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Help understanding this car seat heater diagram

john.r2

Mar 27, 2023
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I have two seat heater diagrams side-by-side. These are 12V car seat heater diagrams and I wanted some help understanding the difference between the two and the left would activate and heat up vs the right. Both seats have the Ignition, Ground, TH and SW connections. The SW connection if i'm not mistaken goes to some relay but there is 1 connection that is only on the left side and thats the P connection.

I just wanted to try and figure out how i can make the left one work and heat up based on the right one's wiring. I have in place the wiring from the right hand side diagram but I have the heated seat system of the left side which I cannot activate without the P connection.

Is there a way to bench test it?

Any help or advice would be great

Thanks

SeatHeater.png
 
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Bluejets

Oct 5, 2014
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Insufficient detail..........include complete schematic.
i.e. this.......... "o" is not a drawing of a motor vehicle
 

ivak245

Jun 11, 2021
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You will have to figure out where the thermistor goes on the LHS circuit, whereas the RHS circuit uses a thermistor which is directly connected to the controller. That also has a "TH" connection, so that may be some sort of heat control. Need more of the complete car wiring diagram(S) to figure this out.
 

kellys_eye

Jun 25, 2010
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A lot of problems associated with this form of wiring is that the heating control isn't just a simple switch affair - it's often controlled by a CAN Bus signal via a module locate in the seat itself. As a result you can't just apply a 'voltage' to the SW line and expect the heaters to work.

There will be all sorts of feedback (to prevent burning/overheating etc) as well. Unless you can find the actual elements themselves and make a hardwire connection to them - then apply the appropriate control and safety signals - you are not going to get these working in a simple way at all.
 

john.r2

Mar 27, 2023
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ok so i compared both seats physically and checked the wires. I'll refer to the seats in the diagram as left and right (left being the newer one with the 5 wires and the right being the older seat with the 4 wires which i have connections for)

Both the seats have 4 wires going from a small black box called the Seat Heater Control Assembly which is mounted under the seat very close to the main connector. This box looks a bit like a relay of some kind. The right seat has 4 wires coming from the main connector via a disconnectable plug to this black box and then from the black box there is a 4pin connector with 4 wires going to the seat heating elements.

On the left seat, there is also a identical 4pin connector that goes from this blackbox to the heating elements of the seats. However, the difference is this blackbox does not have the 5 wires coming from the main connector to a seperate input connector. Instead, the 5 wires related to the seat heating seem to come from the main connector and go inside a black conduit, wrapped in a lot of insulation tape and then it's somehow linked to this same 4 pin connector that is the output to the heater elements. But it appears there is only 2 wires going to the seating element itself. Whereas on the right seat, there is 4 wires going to the seating element system.

On the left diagram, that zigzag component between TH and P seems like it's incorporated into the wiring loom and taped up because it doesnt actually go to the blackbox like it does on the right side seat. What is that zigzag component? and how does it differ from it's function on the right seat?

So, what i'm trying to figure out is how to adapt it so I can transfer the blackbox over from the right seat to work on the left seat so that the cars original 4 wired control is able to operate the seat heaters.
 
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ivak245

Jun 11, 2021
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The component would be a thermistor (thermal resistor), to provide temperature feedback to the controller.
 

Delta Prime

Jul 29, 2020
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the 5 wires related to the seat heating seem to come from the main connector and go inside a black conduit, wrapped in a lot of insulation tape and then it's somehow linked to this same 4 pin connector that is the output to the heater elements. But it appears there is only 2 wires going to the seating element i
Confidence is low a viable solution to your inquiry will be successful. The lack of information.The uncertainty of your description no one who is competent will be able to help you unless they have the unit right in front of them.You cannot "perhaps around". But then again you could be a philosopher & that won't work either. A schematic is the language of electronics spoken in the house of Maker Pro.
 

kellys_eye

Jun 25, 2010
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The blocks labelled 'seat heater control' are the stumbling...errr, blocks.

Switching, temperature control and safety protection (in case of over heating) are all achieved by those blocks via CANBus signals and your vehicles main computer. The only way you can 'bypass' them is to access the heaters and thermistors directly and wire your own controller to them.

Sophisticated CANBus reading equipment could give you a route to conversion but there is no simple swap without causing potential problems.

Manufacturers are increasingly making each and every car accessory 'unswappable' by the inclusion of processor modules dedicated to the task. Controlling such tasks as heating seats is simple - VERY simple - by ordinary means but that would allow 'you and me' to do whatever we wanted and thus bypass the lucrative 'stealership' route of charging you an absolute fortune for a simple wiring procedure - so the manufacturers are deliberately forcing users to stealership solutions and this is a simple, but good, example of what we're facing.
 

john.r2

Mar 27, 2023
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OK, so I did some more digging and physically tracing the wires on both seats. So on both seats it appears there are only two wires (red and black) that actually go to the heating elements of the seats from the black box.

On the right seat. There is two additional wires from the black box going to what appears to be an embedded thermistor in the seat itself. So that thermistor provides the feedback back to the black box itself. On this same seat there are 4 wires coming from the car loom. 1 is ignition from battery via 15a fuse. 2 is ground. And the other two come directly from the seat heater switch unit itself. This is a switch on the centre console.

The other seat on the left works very similar in that there are two wires (red and black) going to the heating element. But where it is different is that the thermistor which is also integrated into the seat does not directly feedback to the black box on this seat. Instead the thermistor is connected back to the heater control switch on the centre console. Therefore the difference between the two is that the right seat uses the black box to regulate the temperature based on the feedback it receives from the thermistor. Whereas on the other seat, the regulation seems to be happening directly from the heater switch itself. It explains why the two blackboxes are slightly different size. The one that’s doing the regulating is big and the other is very small.

Based on this. I’ve modified the drawing attached below with the proposal to transfer over the. Black box from the right seat to the left seat and connect the thermistor to that black box.

If anyone can kindly have a look at the diagram I’ve drawn below. Does it look right?

Also, are thermistors polarity sensitive? Because I would need to figure out which way round to connect the two thermistor wires to the old black box

Thanks
 

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john.r2

Mar 27, 2023
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Also, wondering what the dotted line on the thermistor is on the right seat
 

ivak245

Jun 11, 2021
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The dotted line means that it is sensing the temperature of that part of the circuit, i.e. it is probably attached to that particular heating element. If you short "TH" and "SW" the heater should work. Just check that with power applied through the fuse that you have something (probably 12v) at "TH" or "SW".
 

Delta Prime

Jul 29, 2020
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Since we're talking generalities.No leather seats for you. See if you can follow this.
photo_1694307970975.png
 

kellys_eye

Jun 25, 2010
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Without some form of feedback - provided by the thermistor and controlled by the 'box' - the heating elements will just continue to make the person in the seat hotter and hotter. That person would have to regulate the heat themselves by switching the system on/off.

For the passenger this isn't a problem but for the driver it's a distraction therefore the feedback circuit is applied to their seat to stop that distraction.

On top of the issue of distraction is the issue of safety and that's another function of the thermistor - to create a signal to switch the system off in the event of over heating.
 

john.r2

Mar 27, 2023
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Thank you for all your replies and help. I've done some further testing on the old model seat (the right diagram) and i also found manufacturer diagnostics notes on what to check when seat heater isn't working. According to the diagnostics procedure, SW and TH are coming to the black box (Known as Seat Heater Controller) and the diagnostics procedure says to check that with the ignition turned on, to turn on seat heater and adjust setting between 1 and 3 and that apparently TH should change between 10V and 12V depending on the 1 to 3 volume setting. Whereas SW is supposed to be constantly 12V as long as seat heater switch is turned on.

So going based on this info, i decided to disconnect the black box and do some bench testing just on the black box. I attached a 5W car bulb to the red and black output wire from the blackbox (simulating the heating element) I then left the two middle pins on the output that is supposed to be connected to the thermistor disconnected. I then supplied ground and +12v to the input power pins and as it should be, the bulb did not turn on. I measured the red output wire and the ground input wire and i get 12V reading. But if i measure the ground on the output, then I don't get a voltage reading. This leads me to believe it's switching the circuit on the ground. So, I then supplied via a voltage adjuster between 10 and 12v to the TH input pin and surprise surprise the light bulb lit up and would dim as i lowered the voltage. Strangely though, I have no idea why it needs the SW which needs to also be constant 12V when seat heater is ON. But on the bench test it was not making any difference.

I then decided to provide the actual seat heating element with 12v and ground connection via the red and black wires going directly to the heating element. Before I did this, i measured the resistance between the two pins that is supposed to be from the thermistor coming back to the black box and this resistance measured at 10.55k. Once I turned on the power to heat the heated seat element, the seat began getting warm and the resistance reading began to drop slowly.

So this is my findings so far with the right diagram older model seat.

Any advice on how I can get the right system power the left system would be great. Will my proposed idea of transferring the black box over and connecting the thermistor possibly work?
 

Delta Prime

Jul 29, 2020
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and i also found manufacturer diagnostics notes on what to check when seat heater isn't working.
It's Billy goat gruff time!
Pay toll to the troll.. care to share the name of the manufacturer make,model, year.?
 

john.r2

Mar 27, 2023
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It's Billy goat gruff time!
Pay toll to the troll.. care to share the name of the manufacturer make,model, year.?
It’s a Lexus IS 2010

Does anyone know if I was to use the existing thermistor on the new seat to feedback to the old seat controller/black box, is there a chance the resistance parameters could be different between the two thermistors? For example let’s say on the old seat the thermistor read between certain values and that’s what the black box uses to adjust heating but on the new seat if the parameters are different then technically it could throw the black box into thinking something different. Or do all thermistors give same readings relating to temperature?
 

ivak245

Jun 11, 2021
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Thermistors can give different resistances to each other. They are usually identified as "X Ohms at Y degrees C". They are also available as NTC -Negative temp co-efficient-(as yours is) and PTC Positive temp co-efficient. Car manufacturers never want to spend more than they have to, so both thermistors are probably the same, but you can check if you can get into the other thermistor, and compare room temp. resistances.
 
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