Z3 Seat Swap DIY--early seats into newer car

Seat Replacement DIY--Early Version Seats or ///M seats into Later Version Z3

Recently, when I bought second-hand black leather Z3 seats to replace my blue leather seats, I discovered that the electrical wiring had changed during the course of Z3 production. I'd need to rework the electrical wiring if I wanted seat adjustment, heat, and occupancy sensors to work correctly. After many months of stalling, and occasionally taking another befuddled look at the wiring, I decided to just dive in.

Wiring diagrams were of no real help, since the wiring colors didn't match what was present on the seats. I purchased a complete wiring harness in the hopes that I could scavenge the needed parts. In retrospect, that wasn't necessary, but it did help me determine which wires on the seats connect with which wires in the car.

I'll be the first to admit that I'm no wiring expert, and I offer these instructions simply as a helpful guide to you if you're contemplating the same mod. There may be other approaches that work better, and these instructions may not pertain to all seats and all year vehicles. In this case, the seats are 01/97 production and the car is 04/2001 production. I believe the wiring changed in 04/99 with the 2000 model year cars, though there may have been other iterations along the way. Also, I believe the ///M seats maintained the earlier wiring throughout production....


Supplies Needed:

~ 10mm, 13mm, 16mm sockets w/wrench
~ adjustible wrench
~ hammer
~ blankets or towels to place seats on (they'll be upside-down alot of the time)
~ wire stripper and crimper tool
~ butt connectors, blade connectors, wire nuts, or soldering tools
~ portable lighting to see under seats
~ plenty of workspace at the side of the car
~ several hours to complete the task


Step 1:

Use the power adjuster to raise both seats as high as they'll go--this will give you more working room under the seats later on. Slide both seats back far enough to expose the nuts on the floor at the front of the seats' rails. Use the 13mm socket to remove all four nuts (two for each side of each seat). Next, slide the seat forward to expose the nuts at the rear of the rails. If your seatback is reclined, release it to full upright position to get better access to the rear. These nuts will now be removed with the 16mm socket. Now use the adjustible wrench to remove the bolt that holds the seatbelt to the seat base, and in the case of the roadster, work the strap out of the seatbelt guide at the top of the seat.

Step 2:

Disconnect the negative battery cable using pliers or the 10mm socket. Make sure you have your car's stereo code #--once the battery is reconnected, you'll need to reenter the code to use your stereo.

Step 3:

Tilt the seatback rearward as far as possible--this will allow better leverege for working under the seat, and also easier removal from the car. lift the back of the seat forward toward the windshield and get in behind it with lighting so you can see where the connection from the floor meets the seat connection. It is a yellow and black boxlike plastic structure that is fastened to the seat bottom. You can twist it to unfasten it from the seat. Once it's unfastened, the wiring can be disconnected by pulling on the black plastic piece on the side labelled 'a' in this image (The connection is shown already disconnected):



Here is an image of the connection that comes out of the floor (already disconnected):



Step 4:

Remove the seat from the car being careful how you lift and swing it out--It's easy to gouge your sill plate or center console. Set it upside down on the blacket or towel for easy reference to its underside. Set the replacement seat near it so you can compare the connections.

Step 5:

Remove the black plastic 'caps' on the floor's yellow connector for easier access to the wiring. Remove the cap on the seat-side connector as well.

Step 6: Passenger Seat only

Look at the underside of the replacement seat and you'll see the "seat occupancy sensor" which is shown in the following image. This needs to be connected in order for the seatbelt tensioner to fire and passenger side airbag (if equipped) to go off in event of an accident. The airbag indicator lamp will light up on your instrument panel if this is not connected when the battery power is restored. You will then have to pay for either the reset tool or a trip to the dealer to reset it. The driver's side doesn't have the occupancy sensor--the car assumes there is a driver present in the event of a crash. This image shows it disconnected--a white plug inserts where the arrow is:



Step 7:

On both replacement seats you'll see connections like the following:



The black plug is for the heating feature, and the white plug is for the seat motors. Because I had bought a separate wiring harness, I was able to salvage the parts that plug into these and make the following connection using butt-connectors (you can use blade connectors, wire nuts, or solder if you prefer). If you use butt-connectors, gentle pulling will ensure that you've crimped them sufficiently.




As I hope you can see, I only cut and rewired what was necessary to make the circuits complete. Some of the wiring from the old seats was compatible with the wiring in the yellow floor connector, and I plugged those seat wires right into the yellow seat-side connector as shown is this image (the central three blocks of wiring in the connector):



The outside blocks of wiring--one at each end--are what were converted to the old style plugs.

Place the replacement seats back into the car and get in behind them to reconnect everything. Reconnect the battery cable and test your connections.

I don't know what some of the connections were for, but none of it was redundant, and they've worked well since completion. This isn't the complete story, as I've forgotten some details in the 2 weeks since I did this. Questions, comments, or corrections are welcomed.

Glad you were able to get this resolved, and good write-up and pictures. You should post or link this in the Z3 FAQs and How-tos sticky.

Great post. Thanks for taking the time to share. I can move it into the FAQ post or you can post a link there to this thread.

Hello all. I am new to this forum. I just recently acquired a 99 z3 2.8 and am in the process of replacing the seats with ones from 2001. This the same situation I am in however our seats and model year are reversed. I notice that the motor wiring is Brown/red from the 2001 floor adapter which I can reverse for the 2001 seats. But how do I determine the heater wiring coming from the 99 to the 2001 seats? Also if I disconnect the battery I wouldn't have to worry about the seat tensioner cable? Would you know which wire is responsible for the ABS sensor. I would assume that one of the wires from the seat has to be properly connected for the car to know that the seats are installed. Or would that be controlled by the motor wires? Any further info would be greatly appreciated.

Originally Posted by bmwz328

Hello all. I am new to this forum. I just recently acquired a 99 z3 2.8 and am in the process of replacing the seats with ones from 2001. This the same situation I am in however our seats and model year are reversed. I notice that the motor wiring is Brown/red from the 2001 floor adapter which I can reverse for the 2001 seats. But how do I determine the heater wiring coming from the 99 to the 2001 seats? Also if I disconnect the battery I wouldn't have to worry about the seat tensioner cable? Would you know which wire is responsible for the ABS sensor. I would assume that one of the wires from the seat has to be properly connected for the car to know that the seats are installed. Or would that be controlled by the motor wires? Any further info would be greatly appreciated.

If you look at the first photo in my info above, I've disconnected the heat and motor plugs from the yellow connector. You would need to do the same, and adapt them to the floors wiring.

Just remember that brown wiring always connects to brown. You should be able to look at some of my other pics above to determine what other connections to make--particularly the pic that shows the yellow floor connector with the old-style wiring attached to it.

I think you may be able to use the process of elimination to determine alot of this.

So basically through your experience I will only have to worry about the wiring coming from the old seats and match it up with the new. I guess the older style wiring does not have as many wires as the 2001? So judging from the pictures there are only four wires I need to worry about when connecting the 2001 seat to the 99 Z3. In essence I can disregard the remaining wires with the 2001 seat adapter since I the 99 does not have that many wires in comparison. I'll let you know how I make out.

Keep in mind you have the extra wire on the passenger seat for the occupancy sensor (the little white plug near the top of the photo. Here's a shot of what you should have coming out of your floor. The little black and burgundy plugs are compatible with the new wiring from what I recall.

I am trying to put Late model M coupe seats in a Z3 roadster. I have done the bushings on the new seats and just removed and cleaned under the roady driver seat. My problem is the rear bolt holes are off enough so the bolts will not line up. Has any one else done this exchange? I'll take pictures & post.

Carl, are the rear holes off while the front studs are inside the holes in the sliders?

Originally Posted by Vinci

Carl, are the rear holes off while the front studs are inside the holes in the sliders?

After having the seat in & out 3 times, I took some measurements and found the distance from frt holes in seat frame to the moving seat chassis to be different by half an inch. Took the screw drive apart and turned the shiskabob 2 turns and remeasured to varify that they were within a 1/16th of each other.then Reinstalled. Wah Lah bolts lined up. The masking tape method on the bushing install instructions pages is not as acurate as the tape slides on the greased drive rods.
Always Measure prior to attempting to reinstall the seats. Also the sanding is teadious.
This was an all day project. Sort of. Started the AM watching finish of the 24 hour at LeMan while taking the coupe seats apart, Racing diesils Amazing
Then Sanded the bushing (and fingere tips) during Sprint Cup from Irish Hills Michigan- JR got first win!

Now putting passenger seat in - Not a bad day..

That's what I figured happened. I used a 12v power supply when I did mine so I wouldn't have to keep track of the original position.

I didn't get to see the end of the Le Mans race, but I watched a couple hours on and off between 10 and 2am. Great race, from what I saw. I was glad to see the "slower" Audi come out on top.

Originally Posted by Vinci

I didn't get to see the end of the Le Mans race, but I watched a couple hours on and off between 10 and 2am. Great race, from what I saw. I was glad to see the "slower" Audi come out on top.

I started watching LeMans last night about 9 PM dropped out at midnight as the sky got brighter-(AM in France) a Rain Race at night & into the AM. Tire strategy was interesting. Was impressed that for the second year in a row the Aston Martins w/ Gulf colors won their class. The only race they run all year. Reminded me(I'm showing my age) of the Ford GT 40's that beat Farrari

I love my Z3 but a DB9 is my ultimate GT car
Micheal Jordan is even driving one in his latest Hanes T shirt comercial

Even though I did not have key in ignition during the R&R of the seats I now have the airbag light lit. I guess I need to get a reset tool

I have one, if you'd like to meet somewhere.

Originally Posted by Vinci

I have one, if you'd like to meet somewhere.

Thanks I'll PM you next week and see what's time & day is good for you.
it will give me an excuse to cross the Sunshine bridge at above average speed. I love crossing that bridge with the top down. ..

No problem. If you just need a reset, I can meet you wherever. If you need to do any more to it, you're welcome to my garage and tools.

Awesome, thanks! This is exactly what I was looking for. I've had some '00 M coupe heated sports seats I'm going to put in my '01 roadster. I'll post my notes when I'm done.

Ok, so the connectors for my replacement seats were just a bit different. The new seats came from an '00 M coupe. The middle connectors wouldn't fit into the plastic yellow frame like yours.

I had to cut the connectors from the old seats and solder them onto the new seats. Pretty easy task, as the wiring was identical (for the middle connectors and the main power). Like yours I had to combine the brown and white wires for the heated seats.

I'm having trouble finding the heated seat switch connectors. They're supposed to be in the front portion of the console, right?

I found the heated seat switch connectors in the front part of the console. They were tucked up under the carpet near the ECU. I previously moved the top up/down switch to the front console, so I had to move the DSC switch to the back portion of the console to make room for the heated seat switches.

The swap went pretty smoothly. I used a Peak Research tool to reset the SRS system and clear the codes (I had to use my car when the passenger seat was out). There were no codes thrown with the new seats in place.

I have to say that the sports seats are VERY comfy.

I am trying to replace seat adjustment switch on a passenger side, and to remove the seat to do this. After removing 4 nuts/bolts, and flipping the seat, I tried to disconnect the wiring connector. Easy at this point, but I found it extremenly difficult to disconnect as it is tucked behind the rails and other supports. It seems to me that removing seat itself from the rail is the only way for me to have an access to the connector. Is it so, really, or any other ways? Help, please.