Saab seat heater diagnosis

This article focuses on the saab adjustable seat heater. This is the system that has an in dash switch with 3 temperature settings. If you have a classic Saab 900 or a Saab 9000 model you may have this type of seat heater. As with many electrical systems the best way to diagnose is to get your wiring diagram out, and find a easy to access spot that most of the circuit runs to. In this case we will start our diagnosis at the plug for the in dash switch. You will need your volt /ohm meter to run these tests. The switch on your dash supplies current to a heating pad under the upholstery of your seat, it's like the electric blanket you may use in your bed.

Before I get to the diagnosis I want to describe how this system works. There are two heating pads in the seat one for the bottom and one for the back they are wired in series. This means if there is a break in the wiring of one heater pads both stop working. There is a ntc resistor (this is a resistor that goes down in resistance the hotter it gets) embedded in the lower pad to control temperature. This ntc resistor is wired to the in dash switch/relay. Inside the switch is a relay that switches off the current to the heat pad as it gets up to selected temperature.

With all electrical diagnosis you will need the wiring diagram for your model. With a diagram and this procedure you should be able to pin point the problem in no time. If you have a classic Saab 900 first check fuse #12 on the Saab 9000 check fuse #10. Remember to not just look at the fuses but make sure they are hot with 12v with the ignition on.

The next thing you will do is pull the in dash seat heater switch out to access the plug socket. When making measurement probe from the back of the plug to avoid damaging the small metal contacts inside.

1. Check terminal #2 you should find battery voltage here with the ignition on. This is the positive supply from the fuse. If this is dead and the fuse is powered up you must trace your wiring back to the fuse box. The classic 900 has a connector between the fuse and switch above the hood release handle this should be checked.

2. Check pin #9 for ground this you will test with your ohm meter you should have continuity from #9 to chassis ground. Near zero ohms 1 or 2 ohms will not keep this from working. If you find no ground trace this through the connectors to it's chassis connection (900 this will be near the parking brake under the center console, 9000 on left front seat member).

3. Now turn on the switch and check for voltage on pin #10 this is your power supply out to the heater pad. If you find no power here you may have a bad switch or a bad ntc resistor. If in step 4 you find the ntc resistor good most likely you have a bad switch.

4. Next we will check the ntc resistor this should be done with the switch removed from the plug. Place your ohm meter probes across pin# 6 and pin # 9. What you are checking for is that you don't have an open circuit (open refers to the circuit having no continuity).You should have some resistance but your reading will vary depending on the temperature of your ntc resistor at 70 degrees you should have around 1000 ohms. The main thing is that you don't have an open. If you find an open circuit here you must change your lower seat heater pad.

5. You now want to test the seat heater pads themselves. This is a continuity test with your ohm meter. Check across pins # 10 and # 9 with the switch removed from the plug. This test is similar to the ntc test. A good reading would be 2 - 3 ohms (with an older pad you may find up to 5 ohms). If you find an open circuit here you have a broken wire in one of the seat heater pads. You will now have to remove the seat pull back the upholstery and replace the offending heater pad.

Most of the time when I diagnose a seat heater I start at the connector under the seat. Turn on the switch and check for 12 volts on the connector under the seat. If I find power I know one of the seat heater pads is open, but if I don't find power I go to the switch and make the checks above. The most common fault is the seat heater wiring burning or breaking but do the diagnosis and have confidence in your repair. If you have problems with the back lighting in the switch pin # 7 is power from the dash rheostat and the bulb is grounded pin # 9. If your bulb is burned out, you now can get a small kit from Saab you must solder the new one in.

(This article is meant to educate a consumer,or as a guideline for professionals. You can cause serious damage to your vehicle and/or cause yourself injury. Only those qualified should attempt repairs. I do my best to assure that the above info is correct but take no responsibility for any damages incurred.)