# Help understanding this car seat heater diagram

#### john.r2

Mar 27, 2023
18
In my bench test of the black box where I have a light bulb hooked up to the red and black output of the black box simulating the heating element, while providing power to the TH pin so that the bulb comes on, if I was to then short the two pins that are supposed to be connecting the thermistor to the black box then technically the black box should cut or lower the power to the bulb right? Because as the temperature increases the resistance decreases of the thermistor. So shorting it is essentially being resistance down to 0 which essentially should make the black box believe it’s very hot. Not sure if that’s correct but thought I’d ask first

#### john.r2

Mar 27, 2023
18
OK, so i've managed to get hold of a more colourful and in-depth diagram of the right seat (old model) Which i've attached below. I've removed the diagram from the illumnation circuit going to ILL+ as that diagram is a another big diagram itself related to relays that turn the backlight on. And also i've left out the bottom connections on the heater switch as those connections are related to cooling seats where there is a fan activated by another circuit and that all works fine. So for the purpose of trying to figure out how the heated seats are working on this, you can see that the main heater switch (which is a switch with a dial between 1 to 3 setting for temperature) sends two wires to the heater control (black box). The heater control black box has it's own 15A fuse and earth connection which it uses to heat the heating element when the correct signal is received from the switch. Now the question is, i've figured out that the wire that goes from the heater switch to what is called THL/THR (L for Left and R for Right) gives a varying voltage between 10 and 12v which lets the black box determine how hot to make the heating element. So unless there is a voltage coming to TH the heating element isn't activated.

What I don't understand is what role the SW connection makes to the black box. Because according to the diagnostics document SW is supposed to give a voltage of 12v when heater switch is turned on regardless of what the temperature dial is set to. So TH gives a voltage range between 10 and 12v depending on heat setting and at the same time SW gives 12V. So essentially the two work together. However, when i did the bench test by connecting a bulb to the output, i was able to make the bulb light up just by providing the 10-12v on the TH line.

#### Attachments

• Old-SeatHeaters.png
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#### Delta Prime

Jul 29, 2020
2,067
OK, so i've managed to get hold of a more colourful and in-depth diagram of the right seat (old model) Which i've attached below. I've removed the diagram from the illumnation circuit going to ILL+ as that diagram is a another big diagram itself related to relays that turn the backlight on. And also i've left out the bottom connections on the heater switch as those connections are related to cooling seats where there is a fan activated by another circuit and that all works fine. So for the purpose of trying to figure out how the heated seats are working on this, you can see that the main heater switch (which is a switch with a dial between 1 to 3 setting for temperature) sends two wires to the heater control (black box). The heater control black box has it's own 15A fuse and earth connection which it uses to heat the heating element when the correct signal is received from the switch. Now the question is, i've figured out that the wire that goes from the heater switch to what is called THL/THR (L for Left and R for Right) gives a varying voltage between 10 and 12v which lets the black box determine how hot to make the heating element. So unless there is a voltage coming to TH the heating element isn't activated.

What I don't understand is what role the SW connection makes to the black box. Because according to the diagnostics document SW is supposed to give a voltage of 12v when heater switch is turned on regardless of what the temperature dial is set to. So TH gives a voltage range between 10 and 12v depending on heat setting and at the same time SW gives 12V. So essentially the two work together. However, when i did the bench test by connecting a bulb to the output, i was able to make the bulb light up just by providing the 10-12v on the TH line.
I have two seat heater diagrams side-by-side
The two diagram you have submitted are arguments; composed of one incorrect premise and an preconceived ultimate conclusion.
You are not drawing inferences from the data we are providing you.

#### john.r2

Mar 27, 2023
18

The two diagram you have submitted are arguments; composed of one incorrect premise and an preconceived ultimate conclusion.
You are not drawing inferences from the data we are providing you.
I am only able to take screenshots as the diagram is paid access on Toyota's Tech portal I got 30minute access for a fee and was able to get this diagram and took a screenshot. This diagram is for the old model seat (the one on the right of the original diagram I first posted)

#### john.r2

Mar 27, 2023
18
In my bench test of the black box where I have a light bulb hooked up to the red and black output of the black box simulating the heating element, while providing power to the TH pin so that the bulb comes on, if I was to then short the two pins that are supposed to be connecting the thermistor to the black box then technically the black box should cut or lower the power to the bulb right? Because as the temperature increases the resistance decreases of the thermistor. So shorting it is essentially being resistance down to 0 which essentially should make the black box believe it’s very hot. Not sure if that’s correct but thought I’d ask first
I tried this and shorting the two thermistor pins turns off the bulb

#### ivak245

Jun 11, 2021
111
Because you have presented it with an extremely low resistance which it interprets as a very high temp. Put a 10k or 8.2k resistor in there instead, the unit should keep supplying power to the heater.

#### john.r2

Mar 27, 2023
18
Ok so I wired the new seat with the old seat black box like the picture below. It’s all working now. Only thing when I was testing is that the inbuilt thermistor on the old seat was reading a resistance of 10.55k ohms in room temperature and decreases as heater heats up, gradually going towards 0. Whereas the new seat thermistor starts at 8.55k ohms. It’s working good but I’m assuming the new thermistor will possibly tell the seat controller to shut down earlier than the previous one would have.

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