• Hello Guest, welcome to the initial stages of our new platform!
    You can find some additional information about where we are in the process of migrating the board and setting up our new software here

    Thank you for being a part of our community!

Heated Seats

bmos

bmos

New member
Joined
Feb 16, 2018
Location
Cumberland, ME
Does anyone have info on how the heated seat temp protection relay works on a 1991 740?
I have non-op heated seats. They have power and the relay under the seat clicks after a second and the seat never heats up. Seems like something is tripping the relays and they are shutting off the circuit?
 
There is a thermostatic control inside the seat. It regulates the temperature so the seat doesn't get too hot. It is usually the point of failure in the seat heaters. It's attached to the grid and you need to remove the seat bottom cover enough to get to the thermostat. It goes open circuit when it fails. Volvo may still supply it. It is also probably available at an electronics supply house. I think you would still want to reference the volvo green book for the temp range used by the sensor.

Edit:Recent thread on this very subject.
https://forums.turbobricks.com/showthread.php?t=353317
 
Last edited:
dl242gt said:
There is a thermostatic control inside the seat. It regulates the temperature so the seat doesn't get too hot. It is usually the point of failure in the seat heaters. It's attached to the grid and you need to remove the seat bottom cover enough to get to the thermostat. It goes open circuit when it fails. Volvo may still supply it. It is also probably available at an electronics supply house. I think you would still want to reference the volvo green book for the temp range used by the sensor.

Edit:Recent thread on this very subject.
https://forums.turbobricks.com/showthread.php?t=353317
Click to expand...

Is the 700 seat heater same-same as 240?

https://www.brickboard.com/FAQ/700-900/InteriorSeats.htm#SeatHeaterFunctionalDiagnosis
 
In researching a related issue, I found thermostatic cut outs could be found on the interned. Looked like a diode, only larger; seems like it could be soldered in line....
 
Temp

BTW the heaters have two outputs (dual output) that pulse. I am not 100% sure on the output temps on of the top of my head:
Below 50 High, below 65 Low, above 65 off
If the cabin is warmed up you will not have any heaters on.
 
My 92 740 with cloth seats have a plunger type switch you activate when sitting. Never saw that before on my other 7/9?s, but all those had leather
 
dl242gt said:
Thanks for clarifying that Art! I've only had experience with the cloth type seat heaters. Not those fancy pants leather seat version.
Click to expand...

In the 240's I'm familiar with, the seat heaters were the same for fancy pants leather cars and for easily washable cloth seats in DL cars. Yet there were changes for '93 in the grid used, possibly made for the same inventory reasons so many other '93 240 uniquenesses occurred. I only posted the FAQ reference because 7/9 cars are covered there better than I've done with 240's. I've learned to stay out of 7/9 threads with any notion my 240 experience applies.

In the 240, there's no relay, no two-level heating scheme, just two grids of nichrome wire wrapped on plastic grids in series with a Klixon thermostat in the butt pad. The terminal on the Klixon is where the wire usually breaks.
 
Do you have a 240 seat heater troubleshooting link? A customer of mine would like to have her driver's seat working again. I have several used seat grids around that I have salvaged over the years.
 
I have had leather seat 240 turbos but the grids and setup in those were the same as the cloth cars. I also changed those cars to cloth seats because they are so much better than the leather. Especially the GL cloth seats. Practically indestructible velour that no car maker will use nowadays.
 
dl242gt said:
I have had leather seat 240 turbos but the grids and setup in those were the same as the cloth cars. I also changed those cars to cloth seats because they are so much better than the leather. Especially the GL cloth seats. Practically indestructible velour that no car maker will use nowadays.
Click to expand...

The velour is indeed nice. I have two 83 Ti with that material. On the subject of seat heaters, I added some experimental modifications about 18 years ago. Installing something like what Dave Samuels offers would be an enjoyable mod as well. The passenger seat surprised a few of my passengers in the day. Cars can provide fun in many ways.

seat145.jpg


seat154.jpg


seat155.jpg


seat156.jpg


seat157.jpg


seat158.jpg


seat159.jpg


seat160.jpg
 
Wow, looks like I got lucky that both of my seat heaters work in my first 740.

Weird that I don't see anything like this on the 1991 740 heated seat circuit diagram (or I'm misreading it). It looks like the green relay on the seat bottom is the thermostatic relay based on wire colors and location. Was leather standard in the US for 1991? Almost had to be or there would be two diagrams.

Looks like my next step is swapping in a relay from my other car to confirm that the relays have failed.
 
Use a test light to test that the relay pulses as it's supposed to. Then you can check that power is going to it and supports the current flow in the circuit. What you are seeing in the photos is a heater upgrade added to the seats. You can see the box.

Since a 740 was a higher line car leather was pretty common. If yours was a GL model there was a chance it may have cloth interior.
 
dl242gt said:
Use a test light to test that the relay pulses as it's supposed to. Then you can check that power is going to it and supports the current flow in the circuit. What you are seeing in the photos is a heater upgrade added to the seats. You can see the box.

Since a 740 was a higher line car leather was pretty common. If yours was a GL model there was a chance it may have cloth interior.
Click to expand...
Should have mentioned that I did that. The switch turns on/off power to the relay but the relay never sends power to the seat. Since the diagram shows the relay having non-standard internals, I wanted to ensure I wasn't going to replace a relay that was protecting the circuit as intended based on a poor assumption.

Is it just a normal relay? How does that protect against overtemp?
 
I took it apart and WOW is there a lot going on in there.
<blockquote class="imgur-embed-pub" lang="en" data-id="a/pDlB3N1" ><a href="//imgur.com/a/pDlB3N1">3523890 Heated Seat Temperature Control Relay</a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>

I tried jumping power and ground to the appropriate pins and measured 0V at the output so I'm reasonably confident its a bad relay.
I'll replace it and update this post for posterity's sake.
 
You must log in or register to reply here.
Back
Top