Friday, October 8, 2010

BMW 740iL seats retrofit in my 1973 BMW: wiring

I installed front seats from an E38 BMW 740iL in my 1973 BMW Bavaria.  This car of course did not have power (or heated, for that matter) seats so here are some notes on how to connect them.






The basic parts needed are:
  • two fuse holders and fuses (one 30A and one 15A).
  • a relay and relay holder (recommended).
  • some wire: there are several colors involved but the power wiring could just be red and brown.  The heater circuit uses white/blue and white/yellow but I suppose white and yellow would look OK.  The power wiring for the seat power is very thick (like 10ga) and the rest of the wiring is thinner, I think that 14ga or 16ga should do.
  • wiring "pigtail" for each seat's wiring connector from the donor car.  These seats use a very complicated AMP connector so it's important that you have it since it's nearly impossible to make your own.  On the plus side, it's a very nice connector.
  • two of BMW part 61318352259, "seat heating switch". This would certainly be a good thing to buy used (I found mine on eBay).  I would get the pigtails for the small connector on these switches as well.  Here's a list of donor cars.
  • some typical wiring splice connectors.
Seat Power


The seat power is connected to the battery at all times through a single 30A fuse which is shared between the two seats.  I suppose that this could be divided into two 15A circuits but I don't know the headroom that the designers considered so it's probably safer to wire it up like BMW did.  There are potentially three sets of circuits to power on each seat:
  1. The main set of motors (the thick red and brown wires are +12V and ground respectively and there is also a thinner brown wire that I believe is a also a ground).  All seats have this.
  2. The lumbar support motor (red/yellow is +12V)
  3. The thigh support motor (red/white is +12V)
These are found on pages 40 through 42 on the BMW E38 wiring diagram but the internals of the seat wiring are not particularly important.  Some of these seats have a memory function which is shown on page 39 and needs an additional fuse.


From what I understand, the regular ("comfort") seats have 16 adjustments and just use the first power supply.  There are "sport" seats with 18 adjustments that have the other two as well.  There is also a bizarre "active" (massaging) seat that I don't know anything about.

Seat Heaters


The BMW E38 wiring diagram shows the seat heater circuit on page 23.  Each seat has a heater switch which consists of an on/off button (with backlight) and a temperature knob.  The switch has six numbered pins corresponding to:
  1. interior lights system (connect it to the dash lights dimmer circuit, ie: the one that lights up the in-dash clock)
  2. seat harness white/blue
  3. power supply: switched ignition power through a 15A fuse (common between both seats)
  4. ground
  5. seat harness: white/yellow
  6. not connected
The power supply pin is supposed to be connected straight to the car's ignition or "run" circuit however on older cars that circuit isn't designed to have many accessories on it.  I therefore connected it through a relay which switches power to the heater circuit (via the 15A fuse as per the diagram) when the ignition is on but then allows the seat heaters to draw current directly from the battery.


The seat heaters have a very nice mechanical safety system built in.  It consists of a clear tube that contains two springs on either side with a significant air gap between them.  Presumably as the seat heats up the springs expand and eventually they will make contact, cutting power to the circuit until they've cooled enough to break contact.


Additional / Unused Wires


There are additional wires on the seat harness that are not connected to the seat motors or heaters.  These, I believe, are for the car's SRS system (airbags) and functions such as the seatbelt warning and they're probably connected to an "occupant switch" and the seat belt buckle.  My Bavaria used to have the USA-mandated "fasten seatbelts" switch so I could even connect some of this up but it doesn't really interest me.  In addition the built-in seat buckle has a mechanism to tighten the belt during a crash and that seems to be connected to the car via CAN or some other bus (it uses a twisted pair).  I recommend removing that whole assembly since it won't work in the E3.


Seat Mounting


BMW mounted the E3 seats on "perches" up through 1973 and then switched to mounting seats almost directly to the floor in the 1974 models.  As a result it's difficult to install later-style seats in a 1973 or earlier E3.  The E38 seats have nearly identical rail-to-rail widths to the stock E3 seats and their rails are only a little longer.  I first tried simply bolting the E38 seats to the E3 mounting "perches" but found that the seats sat too high so I had the seat "perches" removed and John at Nameless Performance made brackets to bolt the E38 seats straight to the floor in the 1974 and above style.  I then re-routed my carpets to cover things up.  The E38 seats wound up at a perfect driving position, matching the originals, with plenty of adjustment left.


To remove the "perches", take out the carpet pieces and drill out the spot welds and pull them out.  I had someone do this for me but you can do it yourself if you know what you're doing (when it comes to metal, I typically don't).  Please contact Nameless Performance if you'd like to try E38 seats in your E3 as they can sell you some brackets.
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