W124/R129 MAS control unit defective? Testing & replacement (300 24V)

The car runs, but something in the electrical system no longer works reliably: one accessory doesn't switch, another keeps running after the fact, in the morning everything works, after twenty minutes of driving it doesn't. Anyone who starts swapping individual components on a hunch quickly spends a lot of money for nothing. Because behind several accessory units sits one shared control unit: the engine accessory control unit, MAS for short, with part number 0115458332. If it fails, the fault pattern looks like five different defects – but it's only one.

This guide shows, from a workshop perspective, how to narrow down the control unit – and how to tell you're looking in the wrong place.

Symptoms

What's typical of a control-unit problem isn't a single dead accessory, but the pattern: several functions at once, sporadic, or temperature-dependent.

Symptom	Likely cause
Several electrical accessory units fail together, but fuses are intact	Central control unit without supply or without output signal – check MAS control unit
Function comes and goes, often after warm-up or on rough roads	Cold solder joint in the control unit or oxidized connector contact
Fault occurs only in wet weather	Moisture in the connector area or a leaking electronics box
Not to be confused with: simultaneous misfires in the engine electrics and fuel pump, fault occurs abruptly	More likely the OVP relay – a classic fault candidate for these model years, significantly cheaper
Not to be confused with: A/C doesn't cool, otherwise everything normal	More likely the A/C relay – a single system pointing away from a central control unit

Which accessory units run through the MAS in any given case depends on model year and equipment. So don't rely on forum lists – rely on the wiring diagram for your VIN (chassis number).

Causes

The electronics of these model years are built to be robust, but they are now over three decades old – and age in a well-known pattern. Electrolytic capacitors dry out and lose capacitance. Solder joints on heavy components and on the connector strips develop fine hairline cracks from the constant temperature swings in the engine bay; the contact still holds when cold and then fails when warm. On top of that comes moisture: through leaking seals and porous cable grommets, water migrates into the connector area and the contacts oxidize.

This explains why such faults are almost never cleanly reproducible. A control unit that works sitting on a bench can still cut out once installed at 80 degrees engine-bay temperature. Anyone who takes just one quick measurement and finds nothing has not ruled out the fault.

Testing it yourself

Work through the steps in order – starting with the cheapest. Disconnect the battery before any work on control units.

1. Fuses and ground: Visually inspect all relevant fuses, then loosen the ground points in the engine bay, clean them to bare metal and bolt them back down tightly. Corroded ground is by far the most common cause of seemingly unexplainable electronics faults – and costs nothing.
2. Pull the OVP relay and inspect it: Charred contacts or a blown fuse on the relay explain many failures immediately. The OVP relay (ÜSR) is significantly cheaper than a control unit and should therefore be checked first.
3. Unplug the connector and inspect it: Verdigris, white deposits or moisture in the connector. Clean the contacts, apply contact grease, lock it in securely – then test-drive again.
4. Measure the voltage supply at the control unit: Use a multimeter to check whether permanent positive, switched positive and ground are present at the connector. If nothing is present here, the fault is in the wiring harness, not in the control unit.
5. Warm test: If the fault only occurs once at operating temperature, repeat the test in the warm state. Only once the supply and ground are clean both warm and cold, but the outputs remain dead, is the control unit the justified suspect.

Solution

If the suspicion is confirmed, replacement with a tested genuine part is the safe way forward. We stock the engine accessory control unit with the part number 0115458332 as a genuine part, personally tested and functional, at a price of 169,00 €. There are effectively no aftermarket versions – an untested used part of unknown origin means: swapping an old component for an equally old one.

You'll find more tested control units and relays for classic Mercedes cars in the category Mercedes relays & control units, all parts for your model series in the category W124. How a control unit that looks identical but is completely different can produce its own distinct fault pattern is shown in our guide on the ASD control unit.

Compatible vehicles

Model series	Engine	Part number
Mercedes-Benz W124	300 24V	0115458332
Mercedes-Benz R129	300 24V	0115458332

This control unit is specified for the W124 and R129 with the 300-24V engine. No other model series are documented for this part – please don't order on a hunch. Send us your VIN (chassis number, FIN), and we'll check the fit for you before you buy.

FAQ

How can I tell whether the MAS control unit is really defective?

A central control unit gives itself away through the pattern: several functions disrupted at the same time, sporadically or depending on temperature. If, on the other hand, only a single system fails, the cause is almost always in its own relay, switch or wiring harness. Before replacing anything, fuses, ground, connector contacts and the OVP relay (ÜSR) should be checked.

Does the control unit 0115458332 fit every Mercedes 300 24V?

Fitment is documented for the W124 and R129 with the 300-24V engine. However, changes were made within a model series over the production period, so the VIN (chassis number) is what ultimately decides. Send us the FIN and model year – we'll check before you order.

Does the control unit need to be coded or taught in after installation?

Control units of this vehicle generation are plugged in and bolted down; software coding as with modern vehicles is not provided for. This can be confirmed definitively via the vehicle-specific documentation for your FIN. In any case, disconnect the battery when removing or installing it.

What does a tested MAS control unit 0115458332 cost?

The genuine part costs 169,00 € from us and is available from stock. Stock of these control units is naturally limited, as they are no longer produced.

Can I have a defective MAS control unit repaired?

Whether a unit can be repaired depends on the damage pattern: cold solder joints and aged capacitors are often manageable, a burnt-out trace or water damage usually isn't. Give us a call and we'll assess it based on your fault pattern – replacement with a tested part is often the faster route.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches: every control unit that leaves our stock has been personally tested beforehand. Over 1,000 eBay reviews (almost all positive) show that this effort pays off. Not sure about the fit, or want to discuss your fault pattern beforehand? Call us at 0179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form – with the FIN and model year, we'll clarify the fit before you order.