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About cabin temperature sensor

 
Old 10-31-2012, 10:18 PM
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Bjbpe
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Default About cabin temperature sensor

My '87 S4 will only deliver heat when the slide is all the way over to the right. I suppose that means that the cabin temperature sensor is on the fritz. Are these things serviceable like the outside temperature sensor? I use the car daily so if I'm going to tear the dash apart I'd like to get some idea as to whether I will have to replace it or clean up some things like with the outside unit.
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Old 10-31-2012, 11:01 PM
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Check the outside sensor first. It can be reached in front driverside splash shield by either removing tire, removing inner shield. Or by raising lights, remove cover for driver light, remove light itself, and you can see the fabric corrugated tube with the sensor in it.
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Old 10-31-2012, 11:06 PM
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Thanks for your reply Joe. I've done that and the unit is functioning well. Now I'm on the inside of the car.
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Old 10-31-2012, 11:56 PM
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From Wally's HVAC write up: TROUBLESHOOTING THE SENSOR CIRCUIT

The usual effect of a bad sensor string is “all or nothing” temperature control. If this occurs, the first thing to check is the connection for the outside air temp sensor under the left headlamp. If that doesn’t help, go to the next paragraph.

Remove the left side panel from the central console by moving the driver’s seat all the way back and down, and removing the two Phillips screws.
Locate the setting unit, and remove the inner electrical plug.
Turn ignition switch on, and move the temperature control lever to the lowest temperature setting. Use an ohmmeter to check the resistance between terminals 4 and 12 - the resistance should be approximately 3.7 Kohms.
Move the temperature lever to the highest temperature setting - the resistance should smoothly change to approximately 4.7 Kohms.

If the resistance is infinite (no connection), the sensor string is broken - the most likely place is the connector for the outside air temperature sensor. If the resistance is much lower than specified, there is a short circuit in one of the sensors or in the control head.
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Old 11-01-2012, 02:12 AM
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I'm having the same trouble. My outside temp sensor tested good. Do you have a photo of the pin 4 and 12 area of the setting unit? Do you mean the setting unit above the radio? I've removed that just by the 4 screws above the radio in my 89 car.
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Old 11-01-2012, 06:42 AM
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Charley B
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It's important that you provide all needed information for trouble shooting. For example, how old are you?
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Old 11-01-2012, 11:28 AM
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Hey Charlie:

Gimme a break!! I'm 77 years old so I miss a bunch of things but the car is an '87 S4 like it says in the beginning of the thread. Also, I refuse to give information regarding various parts of my badly shrunken anatomy.

Barney
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Old 11-01-2012, 11:33 AM
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To five elements:

I haven't yet developed the intestinal fortitude to start tearing that dash apart yet. Gonna take a stiff shot of Jack Daniels before I start that litttle task. The gist of my question, however, was "Can you service the unit inside the car like the one on the outside? i.e. clean the terminals, etc.
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Old 11-01-2012, 12:43 PM
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jcorenman
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Originally Posted by Bjbpe View Post
To five elements:

I haven't yet developed the intestinal fortitude to start tearing that dash apart yet. Gonna take a stiff shot of Jack Daniels before I start that litttle task. The gist of my question, however, was "Can you service the unit inside the car like the one on the outside? i.e. clean the terminals, etc.
Yes, it is a temperature sensor (thermistor) similar to the outside sensor in the left front wheel well. Once you get to it, you can clean it and check the contacts.

However, the reason for the advice to do other checks first is that it is a PITA to get to. You need to remove the center console, or at least shift it back a few inches for access. The sensor itself is behind the vent opening, connected to two yellow wires, and attached to a small fan to pull in cabin air. The other connection points are easier to get to, for testing. You are looking for yellow wires in each case.

Do you have a copy of the WSM? The testing instructions (including the resistance test outlined by Wally, above) are on pages 87-113 & 114, along with a picture of the connectors on the setting motor.

Five, the connectors in question are on the "setting motor", not the control head. You get to the setting motor by removing the left (driver's) console side-panel. This is the motor unit that operates the temperature-mixing flaps. More pic's on page 87-115.

Here's a marked-up copy of the wiring for the sensor chain. The test-points are pins 4 & 12 on the setting-motor connector-2 (called "control unit" on the schematic, the control-head is called the "control switch" at the top).

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Last edited by jcorenman; 11-01-2012 at 01:35 PM. Reason: add wiring diagram; correct pin#
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Old 11-01-2012, 01:22 PM
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What Jim says is the best approach here - test the whole loop first - then you will know if you have a problem, (though seems its pins 4* & 12* on the setting motor to me)

Then you can test the individual components.

Head unit only - on head unit module pins 13 & 14

Inside Sensor + Outside Sensor in series - on head unit plug (car side) pin 14 and Setting Motor Pin 4*

Outside Sensor alone - 2 yellow wires under drivers headlamp to sensor in alternator feed hose

Inside Sensor alone - 2 yellow wires on T31 Connector to the Inside Sensor in dash
- This is the hardest one to reach/replace - so test by elimination on the others.

* These are all on the connector (car side) with the largest number of pins (12 pins)

BTW with the head unit unplugged and connector pulled off the setting motor you need to test the resistances between these AND also that there is no path to ground or power from either side.

Alan

Last edited by Alan; 11-01-2012 at 07:27 PM.
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Old 11-01-2012, 01:54 PM
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The temp sensors aren't that hard to test if you access them via the plugs under the centre console, which you can reach by removing one of the carpeted sides (same side as the setting motor), and trace as they connect into the thick loom heading for the setting motor.

Using the wiring colours on the diagram posted, you can check them individually. This also has merit of checking wiring to the sensor(s), though I've never heard of connection problem to cabin sensor, but lots of cases of failed connections to external sensor (at the sensor, not in wiring from cabin).

With the cabin sensor you can put a meter on the sensor then just blow into the opening on dash to check it's functioning. But it may be out of spec - I had heating prob where it would go to full heat at any position off cold override, and it was the cabin sensor had gone too high in resistance.
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Old 11-01-2012, 02:38 PM
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Just like in the other thread of FiveElements, first check that the setting motor is working before tearing into the dash. Its easy to test by moving the temp selector from hot to cold while lying on your back in the drivers footwell and you can see the arm from the motor travel up and down. you can also reach up under the dash and feel the arm from the motor move.
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Old 11-01-2012, 04:04 PM
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You guys are awesome. I am always amazed at the volume of information that I get when I ask one of these inane questions. See my question on a new thread about stereo sound.
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Old 11-01-2012, 06:00 PM
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The setting motor, if the arm isn't moving can be repaired quite easilly.....
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Old 11-01-2012, 07:18 PM
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It is possible that your microswitch for max cooling is not operating right (as in always closed regardless of slider position). This would explain the no heat until max hot position - as would also a short to ground anywhere in the sensor loop for all the same reasons. These are easy to check.

If either of the sensor loop connections on the AC head unit plug (car side) pin 14 and Setting Motor Pin 4* have connectivity to ground (pin 4) - you have an issue.

If the head unit pin 13 has connectivity to ground (pin4) or power (pin 15) when the slider is in the middle of the temperature range you have an issue in the head unit. Switches can be replaced or commonly its just a mechanical activation/alignment issues. The mechanism on these are easy to evaluate its all quite visible...

Basically the head unit failsafes are designed as follows:

If the sensor loop goes open circuit you get max heat except in the max cooling position

If the sensor loop is short circuited to ground you get no heat except in the max heating position

If the max cooling switch is always closed you get no heat except in max heating position

If the max heating switch is always closed - you will get roasted (but you can set fan to 0 and open the windows.

My presumption is that since the German climate is more cold than hot they default their failsafes to ensuring heat is always available somehow... makes some sense - while miserable to be too hot - it could be deadly to be too cold...

Alan
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