Making
your Seat Heaters Hotter (Range Rover 4.0/4.6/P38)
Introduction
Diagnosis
Disassembly
Temperature Testing
Replacing the Thermal Switch
More Information
Introduction
Brett Allison reported that "After enough complaints from my wife that the seat heater wasn’t working, I decided to dig into the problem with my heaters. I found that my heaters did in fact work, but they didn’t get very hot. It turns out that a simple, cheap electrical part swap can make your seats heat up to a higher temp. My car is a 2001 Range Rover. Keep in mind that other model years could be different."
Brett has kindly provided the following commentary and illustrations covering the operation.
More Information
Main Heated Seat Diagnosis and Repair Page
4.0/4.6 Heater Element Repair Page.
Introduction
Diagnosis
Disassembly
Temperature Testing
Replacing the Thermal Switch
More Information
Introduction
Brett Allison reported that "After enough complaints from my wife that the seat heater wasn’t working, I decided to dig into the problem with my heaters. I found that my heaters did in fact work, but they didn’t get very hot. It turns out that a simple, cheap electrical part swap can make your seats heat up to a higher temp. My car is a 2001 Range Rover. Keep in mind that other model years could be different."
Brett has kindly provided the following commentary and illustrations covering the operation.
Disassembly
I was able to disassemble my passenger seat completely, all the way down to separating the foam seat bottom cushion with the integrated heater wire circuit from the frame and leather seat cover. I found the directions in the factory workshop manual to be adequate so I won’t cover that. The biggest pain was removing the plastic plug trim pieces. You pretty much have to damage them to remove them, which is ridiculous, so I’ll be buying 9 new ones shortly.
You only need to get to the seat bottom part of the heater, so don’t take apart the seat back. When I removed the leather seat cover from the cushion this is what I found.
The heater wires are sandwiched between two layers of thin foam glued together with a mild adhesive. Fortunately the foam layers can be peeled apart cleanly as I have done in the above pic. After plugging in the disassembled seat cushion and playing with it for a while, I finally discovered the thermal switch and its function. Here’s a close up of the switch mounted in place.
The thermal switch (thermostat), mounted in the seat cushion in-line with the heater wire circuit, regulates the max temperature that the seat heater reaches and also determines the temperature at which the heater switches back on as it cools. The switch contains a temperature sensitive bi-metallic disc mounted inside the plastic housing. When the seat gets hot enough, the disc clicks audibly as it inverts its shape allowing the heater wire circuit to open. When the seat cools to a certain temp., the switch pops back to normal, and the heater turns on again. Pretty simple.
Temperature Testing
So, I tested the temperature set points of the original thermal switch. I borrowed my digital thermocouple from work and put the probe directly on top of the thermal switch. I then put the leather seat cover over the cushion and put a folded towel on the seat with a 12-pack of Coke on top of the towel (to simulate someone’s butt sitting in the seat). I plugged in the heater connector, started the car, and turned the heater on. I found that both the seat back and seat bottom have to be plugged in for the heater to work. The two white 2-wire connectors visible under the seats from the rear are the seat heater circuits. I found the heater turned off when the temp. at the sensor reached 92 degrees F and didn’t turn back on until the temp. had dropped all the way to 78 degrees F. And these temps are INSIDE the seat. I didn’t measure it, but the temp. at the surface of the seat cushion is much lower. By placing my hand on the seat, the heat was just perceptible. Basically, the heater would heat mildly once, and then never come back on since it would never drop back to 78 degrees. My wife likes to toast her buns, so this was unacceptable. I considered removing the switch altogether and just jumping the connectors together so that the heater would never turn off. So I jumped the wires and did the same temp. test. After about 40 minutes of continuous operation, the seat had reached 133 degrees F inside the seat (nice and toasty), but I noticed the white connectors under the seat were really, really hot. It didn’t seem safe to let the heater operate continuously, so I don’t recommend this type of “fix” at all.
Replacing the Thermal Switch
When I surfed the web about thermal switches, I found they are available in a wide range of temperature set points. Simply plugging in a new thermal switch with a higher set point should give hotter seats and maintain the thermal cut-off safety mechanism built in to the system. But, you don’t want to get switches with too high a set point such that the heater doesn’t cycle off. I ended up installing switches with an advertised 45 degree Celsius (113 degrees F) cut-off, 20 degrees F hotter than my factory switch but still below the max temp of 133 degrees F that I measured on continuous operation for 40 min.
You want normally closed thermal switches. I bought CanTherm brand ( www.cantherm.com ) switches through www.digikey.com. For the 45 degree C cut-off switch: Digikey part # 317-1080-ND, Cantherm part # R2004525, $8.40 each. Search the digikey site for thermal switch. (Just for kicks, I called the Land Rover dealer and asked the cost of new seat cushion with new heater. They’re about $1,200, partly because you have to buy the leather cover and foam cushion as one piece. I think I’ll spend $8.40 instead.) Cantherm has hotter set points, if you want to go hotter, but again I wouldn’t recommend going much above 120 degrees F (49 degrees C) so as to avoid continuous operation of the heater circuit.
These particular switches had wider spade connectors than the factory switch, so I used a Dremel cut-off wheel to trim them down. As you can see, they’re pretty much identical to the factory switch. In the pic below, left is the factory switch, middle is the unmodified Cantherm switch, and right is my hack job to make it fit. When I first started cutting, I noticed I was melting the solder on the switch spade, so grind away slowly so as not to damage the switch from excess heat. Cantherm does make the narrow spade switches, but digikey didn’t seem to have them offered on their web site.
I simply swapped in the new switch, carefully arranged the heater wires, and glued the foam back over the wires using 3M 77 brand spray contact adhesive. After installing the 45 degree C switch, I did the temp. test again. The heater heated to 113 degrees F max temp, and more importantly, the new switch turned back on at 91 degrees F. So, now the temp never falls below 91 degrees. Much better.
And finally, I’m pretty sure the switch swap can be accomplished without removing the leather cover from the seat cushion. I’ll be trying this when I get around to tackling the driver’s seat. It’s a real pain to remove and reinstall the 15 little rings that hold the leather seat cover to the cushion. Don’t do it if you don’t have to. I hope this is helpful to someone.
I was able to disassemble my passenger seat completely, all the way down to separating the foam seat bottom cushion with the integrated heater wire circuit from the frame and leather seat cover. I found the directions in the factory workshop manual to be adequate so I won’t cover that. The biggest pain was removing the plastic plug trim pieces. You pretty much have to damage them to remove them, which is ridiculous, so I’ll be buying 9 new ones shortly.
You only need to get to the seat bottom part of the heater, so don’t take apart the seat back. When I removed the leather seat cover from the cushion this is what I found.
The heater wires are sandwiched between two layers of thin foam glued together with a mild adhesive. Fortunately the foam layers can be peeled apart cleanly as I have done in the above pic. After plugging in the disassembled seat cushion and playing with it for a while, I finally discovered the thermal switch and its function. Here’s a close up of the switch mounted in place.
The thermal switch (thermostat), mounted in the seat cushion in-line with the heater wire circuit, regulates the max temperature that the seat heater reaches and also determines the temperature at which the heater switches back on as it cools. The switch contains a temperature sensitive bi-metallic disc mounted inside the plastic housing. When the seat gets hot enough, the disc clicks audibly as it inverts its shape allowing the heater wire circuit to open. When the seat cools to a certain temp., the switch pops back to normal, and the heater turns on again. Pretty simple.
Temperature Testing
So, I tested the temperature set points of the original thermal switch. I borrowed my digital thermocouple from work and put the probe directly on top of the thermal switch. I then put the leather seat cover over the cushion and put a folded towel on the seat with a 12-pack of Coke on top of the towel (to simulate someone’s butt sitting in the seat). I plugged in the heater connector, started the car, and turned the heater on. I found that both the seat back and seat bottom have to be plugged in for the heater to work. The two white 2-wire connectors visible under the seats from the rear are the seat heater circuits. I found the heater turned off when the temp. at the sensor reached 92 degrees F and didn’t turn back on until the temp. had dropped all the way to 78 degrees F. And these temps are INSIDE the seat. I didn’t measure it, but the temp. at the surface of the seat cushion is much lower. By placing my hand on the seat, the heat was just perceptible. Basically, the heater would heat mildly once, and then never come back on since it would never drop back to 78 degrees. My wife likes to toast her buns, so this was unacceptable. I considered removing the switch altogether and just jumping the connectors together so that the heater would never turn off. So I jumped the wires and did the same temp. test. After about 40 minutes of continuous operation, the seat had reached 133 degrees F inside the seat (nice and toasty), but I noticed the white connectors under the seat were really, really hot. It didn’t seem safe to let the heater operate continuously, so I don’t recommend this type of “fix” at all.
Replacing the Thermal Switch
When I surfed the web about thermal switches, I found they are available in a wide range of temperature set points. Simply plugging in a new thermal switch with a higher set point should give hotter seats and maintain the thermal cut-off safety mechanism built in to the system. But, you don’t want to get switches with too high a set point such that the heater doesn’t cycle off. I ended up installing switches with an advertised 45 degree Celsius (113 degrees F) cut-off, 20 degrees F hotter than my factory switch but still below the max temp of 133 degrees F that I measured on continuous operation for 40 min.
You want normally closed thermal switches. I bought CanTherm brand ( www.cantherm.com ) switches through www.digikey.com. For the 45 degree C cut-off switch: Digikey part # 317-1080-ND, Cantherm part # R2004525, $8.40 each. Search the digikey site for thermal switch. (Just for kicks, I called the Land Rover dealer and asked the cost of new seat cushion with new heater. They’re about $1,200, partly because you have to buy the leather cover and foam cushion as one piece. I think I’ll spend $8.40 instead.) Cantherm has hotter set points, if you want to go hotter, but again I wouldn’t recommend going much above 120 degrees F (49 degrees C) so as to avoid continuous operation of the heater circuit.
These particular switches had wider spade connectors than the factory switch, so I used a Dremel cut-off wheel to trim them down. As you can see, they’re pretty much identical to the factory switch. In the pic below, left is the factory switch, middle is the unmodified Cantherm switch, and right is my hack job to make it fit. When I first started cutting, I noticed I was melting the solder on the switch spade, so grind away slowly so as not to damage the switch from excess heat. Cantherm does make the narrow spade switches, but digikey didn’t seem to have them offered on their web site.
I simply swapped in the new switch, carefully arranged the heater wires, and glued the foam back over the wires using 3M 77 brand spray contact adhesive. After installing the 45 degree C switch, I did the temp. test again. The heater heated to 113 degrees F max temp, and more importantly, the new switch turned back on at 91 degrees F. So, now the temp never falls below 91 degrees. Much better.
And finally, I’m pretty sure the switch swap can be accomplished without removing the leather cover from the seat cushion. I’ll be trying this when I get around to tackling the driver’s seat. It’s a real pain to remove and reinstall the 15 little rings that hold the leather seat cover to the cushion. Don’t do it if you don’t have to. I hope this is helpful to someone.
More Information
Main Heated Seat Diagnosis and Repair Page
4.0/4.6 Heater Element Repair Page.