Overvoltage protection relay faulty? Testing and replacement (W126, W201)
Hardly any component causes as much troubleshooting on classic Mercedes as the overvoltage protection relay. It sits hidden in the electrics box, yet still switches key loads. When it fails, people often search the injection system and wiring harness for days – when in the end, one socket is all it takes. This guide shows the typical symptoms, how to narrow it down in five steps, and the matching tested genuine part in stock.
Original Mercedes-Benz overvoltage protection relay A1265404745 – tested and functional, scope of delivery as pictured.
Symptoms: how do you recognize a faulty overvoltage protection relay?
The OVP relay protects the connected electronics from voltage spikes and routes the power supply for several control units through a shared fuse plugged in on top. That's exactly what makes the fault pattern so confusing: a small relay knocks out seemingly unrelated systems at the same time. Which loads run through it depends on the series, engine and equipment – the basic principle is the same for all of them.
| Symptom | Likely cause |
|---|---|
| Engine suddenly cuts out at idle or while driving and won't restart afterward | OVP relay: fuse blown or contact in the relay broken – power supply to the injection electronics is lost |
| Idle hunts, fluctuates, or stays continuously too high; the engine responds poorly to the throttle | OVP relay: idle control without a clean power supply (equipment-dependent) |
| Warning light for ABS or traction control stays on, often together with engine misfires | OVP relay: several control units are connected to the same fuse |
| Fault occurs sporadically – after vibration, in wet conditions, or only at certain temperatures | OVP relay: corroded connector contacts or a cold solder joint in the relay |
| Engine starts cold poorly, but then runs cleanly afterward | More likely cold start valve – it only operates during the starting process |
| Poor starting only after the car has stood for a longer time, no problem after a short stop | More likely fuel accumulator |
| Only individual warning lights in the instrument cluster act up, the engine runs perfectly | More likely the check control unit |
Rule of thumb from workshop practice: if the fault affects several independent systems at the same time, it's rarely the case that each control unit is individually faulty – then it's worth looking at the shared power supply. If the trouble stays limited to one system, you're better off continuing to search there.
Causes: why does the relay fail after decades?
Inside sit a circuit board with solder joints, components that protect against voltage spikes, and sturdy connector contacts. After decades in the engine bay, a lot adds up: constant temperature changes cause fine cracks to form in solder joints – contact is then only intermittent, typically not there when it's shaking or hot. Moisture in the electrics box oxidizes connector tabs and sockets, and the spring force of the contacts weakens.
Important for context: a blown fuse on the relay is initially just a symptom. A one-off blow can simply be due to age or a voltage spike. If it blows again immediately after replacement, there's a short circuit in the downstream circuit – then a new relay won't help, and the fault must be found in the wiring harness or at the load itself.
Test it yourself: 5 steps before you replace it
- Check the fuse on the relay: the cheapest step. Pull off the fuse, measure it, clean the contact surfaces and reseat it firmly. The installation location differs depending on the series – look for the relay carrier in the engine bay.
- Inspect the connector contacts: Pull off the relay and check the pins, socket and connector tabs for verdigris, corrosion or scorch marks. Moisture in the electrics box is a warning sign and should be addressed as well.
- Measure voltage at the socket: with the ignition switched on, check the supply at the socket against ground. If no voltage arrives there at all, the fault lies upstream – in the pre-fuse or the wiring harness – and not in the relay.
- Wiggle and tap test: with the engine running, carefully wiggle the relay or tap gently against it. If the engine misfires or dies during this, that points clearly to a cold solder joint or a worn connector contact.
- Swap test: the definitive proof. Plug in a relay with the same part number that's known to work, and try to reproduce the fault. If it disappears, the old relay was the cause.
Solution: tested genuine relay A1265404745
Because the power supply for sensitive control units depends on this relay, we rely on genuine parts: every relay is individually tested before sale and offered only if functional. The genuine overvoltage protection relay A1265404745 costs 69,00 € and is currently available in five units from stock; the scope of delivery matches the photos in the listing.
Overvoltage protection relay A1265404745
Original Mercedes-Benz, tested and functional. Fits, according to the listing, W107, W116, W123, W126 and W201 – 69,00 €, five units in stock.
View offer →Looking for further switching relays and control units for your model series? You'll find an overview in the category Mercedes relays and control units; also tested and in stock, for example, is the ASD relay (automatic locking differential) A0085452432 for 44,00 € (W201, W124, W126).
Compatible vehicles
| Model series | Model series |
|---|---|
| W107 / R107 | SL and SLC |
| W116 | S-Class (listed as compatible in the listing title) |
| W123 | Sedan, coupe, wagon |
| W126 | S-Class |
| W201 | 190 |
This information comes from our product data for part A1265404745. What always matters is the number on your fitted relay – depending on model year and equipment, different versions existed. If in doubt, send us a photo of the old part or your chassis number, and we'll check the fit free of charge.
FAQ
What does the overvoltage protection relay do on a Mercedes?
It protects the connected electronics from voltage spikes in the electrical system and routes the power supply for several control units through a shared fuse plugged in on top. Exactly which loads run through it depends on the series, engine and equipment.
What symptoms does a faulty overvoltage protection relay have?
Typical signs are the engine suddenly dying followed by starting problems, a hunting or too-high idle, and warning lights that appear together seemingly unrelated. It's also characteristic that the fault comes and goes sporadically.
Can I test the overvoltage protection relay myself?
Yes, largely. Checking the fuse, inspecting the plug contacts for corrosion, measuring the voltage at the socket, and the knock test with the engine running can all be done without special tools. The definitive proof comes from swapping in a working relay with the same part number.
The fuse on the relay keeps blowing – is the relay defective?
Not necessarily. If the fuse blows again immediately after replacement, that points to a short circuit in the downstream circuit. That fault must be found first, otherwise it will destroy a new relay or its fuse too.
Does the relay A1265404745 fit my vehicle?
According to our product data, it fits W107, W116, W123, W126 and W201. What matters is the number on your installed relay. If in doubt, send us the chassis number or a photo of the old part via the contact form – we'll check free of charge.
KFZTeile-Thot is a specialist workshop for tested and refurbished Mercedes parts with our own test benches, specializing in Mercedes classic cars, instrument clusters and electrical components – with over 1,000 eBay reviews (almost all positive). Questions about the overvoltage protection relay or fitment? Call us: 0179 8991069 (Mon–Fri 8 am–6 pm) or use the contact form. More electrical guides: Control unit defective and Check the turn signal flasher control unit.