Memory seats loosing the plot
Thread Starter
Intermediate
Joined: Apr 2009
Posts: 28
Likes: 0
From: Ludlow, Shropshire, England
Memory seats loosing the plot
Hi guys, I'm trying to get to grips with the function of the various components that make the memory seat/mirror work correctly on my Euro 90 S4 that has a 12 position memory drivers seat (M537?) and an 8 position passenger seat with no memory, in order to help me diagnose a worsening problem with the memory function.
I have read all the posts I can find on the subject but am still very confused Are there any electrical wizards amongst you who can shed any light on the subject ?
Problem - Three years ago all three memory positions worked fine then P1 stopped working followed about a year later by P2 and now finally P3 has given up the ghost. The seat & mirrors works fine in all planes when driven manually.
I have stripped and cleaned the door switch, checked all the connections on the seat control (for continuity and supply/earth) and replaced the entire seat control unit all with no change to the symptoms.
I have studied the circuit diagrams and the actual components themselves but am still pretty confused as to how it actually works; if I can get my head around that it might be easier to figure out why it is not working
As far as I can see the position of the seat/mirror is defined by the value of the potentiometers on each of the control axes. These values (voltages?) are stored somewhere against the memory position chosen P1, P2 or P3. If the seat or mirror is then moved to a different position then when the memory recall function is used the control unit compares the current pot value with the stored one and drives the necessary motor in the correct direction to reduce the difference and continues until the values match when it stops hopefully in the correct position.
My 1st question is where are these values stored?. I cannot see any evidence of any silicon based logic components present on the under seat control unit. This appears to be just a simple distribution circiut that directs power to the correct motor and collects the analogue feedback from the postion sensor pots.
My second question relates to the door switch unit itself. As far as I can see from Sheet 70 A51 the unit consists of 4 swiches (3 for P1,P2,P3 and 1 for Mem) two resistors, 3 LEDs (for P1,P2,P3) and a bulb (for Mem). The function of the three switches P1,P2,P3 is to take the earth from pin 1 (from pin 3 of connector 15 on seat unit) and apply it to pin 4 (connected to pin 4 on connector 9 on seat unit) When P1 is pressed pin 4 gets full earth when P2 is pressed one of the resistors is introduced and when P3 is pressed the 2nd resistors comes into play.
The upshot of this is that depending which button is pressed pin 4 on connector 9 on seat unit sees either earth, R1 or R2 which I guess causes a proportionate voltage drop on this pin which I am assuming is the method that the controller uses to distinguish which memory position is in operation. Say for arguments sake P1=12v P2=8v and P3=4v
I have tested the switch unit and it seems to agree with my interpretation but my memory still does not work so my 2nd question is does anybody a) agree with my interpretation and b) have actual values for the resistances in the switch or the expexted voltages at pin 4 on connector 9?
Sorry for the extremely boring nature of this post, I'll try not to do it again because I know I wouldn't want to read the ramblings of a madman however I hoped that writing it down may help to clarify my own thought process.
Regards
Stuart Waite
I have read all the posts I can find on the subject but am still very confused Are there any electrical wizards amongst you who can shed any light on the subject ?
Problem - Three years ago all three memory positions worked fine then P1 stopped working followed about a year later by P2 and now finally P3 has given up the ghost. The seat & mirrors works fine in all planes when driven manually.
I have stripped and cleaned the door switch, checked all the connections on the seat control (for continuity and supply/earth) and replaced the entire seat control unit all with no change to the symptoms.
I have studied the circuit diagrams and the actual components themselves but am still pretty confused as to how it actually works; if I can get my head around that it might be easier to figure out why it is not working
As far as I can see the position of the seat/mirror is defined by the value of the potentiometers on each of the control axes. These values (voltages?) are stored somewhere against the memory position chosen P1, P2 or P3. If the seat or mirror is then moved to a different position then when the memory recall function is used the control unit compares the current pot value with the stored one and drives the necessary motor in the correct direction to reduce the difference and continues until the values match when it stops hopefully in the correct position.
My 1st question is where are these values stored?. I cannot see any evidence of any silicon based logic components present on the under seat control unit. This appears to be just a simple distribution circiut that directs power to the correct motor and collects the analogue feedback from the postion sensor pots.
My second question relates to the door switch unit itself. As far as I can see from Sheet 70 A51 the unit consists of 4 swiches (3 for P1,P2,P3 and 1 for Mem) two resistors, 3 LEDs (for P1,P2,P3) and a bulb (for Mem). The function of the three switches P1,P2,P3 is to take the earth from pin 1 (from pin 3 of connector 15 on seat unit) and apply it to pin 4 (connected to pin 4 on connector 9 on seat unit) When P1 is pressed pin 4 gets full earth when P2 is pressed one of the resistors is introduced and when P3 is pressed the 2nd resistors comes into play.
The upshot of this is that depending which button is pressed pin 4 on connector 9 on seat unit sees either earth, R1 or R2 which I guess causes a proportionate voltage drop on this pin which I am assuming is the method that the controller uses to distinguish which memory position is in operation. Say for arguments sake P1=12v P2=8v and P3=4v
I have tested the switch unit and it seems to agree with my interpretation but my memory still does not work so my 2nd question is does anybody a) agree with my interpretation and b) have actual values for the resistances in the switch or the expexted voltages at pin 4 on connector 9?
Sorry for the extremely boring nature of this post, I'll try not to do it again because I know I wouldn't want to read the ramblings of a madman however I hoped that writing it down may help to clarify my own thought process.
Regards
Stuart Waite
Electron Wrangler
Lifetime Rennlist
Member
Lifetime Rennlist
Member
Joined: Mar 2002
Posts: 13,657
Likes: 616
From: Phoenix AZ
Your assessment of the operation is actually quite accurate. The memory storage for the seat is in the seat controller.
The most common failures are due to additional switch resistance build up (often due to the ingress of armor all or similar...) use switch cleaner to improve this.
Does the memory control pad appear to work otherwise (does mirror memory work from buttons 1,2,3 for example & does the memory button work (e.g. illuminate).
If this doesn't work you need to target this first. If the mirrors work fine - then its isolated to the seat memory unit...
Alan
The most common failures are due to additional switch resistance build up (often due to the ingress of armor all or similar...) use switch cleaner to improve this.
Does the memory control pad appear to work otherwise (does mirror memory work from buttons 1,2,3 for example & does the memory button work (e.g. illuminate).
If this doesn't work you need to target this first. If the mirrors work fine - then its isolated to the seat memory unit...
Alan
Last edited by Alan; May 5, 2009 at 07:45 PM.
