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2004 Saab 9-3 Aero Cabrio
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Discussion Starter #1
New to the forum and to SAAB. Very fun car! Have an '04 Aero Convertible, 292 - Metallic Lime Yellow. Lots of annoying electricals to get sorted, but she runs good and shifts smooth. Here's the latest diagnostic venture. Let me know if you have run into a similar issue.

Have a B2172 and B1927 - L/R seat temperature sensor circuit open. Heated seats do not turn on when requested through the Automatic Climate Control (ACC) module. First hack guess is a broken wire somewhere. I can command both heated seats on with a Tech2 and observe the elements warming with a thermal imager. Let's start under the L seat. Mind you I'm 200 lbs and 6' in a SAAB 9-3 and there is NOT MUCH ROOM BACK THERE. Bless you convertible top, for I am not graceful in these situations.

KOEO and Tech2 shows 0V on the sensor circuit PID, which translates to 176*F. Makes sense why the ACC logic will not allow the seat warmers to turn on.
Unplug connector H8-14 which has the two each heated seat elements powers and grounds and sensor power and ground wiring, and Tech2 shows 5V (-40*F). We now know that the ACC module is capable of seeing circuit changes.​
Reconnect H8-14 and backprobe pin 3 (heated seat sensor 5V supply) and find 2.5V. Tech2 returns to 0V. Odd.​
Unplug connector H8-14 and meter reading jumps to battery voltage on pin 3 !?!​

Back up-right and up front, I remove the ACC module from the dash. KOEO and backprobe the larger of the two connectors (K42) at the module.
Pin 21 (Left heated seat sensor 5V supply) - Expecting 5V, reading 12V.​
Pin 31 (Left & Right heated seats sensor ground) - Expecting 0V, reading 12V.​
Pin 23 (Right heated seat sensor 5V supply) - Expecting 5V, reading 5V.​

I have voltage on the ground leg that is throwing both sensor signal circuits out of range. No wonder the car thinks the seats are on fire.

Disconnect connector K42 at the ACC module, which is still powered and grounded from the other, smaller connector. Still KOEO and testing pins directly at the module
Pin 21 (Left heated seat sensor 5V supply) - Expecting 5V, reading 12V.​
Pin 31 (Left & Right heated seats sensor ground) - Expecting 0V, reading 0V.​
Pin 23 (Right heated seat sensor 5V supply) - Expecting 5V, reading 5V.​

Suspecting ACC module failure at this point.

Remove and disassemble the ACC to expose the PCB.
Found an open circuit between K32, pin 16 (12V Battery) and K42, pin 21 (L heated seat sensor 5V supply).​
Found 14.7M ohm circuit between K32, pin 16 (12V Battery) and K42, pin 23 (R heated seat sensor 5V supply).​
Found 547 ohm circuit between K32, pin 15 (module ground) and K42, pin 31 (Heated seat sensor ground).​

Next stop... LKQ, because for $16... why not? Plus my headlight switch is kinda floppy because the metal tangs are long gone.

Stay tuned SAAB fans.
 

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Most often it is a short in the heating pad. Sometimes you get a little burn spot in the seat too.
 

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2004 Saab 9-3 Aero Cabrio
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Discussion Starter #3
Agree, a common pattern failure, and where I thought I would wind up when I started down this path.

But... I can command both heated seats on with Tech2 and they work perfectly, both seat and back.
I can also light a 4A headlight through the heater circuits, so I know I don't have a wiring quality issue in the element.
And... I should have 5V on pin 21, not 12V. My working theory is that the 10k Ohm resistor for pin 21 on the PCB is bad, allowing it to send 12V through the L seat temp sensor, which returns to the ACC on ground pin 31 and knocks out the R seat sensor reading as well. Also my sun sensor and internal ambient temperature sensor have maxed out readings on Tech2. They all share a common ground splice, J76.

I could de-pin the heated seat sensor 5V circuits from connector K42 and see if the sun sensor and internal ambient temperature sensor report normal values on Tech2 just to confirm...
 

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My working theory is that the 10k Ohm resistor for pin 21 on the PCB is bad, allowing it to send 12V through the L seat temp sensor, which returns to the ACC on ground pin 31 and knocks out the R seat sensor reading as well.
The 10k resistor is not a dropping resistor for 12V. The ACC computer (like the other computers in the car) runs at 5V. The resistor should be connected internally to a 5V regulated supply for the computer. If that 5V is now 12V, it means a voltage regulator has failed in the ACC or there's an internal short elsewhere, and that A/D input is probably fried and always reads full scale.

EDIT: This assumes the seat thermistor itself is good.
 

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Discussion Starter #5
So you're saying the entire module takes it's 12V and steps down to 5V at the input, rather dropping to 5V at an output?
 

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Some Most of the stuff the ACC controls are 12V parts, like the damper motors and seat heaters. So the power switching and control output parts of the ACC (and most modules) just run at 12V. The internal computer circuits are what runs at 5V (usually including all the input circuits) and they have voltage regulation inside the module to supply that.
 
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Discussion Starter #7
Follow up. Replaced and programmed the ACC with a junk-yard unit. 5V at the left seat sensor circuit. Reading actual values on Tech2. Heated seats turn on normally.
 
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