BBB (BeauKey's Beemer Blog)

WARNING: do not work with airbags if you are not certified. Effectively airbags are explosives that can cause significant harm when not handled properly.

WARNING: DO NOT modify, cut, or change anything about the car- of seat wiring or connectors. All procedures and methods here can/must be done with unmodified cables and connectors.

NOTE: modification of the car systems might void warranty, or might be illegal since the car approval was based on the one or more of the systems that are disabled.

Although this post is about Renault Kadjar, the procedures that are described here are quite universal, and could be used for other cars too.

Introduction

Most (all, these days?) cars have airbags, also known as “Supplemental Restraint System” (SRS). Airbags are all located over the car: Steering wheel, passenger seat dashboard, seats, side-airbags (“curtains”), etc. Airbags are engaged when the car is subject to heavy G-forces, e.g. during a collision.

Sometimes the factory seat is replaced by another seat that suits a certain use-case. E.g. for long people (my case), or to support disabled people, or any other use-case where the factory seat does not fit.

Car front seats are usually bolted to the floor of a car with 4 bolts. That is simple.

More complex are the electrical systems that might be present in car seats:

Seat belt warning
Seat heating
Electrical seat adjustment
Back massage (…)
Seat sensors
Airbags

All these functions require one or more connectors to connect the feature to the car battery and/or car computer. Each of these connectors must have a unique mechanical outline to prevent mixing up at the wiring of the seat.

Front seat replacement

A replacement seat usually has no, or maybe a subset of features of the original seat. The “Seat belt warning” is always used (mandatory). Replacement seats usually do not have airbags.

Airbag

When a car ignition is switched on, a whole bunch of tests are executed to check the status of the car. When the original seat is replaced, it will detect that an airbag is missing. A warning light on the dashboard warns the driver, and the car electronics (OBD2) might log the failing airbag. To pass MOT (periodic car check, “APK” in Dutch, “MFK” in German), the airbag warning light MUST be off to pass MOT.

To “mimic” the original airbag, a dummy resistor with the same electrical resistance of the seat’s airbag must be placed on the airbag connector. When the car tests the airbag, it finds the same characteristic as the original airbag, and the car will assume that everything is OK.

Other seat features that require modification

In the Renault Kadjar, these seat features require modification too:

Seat position sensor (will also give a "Check Airbag" warning)
Seat pressure sensor (will give "Check Start/Stop system" warning)

Seat connectors

The Kadjar driver seat has 6 connectors:

Connectors, from left to right (bold items require actions):

White, 2 wires: Seat belt (connector not shown on this picture)
Brown, 2 wires: Seat position sensor
White,2 wires: Seat pressure sensor
Brown, multiple wires: Car seat and backrest adjustment
Brown, 2 (thicker) wires: Seat heating
Yellow, 2 wires: Airbag

Airbag dummy

Message/error: Airbag error symbol lights up on dashboard

Mitigation: place 2.2 Ohm resistor behind yellow car connector

Challenge: both sides of the airbag connector contain switches to prevent the airbag engagement from static electricity or other trigger.

Airbag connector switches

When you compare the two yellow airbag connectors to the brown and white connectors, you might notice that both yellow connectors look "complex". That has a reason. There are switches inside the yellow connectors that prevents the airbag to engage when the seat is disconnected or removed from the car.

The yellow connectors and switches look like this:

When the yellow connectors are connected (upper schema), the switch in the in the seat connector is open and the switches in the car connector are closed. The airbag is active.

When the yellow connectors are disconnected (lower schema), the switch in the seat connector is closed, shorting the airbag wires. This will prevent that the airbag could be triggered. The airbag is not active. The two switches in the car connector are opened.

The magic happens when you connected the two yellow connectors. The car-side switches are closed, and the seat switch is opened.

The Kadjar seat airbag measured 2.1 Ohm, and a dummy resistor (2.2 Ohm) is placed on the car side, behind the car connector:

I made an airbag dummy resistor, using an insulation piercing connector (red on picture). See picture below.

Seat position sensor dummy

Message/error: Airbag error symbol lights up on dashboard.
Mitigation: place a 100 Ohm resistor on the seat position sensor car connector.

The Seat position sensor is used to detect when the seat is in the (most) front position. The information of this sensor is used to manage the airbag in the steering wheel. By using a 100 Ohm resistor the car assumes the seat in a normal position.

NOTE: the workaround above can be dangerous for drivers who are sitting (very) close to the steering wheel, e.g. short people. In this case a 0 Ohm resistor (wire bridge) must be used as a dummy, to limit the impact (power) of the steering wheel airbag, in case of a frontal collision.

This is the schema of the sensor:

Seat pressure sensor dummy

Message/error: Check Start/Stop system
Mitigation: place a 200 Ohm resistor on the seat position sensor car connector.

This sensor is to detect if someone is sitting in the driver seat.

The schema of this sensor is like this:

The use-case for the Seat pressure sensor is, that when Start/Stop has stopped the engine, and the driver leaves the car, the handbrake will be engaged and the Start/Stop system will not start the engine until the driver is back in the seat and the seat belt is on.

Using a 200 Ohm resistor as a dummy, the car assumes that there is a person is on the seat. This will make the Start/Stop system to work normally and it will not show the warning ("Check Start/Stop System") on the dashboard.

Bringing it all together

The next picture shows the driver seat removed and the car cabling with the three dummies attached (2.2, 100 and 200 Ohm).

You see the three dummy resistors before shrink wrapping. The two red clips are the insulation piercing clips, which connect the airbag dummy resistor "behind" the yellow connector of the car.

Before shrink wrap. Yellow tie-wraps applied to fix and show mods.

Final state, after applying shrink wrap. The car cabling will be fixed to the new seat's base plate, to prevent the cabling "lying around" on the floor: