W124 And 190E Relays: Which One Is Failing?

A W124 or 190E that will not start, an air conditioner that blows warm, indicators that stick on: these sound like three expensive problems. On these cars they are very often the same small, cheap problem, a relay. A plug-in black box with its part number printed on top, which has simply worn out after thirty years of switching.

That is good news, because a relay costs a fraction of a fuel pump or an A/C compressor. The catch is knowing which one. Here is how to match the symptom to the relay, what each one does, and how to check before you buy anything.

Before you buy anything

Three checks save most of the wasted money on these cars.

Check the fuse first. It sounds obvious and it is skipped constantly. A blown fuse and a dead relay produce exactly the same symptom.

Read the number on top. Every genuine relay carries a Mercedes part number, printed in groups like 001 545 99 05. The same part is listed online as 0015459905 or A0015459905. They are all the same number. Two relays can look identical and do completely different jobs, so match the number, not the shape.

Look at the pins. Pull the relay and look at the legs and the socket. Green corrosion or brown heat marks mean a poor contact, and a poor contact behaves exactly like a failed relay. Sometimes cleaning the socket is the whole repair. If a pin is burnt, the socket needs attention too, or the new relay will go the same way.

A relay that clicks is not necessarily a good relay. The click is the coil. The contacts that actually carry the current can be pitted and burnt while the coil still works perfectly. If you have an identical relay elsewhere in the car or a spare, swapping them is the quickest honest test there is.

Symptom to relay

Starting and fuel

On a petrol W124 or 190E the fuel pump only runs when a relay tells it to. When that relay starts to fail, the car cranks normally but struggles to fire, or starts and then dies, often worse when hot. It is intermittent right up until the day it is not.

On KE-Jetronic petrol cars, look at the overload protection relay, usually called the OVP, at the same time. It feeds the injection system and the ABS, so a failing one can cause cutting out and an ABS warning together. Many carry a small fuse on top, which is worth checking before anything else.

Diesel glow plugs

The glow plug relay runs the pre-heat cycle before a cold start. When it fails, the engine cranks for a long time on a cold morning, smokes, and runs roughly until it warms up. On later diesels the relay also monitors the plugs, so a glow light that flashes or stays on can point to a failed plug rather than the relay itself. Test the plugs before you condemn either.

Diesel engines and model years use different glow relays, so the number on top of yours is what decides it.

Air conditioning

The A/C relay decides when the compressor clutch engages, and on some cars when the auxiliary fan runs. A failing one gives you a compressor that never engages, or one that cuts in and out while the system is otherwise healthy.

One thing to rule out first: if the refrigerant is low, the system deliberately refuses to run the compressor to protect it. A new relay will not change that. Have the pressure checked, and if it is fine and the compressor still will not engage, the relay is the next suspect.

Lights, indicators and wipers

The combination relay handles the indicators, the hazard flashers and the intermittent wiper. When it goes, the faults are odd and seem unrelated: indicators that stay lit instead of flashing, hazards that do nothing, a wiper that pauses at random. Because one box runs all three, seeing two of those at once is a strong hint.

The light control module watches the exterior lighting circuits and lights the bulb warning in the cluster when something is wrong. If that warning will not clear after you have replaced the bulb, the module is a likely cause. One common trap: LED replacement bulbs draw far less current than the originals, and the module can read that as a failed bulb.

Windows and comfort functions

The comfort modules add the convenience layer on top of the basic switches: one-touch windows, closing everything from the key, sunroof control. The giveaway is that the basic function still works while the clever part does not, such as a window that moves while you hold the switch but ignores a single press.

ASD, if your car has it

ASD is the automatic locking differential fitted to some W124 and W201 models. It locks the rear differential when a wheel spins, and a warning light in the cluster tells you it is working. A faulty relay can light the warning without cause, or stop the system engaging when you need it.

Replace the old one with a known good one

These are thirty year old parts, so every relay and module in this guide is a genuine Mercedes unit that has been tested, with the pins checked, rather than a mystery box from a parts bin. That matters more than it sounds: fitting an untested used relay just swaps one unknown for another.

If you are working through an older car, the other thirty year old parts in the cabin deserve the same attention. Our W124 interior restoration guide covers what to tackle first, and the instrument cluster guide explains the faults that show up on the dash.