mcs51

Hi there,
I have aquestion regarding seat heating setup in a ’89 S4 car.
On both ’88 and ‘90 electrical drawing one can see two different controlsystems for the seat heating system.
On one page a “simple” delay relay, onanother page a control unit that controls not only the heating but also theposition of the seat.
The only difference that I can see is the potentiometer and 2 resistors integrated inthe switch. My seats have that potentiometer, so the ’90 electrical drawing isthe right one to use.
Is that “simple”relay some kind of PWM regulation?
What werethe criteria to choose between a) the “simple” relay and b) the large controlunit back in ‘89?
I have memory positions on driver seat but not on passenger seat, can I expectthe large control unit on the driver seat because of memory positions and “simple”relay on passenger seat because it lacks the memory positions?
Both seats don’tproduce any heat at all.
Before removing the seats I would like to know what I can / should expectunderneath them.
Thanks alot for your insights!
Bestregards
Alain

Tom in Austin

Here's the underside of my car's memory seat. AFAIK, the heating function is entirely separate from the controller and wiring for movement of seat via the motors. On my car there's separate wiring with simple two-pin connectors for heating elements in the backrest and seat cushion ... I don't think this goes through the memory system at all.

mcs51

Thanks alot for that picture Tom

Again “1picture is worth more than 1000 words” is proven!

Since I don’tsee the “seat heater relay” (928 618 521 00) under your seat I expect the seat heating to go through that big box.
According to the electrical drawings at least, it should…
Thanks again!


Just 1 morequestion regarding removing of the seat:
It startswith lossen the 6 bolds but then what?
Can I thenjust tilt the seat sideways, backwards, ????? to disconnect some electricalwiring??


You see, Inever did that so it’s all new to me.
The more I know before hand, the better I feel.

Tom in Austin

After removing the six bolts, tilt the seat backward so you can reach underneath and disconnect the wires. On a memory seat, you've have one large connector and one small connector for the seat belt warning light.

mcs51

Thanks for your inputs Tom!
I know what to do this week-end… …

Alan

It is solely the memory or non-memory feature on the seat. It is rare to have memory on the passenger seat (though it exists on some). Sports seats don't usually have any heating option.

Alan

jetson8859

The sport seats in my 87 have heat, both drivers and passenger side. Recently had seats recovered and had the heating elements refurbished, nice and toasty now. As others said above, a simple two connector plug.

mcs51

Passenger seat is out of the car,
underneath it…………………. one “simple” relay, 928 618 521 02.

I measured 1.8 Ohm on the terminal connected to that relay.
Is that a normal value or not?

To check the relay, is it ok to connect a 40W bulb instead of heating elements?

I need to recover my seats, but people I talked to so far told me to replace the original relay and heating elements by new/modern ones.
But then I need to add a new button to activate this new system.
To be honest I want to keep the car as original as possible, that’s why I learn about that system.
Next time I go see someone I have better background

What did you finally did jetson?
Keep original Porsche hardware or changed to new one?

Thanks to all for your inputs so far… ..

mcs51

Using a 12V / 40W bulb, 2 resistors 2K2, a 1K potentiometer and 2 push buttons I was able to get the relay to work on my bench.
After pressing the start button, the bulb blinks, about 1 second on then 0.1 to 0.2 seconds off depending on the position of the potentiometer.
Is this how the relay drives the heating elements?

If yes, strange the seat doesn’t heat up.
Before you ask: yes, fuse 8 (15A) is ok.
Switch and potentiometer in the car are ok since I measured Ohmic values to know what to use on my bench.

To be continued… … …

jetson8859

I kept original equipment but sent the heating elements off to get refurbished, the passenger side element was broken about 1/3 the length of element. Replaced both the elements with similar and upholster reinstalled.

mcs51

Just in case someone wants to see the difference between seat with and without memory.
Both seats have same functionalities, see picture


Note that one of the connectors on the seat without memory is for the motor that tilts the back of the seat. So, not 2 heating elements as I first thought!


There are 2 additional relays under the seat with memory (928 618 138 00) labelled “Sitzmemory relaiseinheit”, impossible to take a good picture of them.
Is the PCB seriously not big enough that 2 more relays are needed ? ?

Case closed for me:
Both heating electronics are ok, one broken heating element in the back of the driver seat.

jeff spahn

Not to raise the dead but I have a similar problem except I don't have the potentiometer in my switch. Also my seats are 12way power instead of 8 way. I simply have the on/off switch shown in the apparently 1988 wiring diagram. My S4 is a 1990 and I only have the 1990 wiring diragram.

I have no heat working in either seat. The fuse in #44 is good. I have used the WSM testing on page 72-33. As I don't have the variable heat, I can only check output of the two switch posts. I have not modified the seats so they were built this way. There is 5v at both power pins regardless of the position of the switch on the passenger seat and the same on the driver seat. When I press the switch to the corresponding pin, the connection breaks and voltage goes to 0. I do not hear a relay click as I would expect from something using this much power. This goes for both sides.

Ideas. I have tried to figure this out but I am missing some bit of knowledge.
I do know that the driver seat is memory and the memory works. I don't recall if the seat I put in was memory or not. I did install the factory passenger memory button and trim and have connected that up. When pressing the memory buttons, you can hear the seat click but the memory function on the door does nothing and the red light doesn't light up on the memory button as it does on the passenger side. (don't know if related but don't want to leave anything out).

jeff spahn

Also, does anyone have the 9269 cable that they would lend me? Porsche part number 000 721 926 90? I can buy it but it's $145 for something to just use once. I'd be happy to pay it forward with my Porken 32vr if anyone needs it.

jeff spahn

bump... really, no one has any input on this situation. It's like I am posting in the MB world forum.

mcs51

Hi Jeff,
Very confusing story you tell there
You have a ’90 car but without the potentiometer in the heating switch, right?
That system uses a transistor the drive the heating elements, so it’s normal that you don’t hear any relay.

The drawing of ’88 car shows a system without potentiometer. In this drawing there is some kind of circuitry that powers a relay for a fixed time to power the heating elements. As I understand the drawing, each time you press one of the switches you get x seconds of heating.




On both years drawing each heating element has its own 2 terminal connector; all of them are connected in series. That means, one broken all not functioning.



So, I would suggest that you take the seats out of the car and that you measure the Ohmic value at those connectors. I hope for you some will measure a resistance, others not.



That would be a great start to understand what you really have in your car… …



Hope this helps
Regards