Memory Seat Heater Problem Workaround
01-16-2007 | 04:01 PM
#1
Thread Starter
Three Wheelin'
Joined: Apr 2004
Posts: 1,868
Likes: 4
From: San Diego, CA USA
Memory Seat Heater Problem Workaround
I've spent a fair amount of time trying to get my driver's side seat heat heaters to work. This included new OEM heater elements at around $250 each.
It's a memory seat. Memory seat heaters do not have the same large, separate relay as non-memory seats. There is one relay per seat. The passenger seat heater relay only controls the passenger side seat heater. If you do not have a relay on the driver's side, you almost certainly have a memory seat: Memory seats have the seat heat relay built into the memory control unit.
I don't know why on earth Porsche would've built the seat heating relay into the memory seat control circuit board, rather than just running the same separate relay from the switch like they do on a non-memory seat. The memory control unit is fairly complicated, and extremely expensive.
By contrast, you can get a seat heat relay from 928 int'l for $22, if you happen to have the non-memory seat heater switch, which has different wiring.
The WSM is a bit confusing--the writeup for diagnosing pre-89 seat heating problem is combined in the area for diagnosing problems with seat memory, which is several pages earlier, and not listed in the table of contents.
After reassembling my seats with the new seat heaters, I got a brief moment of heat, then nothing.
I pulled the seat (again), and started to dianose the problem. In a nutshell, power for the seat heaters runs into Plug 1 of the memory control unit; the on/off switch runs into Plug 2.
I had no power at the seat heat switch.
I had no power where the seat heat switch goes into the memory control unit at Plug 2.
I had no power at the circuit board where the seat power loom plugs into Plug 1 on the memory control unit.
I did have power at Plug 1.
Ergo, there was (at least one, more on this later) bad solder joint on the circuit board, and this was my intermittant connection.
I resoldered Plug 1 until I got continuity between all three prongs on Plug 1 and the circuit board.
In hindsight, I should've tested all the other prongs/Plugs associated with the seat heat circuit, but I figured I had the problem solved.
Reinstalled the seat, complete with tightening all four hex head screws. In the ultimate act of hubris, took off my sweatshirt in my cold garage so I could enjoy the sensual embrace of heat through my tee shirt. Fired up the car. No heat. No joy.
Pulled the seat again and thought, I'm going to drive myself NUTS trying to isolate the problem with the memory control unit, and I'm not going to spend another $500 buying another damn one.
SOLUTION
A non memory seat has three pins on the 14-or so pins available that actually do something: 1 is constant power, 2 is ground, and 3 is switched power.
I happened to have a non-memory seat heat switch, which has an entire harness attached to it including :
(1) - red and brown pins which go into a plug for power (on an '88 the circuit is fuse #8, which is switched to ignition)
- The red pin ultimately draws its power from pin #3 on the seat loom (which is a black wire on the loom). On a non-memory seat, there is an intermediate cord that takes power/ground from pins 1,2,3 and bridges them to several other pins via another plug, which then splits off to the two motors (8-way, constant power) and seat heater (switched power, separate circuit).
(2) the receptacle for the seat heater relay (part # ends in 00 for on/off seat heat; ends in 02 for adjustable seat heaters (with the wheel on the switch to adjust heat)
(3) two 2-pin plugs for the seat back and seat bottom heat. Neither of my seats had the third plug for bolster heat.
I pulled ground from Pin 2 on the seat loom, switched power from Pin 3 on the seat loom, ran that to my new switch, plugged in the spare relay I had, and routed my seat heaters to the "new" seat heater harness.
I left Plug 1 to the memory control unit intact, even though I no longer have a switch attached to Plug 2.
Voila! Seat heat works! Seat still moves! Never used memory before, but I assume it works as well or as poorly as it otherwise would've. And if the seat heating ever fails again, it's one of four things: fuse, switch, relay, or elements.
It just seemed like a much simpler solution to isolate the task of heating the seats from the memory control unit . The electrics of seat heating on a non-memory seat are really quite simple. It seems so dumb to me to have the heat switching be integrated in the same unit that manages all the complexity of the seat potentieometers and also interfaces with the mirror control unit.
Conclusion
If you're having a problem with memory seat heat, I recommend:
1. buying a non-memory seat heat switch (comes with harness) and relay (prolly ~$100 for a new switch and used relay),
1a. install switch and relay
2. pull switched power from pin 3 (black wire) of the seat harness, and ground from pin 2 of the seat harness (brown wire),
3. plug in your heating elements,
4. and you should be good to go.
Hope this helps someone, someday. I've got most of the cosmetic and mechanical things worked out, and I've finished most of the electrical crap, now, too. Last stop is the hatch release!
It's a memory seat. Memory seat heaters do not have the same large, separate relay as non-memory seats. There is one relay per seat. The passenger seat heater relay only controls the passenger side seat heater. If you do not have a relay on the driver's side, you almost certainly have a memory seat: Memory seats have the seat heat relay built into the memory control unit.
I don't know why on earth Porsche would've built the seat heating relay into the memory seat control circuit board, rather than just running the same separate relay from the switch like they do on a non-memory seat. The memory control unit is fairly complicated, and extremely expensive.
By contrast, you can get a seat heat relay from 928 int'l for $22, if you happen to have the non-memory seat heater switch, which has different wiring.
The WSM is a bit confusing--the writeup for diagnosing pre-89 seat heating problem is combined in the area for diagnosing problems with seat memory, which is several pages earlier, and not listed in the table of contents.
After reassembling my seats with the new seat heaters, I got a brief moment of heat, then nothing.
I pulled the seat (again), and started to dianose the problem. In a nutshell, power for the seat heaters runs into Plug 1 of the memory control unit; the on/off switch runs into Plug 2.
I had no power at the seat heat switch.
I had no power where the seat heat switch goes into the memory control unit at Plug 2.
I had no power at the circuit board where the seat power loom plugs into Plug 1 on the memory control unit.
I did have power at Plug 1.
Ergo, there was (at least one, more on this later) bad solder joint on the circuit board, and this was my intermittant connection.
I resoldered Plug 1 until I got continuity between all three prongs on Plug 1 and the circuit board.
In hindsight, I should've tested all the other prongs/Plugs associated with the seat heat circuit, but I figured I had the problem solved.
Reinstalled the seat, complete with tightening all four hex head screws. In the ultimate act of hubris, took off my sweatshirt in my cold garage so I could enjoy the sensual embrace of heat through my tee shirt. Fired up the car. No heat. No joy.
Pulled the seat again and thought, I'm going to drive myself NUTS trying to isolate the problem with the memory control unit, and I'm not going to spend another $500 buying another damn one.
SOLUTION
A non memory seat has three pins on the 14-or so pins available that actually do something: 1 is constant power, 2 is ground, and 3 is switched power.
I happened to have a non-memory seat heat switch, which has an entire harness attached to it including :
(1) - red and brown pins which go into a plug for power (on an '88 the circuit is fuse #8, which is switched to ignition)
- The red pin ultimately draws its power from pin #3 on the seat loom (which is a black wire on the loom). On a non-memory seat, there is an intermediate cord that takes power/ground from pins 1,2,3 and bridges them to several other pins via another plug, which then splits off to the two motors (8-way, constant power) and seat heater (switched power, separate circuit).
(2) the receptacle for the seat heater relay (part # ends in 00 for on/off seat heat; ends in 02 for adjustable seat heaters (with the wheel on the switch to adjust heat)
(3) two 2-pin plugs for the seat back and seat bottom heat. Neither of my seats had the third plug for bolster heat.
I pulled ground from Pin 2 on the seat loom, switched power from Pin 3 on the seat loom, ran that to my new switch, plugged in the spare relay I had, and routed my seat heaters to the "new" seat heater harness.
I left Plug 1 to the memory control unit intact, even though I no longer have a switch attached to Plug 2.
Voila! Seat heat works! Seat still moves! Never used memory before, but I assume it works as well or as poorly as it otherwise would've. And if the seat heating ever fails again, it's one of four things: fuse, switch, relay, or elements.
It just seemed like a much simpler solution to isolate the task of heating the seats from the memory control unit . The electrics of seat heating on a non-memory seat are really quite simple. It seems so dumb to me to have the heat switching be integrated in the same unit that manages all the complexity of the seat potentieometers and also interfaces with the mirror control unit.
Conclusion
If you're having a problem with memory seat heat, I recommend:
1. buying a non-memory seat heat switch (comes with harness) and relay (prolly ~$100 for a new switch and used relay),
1a. install switch and relay
2. pull switched power from pin 3 (black wire) of the seat harness, and ground from pin 2 of the seat harness (brown wire),
3. plug in your heating elements,
4. and you should be good to go.
Hope this helps someone, someday. I've got most of the cosmetic and mechanical things worked out, and I've finished most of the electrical crap, now, too. Last stop is the hatch release!
01-16-2007 | 11:30 PM
#3
Thread Starter
Three Wheelin'
Joined: Apr 2004
Posts: 1,868
Likes: 4
From: San Diego, CA USA
I've never seen what the electrics for the rheostat-equipped systems look like. The WSM says that everything has to be calibrated if pieces are replaced. It looks like the calibration potentiometer is on the relay (part # ends in 02) itself (as well as on the memory control unit.
As far as I can tell, the only differences between the old vs. new system seem to be that the newer switch's wheel sends some kind of signal to the "calibration potentiometer" on the newer relay, so that the output voltage, thus the amount of heat output by the elements, is variable, depending on the ambient temperature as measured...somewhere; I guess on the relay itself.
I would order a newer model, non-memory heat switch, and get a used relay ending in 02. You don't even need to cut anything on the harness--just wire the new heat switch to 12V (I used a portable jumpstart battery pack), plug the relay into the new harness, and plug your seat elements into the 2 (or 3) 2-pin connectors. If you get heat, you know it works and can then tap into the 2 pins on the seat loom to get ground and switched power, and make the solution more permanent.
Not many other alternatives--It's not like the "relay" that's built into the memory control unit is easily servicable...
Good luck!
As far as I can tell, the only differences between the old vs. new system seem to be that the newer switch's wheel sends some kind of signal to the "calibration potentiometer" on the newer relay, so that the output voltage, thus the amount of heat output by the elements, is variable, depending on the ambient temperature as measured...somewhere; I guess on the relay itself.
I would order a newer model, non-memory heat switch, and get a used relay ending in 02. You don't even need to cut anything on the harness--just wire the new heat switch to 12V (I used a portable jumpstart battery pack), plug the relay into the new harness, and plug your seat elements into the 2 (or 3) 2-pin connectors. If you get heat, you know it works and can then tap into the 2 pins on the seat loom to get ground and switched power, and make the solution more permanent.
Not many other alternatives--It's not like the "relay" that's built into the memory control unit is easily servicable...
Good luck!
01-17-2007 | 10:30 AM
#4
Rennlist Member
Joined: Dec 2004
Posts: 3,411
Likes: 470
From: West Chester, PA
Brian,
Does the memory for seat/mirror positioning still work after changing the heater switch? I may need to do the same thing to my L & R seats (12 way memory/heat). THANKS for your wonderful research, analysis, and solution.
Gary Knox
West Chester, PA
Does the memory for seat/mirror positioning still work after changing the heater switch? I may need to do the same thing to my L & R seats (12 way memory/heat). THANKS for your wonderful research, analysis, and solution.
Gary Knox
West Chester, PA
01-17-2007 | 02:29 PM
#5
Thread Starter
Three Wheelin'
Joined: Apr 2004
Posts: 1,868
Likes: 4
From: San Diego, CA USA
I will double check. Plug 1 to the Memory Control Unit has constant power, switched power, and ground (in that order) across the 3 pins. Plug 2 to the MCU has switching information from the heat switch. I disconnected Plug 2. But everything else remains intact, so I can't imagine that disconnecting the seat heat plug from the MCU would interfere with any of the memory control functions.
Like I said, I will double check and repost.
After thinking about this a bit more, I'm considering buying the 02 relay and the seat heat switch with the rheostat--I like the idea of adjustable heat vs. simply on/off.
Like I said, I will double check and repost.
After thinking about this a bit more, I'm considering buying the 02 relay and the seat heat switch with the rheostat--I like the idea of adjustable heat vs. simply on/off.
