensor loop that has failed.

Verify you have vacuum for sure (it seems you do) - but to get full heat you need the hot water valve open (vacuum failure can get you this) but you also need mixing flaps to full hot and only a sensor loop failure will get you that. Verify the resistance in the total loop.

The entire sensor circuit can be checked for general connectivity on the HVAC main control unit (not the head unit) operating motor after removing the console side cover on the driver’s side. Pull off the 12-pin plug on the control unit operating motor - check resistance between male plug terminals 4 & 12
The absolute resistance value will vary depending on actual inside & outside temperatures but there should be ~1000ohm variation in resistance between the cold 65F and hot 86F temperature settings on the head unit. If this is not correct – test the head unit potentiometer. If the circuit has infinite resistance there is an open circuit connection problem somewhere in the whole loop - test the individual sensor connections.

Alan

Art20c

Where is HVAC main control unit located? I have side panel removed, and dont see anything with 12 pin plug.

Art20c

OK, I found this thing, I was behind driver side shelf under instrument pod. It has a steel arm to which metal link to flaps is connected.
When arm is up, there is no suction from heater valve solenoid, when arm is all the way down, heater solenoid makes clicking noise and suction begins. I found this setting unit with arm up high to max.
The only problem is that when I move temperature control lever from high to low, arm doesn't go down, the only way to bring it down is to pull it down manually with fingers (very painful procedure). So, when it is down, as long as temp lever stays at low temp or up to half way up, this arm is sitting at lower position, everything is fine, I got suction to heater valve. But when temp is set to high, arm starts turning up, then I set temp lever back to lowest temp, and arm stops turning but does not return to its low position where it is shots off the flow of air thru heater and sends signal to heater solenoid to open vacuum to heater valve.
I did check resistance on terminals 4 and 12. It was around 1 kohm temp set to high and 4 kohm temp set to low.
Main setting unit is bad?

Alan

Something IS wrong with your sensor loop - check the resistance again to be sure (there should only be a 1K ohm difference between highest & lowest temperatures).

You may need to check the inside & outside temperature sensors and the head unit resistances standalone.

Alan

Art20c

Alan, how to test these sensors? I will recheck the resistance again tomorrow. Spent all day in garage. Very tired. One more thing, where I should check resistance, on wire side of the plug, or on main unit? I took measurements from wires, yellow to yellow.

Art20c

I rechecked resistance between yellow (4) and yellow (12) wires. Lever to cold - 4K ohms, lever to hot - 5K ohms.
Noticed one thing: yesterday, there was very hot inside the car with heat running and all, and metal arm on main unit started to move up once temp lever was moved very close to hot, 2/3 of the way from low to hot settings, this arm wouldn't react. Today, when I tested resistance again, there was cold inside, and arm was moving as soon as I move temp lever from cold position. So I assume that inside temp sensor is OK, given that it knew when it was warm or cold inside.
I'm running out of ideas here, what could be wrong?

jpitman2

99% it will be the external sensor Been here in the last year. In front of the left front wheel cover. metal tube clipped in the cool air hose for the alternator. Two pin connector not far from the sensor. easy for it to get corroded or dirty in there. Try the simple fix first - separate the connector, clean the male pins, then the sockets with a small drill or wire. Reconnect, test AC system. If that doesnt help, check resistance up at the control unit plug. I found in testing the interior sensor, that once you have a meter on the leads from it (left side cover off console), just blowing through the vent will show a change if the sensor is working.
jp 83 Euro S AT 51k

Alan

Well if you now see a variation of only 1K ohm between hot and cold - seems its working OK - before you said 1kohm and 5kohm which was not OK. Given the reading differnces its possible you have a poor connection somewhere? Moving the lever from hot to cold should cause movement of the mixing flaps as you now see happening...

So maybe Its OK now? I'd check what vaccum you have to the hot water valve - it should be on (for valve closed) when the temp is at min and off (for valve open) when the temp slider is at max.

The other vacuum actuators really just control air distribution & secondary mixing.

Alan

Art20c

Alan, movement of the mixing flaps is unidirectional, from down (cold setting) to up position (hot setting), but it doesn't go back. Lets say, settings motor arm is down (cold setting) and I move temp lever from low inch to the right and back to low. Arm will immediately begin to go up and then stop and stay right there, when lever returned to low. It will just not go back and there is no suction from heater solenoid.
Vacuum is on only when this mixing motor arm is down what could be achieved only manually by pulling it down.
What sends signal to solenoid - temp lever or mixing motor? In my case it looks as temp lever sends signal to mixing motor and then mixing motor suppose to send signal to solenoid to open, only problem is that mixing motor is not listening to temp lever to go back to cold settings, thus there is no changing in air temp from heat to cold.