ASD control unit defective? Testing & replacement (W124, W201, W126)
Short answer: If the ASD warning light stays on permanently after starting, or the locking differential no longer engages on a slippery road, the cause on older Mercedes models is very often the control unit or relay of the automatic locking differential. For the W201, W124 and W126, we carry the tested genuine part with part number A0085452432 for 44,00 €, and for the W201, W124, W126, R129 and W202 the version A0135450132 for 99,99 €. Before replacing it, it's always worth checking the fuse, ground and plug contacts – that costs nothing and resolves half the cases.
The automatic locking differential was a cost option on Mercedes vehicles of the 1980s and 1990s that hardly anyone has on their radar today – until the small ASD light in the instrument cluster stops going out. The fault usually shows up after the car has been standing, after driving through wet conditions, or seemingly for no reason when pulling away. We regularly see this land on our bench as "ASD light on, differential lock dead", and the search almost always starts in the same place: with the electronics, not the rear axle.
Symptoms
The ASD shuts itself off when it detects a fault and signals this via the warning light. The overview below helps you sort out the cause:
| Symptom | Likely cause |
|---|---|
| ASD light stays on permanently after starting the engine | System has shut itself off – control unit, wiring or sensors |
| Locking differential no longer engages on snow, wet roads or gravel, one rear wheel spins | No activation by the control unit, or no pressure in the hydraulic circuit |
| Light flickers or only comes on in wet conditions and over bumps | Contact problem at the relay socket, corroded pins, a cold solder joint in the unit |
| ASD is working, but several warning lights are acting up at the same time | Not the ASD – first check the Checking the overvoltage protection relay |
| Entire warning light panel not working, or no self-test | Not the ASD – see Control unit in the instrument cluster |
Causes
On these model series, the ASD's control units and relays sit in the electronics box or the relay box, and are thus exposed to what wears down plug-in slots after thirty to forty years: moisture, temperature changes and vibration. On the circuit board, solder joints and electrolytic capacitors age, and oxide forms on the connector tabs. Both lead to transition resistances that throw the unit off, temporarily or permanently.
On top of that comes the usage pattern: an ASD that hasn't been called on for years because the car is only driven in nice weather doesn't show up until the first wet autumn day – but the fault has been there all along. And because the system shuts itself off as a safety measure at every detected discrepancy, a wobbly contact is enough to disable the differential lock. A mechanical fault in the differential itself, by contrast, is a considerably rarer explanation.
Testing it yourself
- Check the fuse and ground. Remove the fuse responsible for the ASD, as per the vehicle documentation, and hold it up to the light – hairline cracks aren't visible while it's installed. Then check the ground points near the electronics box for corrosion. Costs nothing and explains a surprising number of failures.
- Inspect the socket and pins. With the battery disconnected, pull the control unit and check the contacts and socket for verdigris, moisture or widened sockets. Carefully clean the contacts, seat the unit firmly again, and take a test drive. If the light disappears, it was the contact.
- Observe the self-test. Switch on the ignition and watch how the ASD light in the instrument cluster behaves: does it come on at all, and does it go off again after starting? If it stays on permanently, the system has shut off – if the light doesn't come on at all, that points more toward the display side instead.
- Rule out the neighbors. Before suspecting the control unit: check the overvoltage protection relay and the control unit in the instrument cluster. If several systems are acting up at the same time, the common cause is almost never the ASD module.
- Swap test with a tested unit. Only once the supply, contacts and surroundings are clean does swapping in a working genuine unit bring clarity. On these model series, it's done in a few minutes and needs no special tools.
Solution
If the suspicion is confirmed, replacing it with a tested genuine part is the clean approach. We personally test every control unit for function before shipping; which version you need is decided by the number on your removed unit – not the model year. So compare the part number directly on the component before ordering.
Not sure of the part number?
Send us the chassis number and model year – we'll match the right version for you. You'll find more modules in the category Mercedes relays & control units.
Have the part number checkedCompatible vehicles
| Model series | A0085452432 (44,00 €) | A0135450132 (99,99 €) |
|---|---|---|
| W201 (190) | yes | yes |
| W124 | yes | yes |
| W126 | yes | yes |
| R129 (SL) | not covered | yes |
| W202 | not covered | yes |
These details refer to the parts we have on hand. Since ASD was an optional-equipment system and different versions were fitted within a single model series, the number on your removed unit is always decisive. You'll also find matching models for your series in the categories W124 and W201.
FAQ
Can I keep driving with a defective ASD control unit?
Yes. If the system fails, it shuts itself off and the rear axle behaves like that of a vehicle without a locking differential. You won't notice anything on a dry road, but on wet roads, snow and hill starts you'll be missing the traction benefit. There is no safety risk in the sense of a brake or steering fault.
How do I know which part number my vehicle needs?
Don't rely on the model year – read the number directly off the removed unit instead. We carry the versions A0085452432 and A0135450132; if you're unsure, send us the chassis number and model year and we'll check it for free.
Is the installation something I can do myself?
Generally yes. The units are plugged in and sit accessibly in the electronics box or the relay box. Disconnect the battery, pull the old unit, plug in the new one – no special tools needed. The only thing that matters is that the contacts are clean and the unit is seated firmly.
Why is the ASD light still on after replacement?
Then the fault wasn't in the module. The most common candidates are then corroded plug contacts, a ground problem, a defect in the sensors, or missing pressure in the hydraulic circuit. Work through the points in the "Check it yourself" section one by one before replacing any more parts.
Are the control units new or used?
These are genuine parts from our stock that we personally test for function before selling. Signs of use on the housing are possible due to age, but the function is tested. The scope of delivery matches the product photos.
KFZTeile-Thot is a specialist workshop for tested and refurbished Mercedes parts with our own test benches. Every control unit, every relay and every instrument that leaves our premises has been personally handled and tested by us beforehand – that's the difference from parts of unknown origin. Over 1,000 eBay reviews (almost all positive) speak for this approach. If you're not sure about the part number, call us: 0179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form – the chassis number and model year are enough, we'll take care of the matching.