engine and drivetrain How To Bypass Seatbelt Interlock System on 1974 BMW 2002A

[freeheelboy]

This is a quick synopsis of my 2 day ordeal wrestling with a little known issue that plagues a few of us here that own the 1974 BMW 2002 Automatic.

It is the dreaded SEAT BELT INTERLOCK SYSTEM that was intended to train U.S. consumers to wear seatbelts in the 70's by putting sensors in our seats and seatbelts that were connected to a relay that cut power to the starter if an individual was sitting in the front seat but did no have his/her seatbelt fastened. As you can see from the diagram attached it was also integrated into the parking brake and transmission as well.

As far as I can tell the feature was quickly removed from the line up in 1975 as it drove people crazy.

Symptoms: You turn the key to start ignition and all your lights light up on the dash board as normal, but when you turn it further to engage the starter nothing happens. And I mean nothing.... You may hear some clicking near your left knee as the other relays engage but your starter does not click or move.

I am putting together this thread to help future wiring NOOBs save themselves a few days sorting this out. With the help of several members of this forum I was able to sort it out despite being electrically impaired.... a special thanks to SCoyote and Pat Allen for their patience and contribution.

Here is the original thread: http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,335018/highlight,seatbelt/

It is a very simple solution and I will illustrate it below in pictures.

Drop me a line if you have any questions.

Cheers,

Freeheelboy

[freeheelboy]

(nt)

[freeheelboy]

(nt)

[freeheelboy]

(nt)

[freeheelboy]

(nt)

[freeheelboy]

(nt)

[M3M3]

nice work!

dang, now i am going to have to play with the wiring under my dash.....;-)

thanks!

[PatAllen]

there is another way to make this, by grounding permanently the wire identified #77 on the relay.

there is one benefit of this method, all the inrush current of the solenoid starter is bypassed by the relay so the ignition switch contact may last longer.

i have pictures of a swap from 74' seat belt interlock spec to 75-76' just seat belt buzzer spec, in my blog for a 4280905 Automatic 02 restoration.

IMPORTANT NOTE: all the above methods are 100% safe for manual cars, but for AUTOMATICS, if you do so the neutral start safety is bypassed, that means you can start the car in any gears...

There is a specific method to retain that feature. Its a matter of jumping the wire #77 directly to the neutral start switch. the wiring exits near the speedometer cable in itsw own gromet. One wire is ground and the other is ground return when car is in park or neutral. connect the wire #77 to the ground return.

hth

[M3M3]

bumping a great thread.

 

i am a bit slow.  four years after i posted that i needed to do this mod, i finally did it today.

 

this rewiring makes the starter circuit for a 74+ manual the same as the wiring of the 73 and earlier cars.  all the power for the solenoid goes through the ignition switch whether the relay is in the circuit or not.  the switch is designed for the small amount it takes to trip the solenoid.

[peterschop]

there is another way to make this, by grounding permanently the wire identified #77 on the relay.

there is one benefit of this method, all the inrush current of the solenoid starter is bypassed by the relay so the ignition switch contact may last longer.

i have pictures of a swap from 74' seat belt interlock spec to 75-76' just seat belt buzzer spec, in my blog for a 4280905 Automatic 02 restoration.

IMPORTANT NOTE: all the above methods are 100% safe for manual cars, but for AUTOMATICS, if you do so the neutral start safety is bypassed, that means you can start the car in any gears...

There is a specific method to retain that feature. Its a matter of jumping the wire #77 directly to the neutral start switch. the wiring exits near the speedometer cable in itsw own gromet. One wire is ground and the other is ground return when car is in park or neutral. connect the wire #77 to the ground return.

hth

I had to make this change to my automatic shortly after buying my car. I must not have reconnected wire #77 to ground return. I have on occasion turned the car off and forgot to put it in park. Big problem when I turn the key to start and it is still in drive and the car jumps forward. Another good reason to get the 5 speed in this winter.

[0257]

To all who have gone before me: thanks! After pulling and replacing carpet on my ‘74 tii (which has aftermarket Recaros without electrics) I could not get the starter to respond.  After two days of troubleshooting I finally got around to reading this, identifying that strange relay (now a spare for my underhood set) and performing the bypass.  Whereupon Fjord fired up fine.  Try saying that three times fast.

[Henning]

On 1/18/2010 at 2:17 AM, freeheelboy said:

(nt)

 

I can do better:

 

heh

 

hen

 

[Brandon]

Ten year old post.

 

 

[Marsattacks]

In connection with the starting issues described in another (revived) thread, I started down the path of disabling the seat belt interlock on my 74tii (if in fact it has not already been disabled).  I didn’t expect it to be completely straightforward, and I think it might not be.  Thanks in advance for any insight.  I know little to nothing about wiring/electrics but here goes based on the above very helpful posts.  I realize that there could be differences in the 74 Auto and the 74 tii which could explain some of the discrepancies here.  (In case it matters, my tii has been changed to twin webers so the tii cold start is not enabled).

 

1.  Overall shot of my ignition switch area.

2.  Shot of the black wire (B) plugged into the ignition switch with the remnants of a red wire that has previously been cut.  Looks to me like this wire enters what I assume is the seatbelt relay.  See next photo.  Pending further input I haven’t fully pulled apart the wiring because it is pretty tidy.

3.  Shot of the black and black/white (BW) wires each going into the seatbelt relay.

4.  Shot of the BW wire going into a connector but then changes to a B wire going to (I assume) the starter.

5.  I can do more tracing but as above I’ll probably need to do some unwrapping/unzipping of the wires to get good access.

 

I realize that this might be impossible for you to assess via photos but if by chance you can shed some light on what you’d do (if anything) I’d be grateful.  For now I will have jumper cables handy in case I need to power the starter directly off the battery again.  I can definitely get this to a local mechanic if you think otherwise I’m playing with fire (literally or figuratively).  Thanks!

Edited May 6 by Marsattacks

[John76]

On 5/6/2024 at 8:12 AM, Marsattacks said:

3.  Shot of the black and black/white (BW) wires each going into the seatbelt relay.

4.  Shot of the BW wire going into a connector but then changes to a B wire going to (I assume) the starter.

The black wire in your pic goes from the wiper switch to the wiper relay via the 8-pin plug connector under the dash.

The black/white wire goes from the wiper switch to the wiper motor through the same plug connector.

 

The ignition switch does not have a black wire but does have a black/white wire from tab #50 to the starter solenoid tab #50.