W124/W201 SRS light on? Check airbag control unit A0048201710
The SRS light in the instrument cluster no longer goes out – and that raises the question of whether the airbag will deploy at all in a crash. This guide shows how to recognize a defective airbag control unit, what you can safely check yourself, and which unit fits your equipment.
Genuine Bosch SRS airbag control unit A 004 820 17 10 for vehicles with a driver airbag and belt pretensioners, without passenger airbag.
Symptoms: How do you recognize a defective airbag control unit?
With its built-in acceleration sensor, the control unit measures the vehicle's deceleration and fires the driver airbag and belt pretensioner in a crash. At the same time, it continuously monitors the restraint system and controls the SRS indicator light.
| Symptom | Likely cause |
|---|---|
| SRS indicator light stays on permanently after engine start | Fault in the restraint system – often the airbag control unit |
| Fault in the airbag control unit cannot be cleared and returns immediately | Airbag control unit defective |
| SRS light stays on after an accident with airbag deployment | Control unit locked after deployment – replacement required |
| Several indicator lights stay dark at the same time or behave erratically | More likely the Indicator unit in the instrument cluster, not the airbag control unit |
| Faults in several control units, rough idle, voltage problems | More likely the overvoltage protection relay or the on-board voltage |
Causes: Why does the airbag control unit fail after decades?
The control units of this generation are 30 years old or older. Electronic components age, solder joints fatigue from temperature changes and vibration, and the unit's self-diagnosis detects such deviations as faults. In addition there is the installation location on the transmission tunnel under the center console (on the R129, in front of the selector lever): moisture in the footwell, for example from leaking doors or windows, stresses the housing and connector strips. And after an airbag deployment, the unit locks itself as a matter of principle and must be replaced.
Test it yourself: 5 steps before you replace it
Important: The restraint system contains pyrotechnic components. Never measure at airbag or belt pretensioner circuits with a multimeter, and work on connectors only with the battery disconnected. If in doubt, have the work done by a specialist workshop.
- Observe the lamp test: After switching on the ignition, the SRS light comes on briefly and then goes out. If it stays on or comes back on while driving, the control unit has detected a fault.
- Check the on-board voltage: A weak battery or voltage problems can trigger faults in control units. Check the state of charge and the charging voltage – it costs nothing.
- Check the connector and installation position: With the battery disconnected, check the connector at the control unit for a firm fit, moisture and corrosion. The arrow on the housing marks the required installation position.
- Read out the fault memory: Only reading it out shows whether the control unit itself or another part of the system is being reported. If an internal control unit fault persists after clearing, replacement is due.
- Compare the part number: Read the number off the unit installed in your car. A 004 820 17 10 belongs to vehicles without a passenger airbag, A 004 820 18 10 to vehicles with driver and passenger airbags.
Solution: install genuine control unit A 004 820 17 10
We have the used genuine Bosch control unit A 004 820 17 10 (Bosch 0 285 001 030) in stock several times over – three identical units, housings and connector strips undamaged, 119,00 € per piece. It is a genuine part, not an aftermarket one. Mercedes-Benz lists A 000 820 68 26 as the successor number.
Note two points of distinction: According to Mercedes, control units and sensors from Bosch and Temic must not be combined. And the W124 and R129 from model year 1994 onward have newer control units (for example A 000 820 71 26 / 72 26) – this unit is not intended for them.
Now in stock: genuine Bosch airbag control units
A 004 820 17 10
Bosch 0 285 001 030 · Driver airbag, without passenger airbag
119,00 € · Genuine, used
View offer →
A 004 820 18 10
Bosch 0 285 001 031 · with driver and passenger airbags
249,00 € · Genuine, used
View offer →You can find more control units and relays for classic Mercedes in the section Mercedes relays & control units as well as sorted by model series under W124 and W201.
Airbag control unit A 004 820 17 10
Genuine Bosch 0 285 001 030 for vehicles with a driver airbag and belt pretensioners, without passenger airbag – 119,00 € per piece, several identical units in stock.
To the control unit →Compatible vehicles
A 004 820 17 10 fits vehicles with a driver airbag without passenger airbag:
| Model series | Models |
|---|---|
| 190-series W201 (1988–1993) | 190 E 2.3, 190 E 2.6, 190 D 2.5 |
| E-Class W124 / S124 / C124 (up to 06/1993) | 260 E, 300 E (also 4MATIC and 300 E-24), 300 D, 250 D Turbo, 300 TE (also 4MATIC), 300 CE, 400 E, 500 E |
| S-Class W126 / C126 | 300 SE / 300 SEL, 420 SEL, 560 SEL, 560 SEC, 350 SD / 350 SDL |
| SL R107 | 560 SL |
| SL R129 | 300 SL, 500 SL |
Does not fit vehicles with a passenger airbag, the S-Class W140, or W202, W210, R170 and W208 (Temic control units). If in doubt, send us your VIN (chassis number) – we check free of charge.
FAQ
How do I recognize a defective airbag control unit?
Typical is an SRS indicator light that no longer goes out after engine start, and a fault in the airbag control unit that cannot be cleared. Whether the cause really is the control unit, or a wire, a connector or another component of the restraint system, only becomes clear once the fault memory is read out.
Which airbag control unit fits my W124 or W201?
What matters is the airbag equipment, not the engine. A 004 820 17 10 (Bosch 0 285 001 030) belongs to vehicles with a driver airbag and no passenger airbag. Vehicles with driver and passenger airbags need A 004 820 18 10 (Bosch 0 285 001 031). Compare the number on your installed unit or send us your VIN (chassis number) via the contact form.
Can I swap A 004 820 17 10 and A 004 820 18 10 for each other?
No. The two units are not interchangeable: 17 10 is for vehicles without a passenger airbag, 18 10 for vehicles with driver and passenger airbags.
Does the control unit also fit a W124 from model year 1994?
No. The W124 and R129 from model year 1994 onward have newer control units, for example A 000 820 71 26 or 72 26. A 004 820 17 10 fits the W124 up to 06/1993.
Does the airbag control unit have to be replaced after a deployment?
After an airbag deployment, the control unit is locked and must be replaced. In this case, have the entire restraint system checked by a specialist workshop.
KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches – specialized in classic Mercedes cars, with over 1,000 positive eBay reviews. Questions about the airbag control unit or the matching part number? Call us: +49 179 8991069 (Mon–Fri 8 am–6 pm) or use the contact form.