R129/W124 surging idle? Check 9-pin OVP relay 2015403745

Anyone looking for the overvoltage protection relay on the SL R129, the 190 E 2.5-16, the 300 E-24 or the G-Class will mostly find information on the 7-pin version – but what is installed there is the 9-pin one with two relay stages. The symptoms are the same, the spare part is not. Here you will read how to narrow down the OVP relay (ÜSR) and identify the right version.

Symptoms

Symptom	Likely cause
Idle surges or is too high, engine stalls when coming to a stop	Idle control unit at terminal 87L without clean supply
Poor starting behavior, up to a no-start condition	Terminal 87E does not switch through: KE-Jetronic and lambda control without power
ABS and ASD/ASR warning light permanently on	ABS control unit is on the same relay stage and fails too
Bucking, misfires, increased consumption – often only when warm	Cracked solder joint in the relay or one of the two 10 A fuses blown
Not to be confused with: Engine does not start at all, pump stays silent when the ignition is switched on, idle and ABS otherwise normal	Often more likely the fuel pump relay – see W126 won't start? Check the fuel pump relay

Causes

The overvoltage protection relay has two jobs: it protects the control units from voltage spikes in the vehicle electrical system and at the same time switches their power supply. Terminal 87E supplies KE-Jetronic, lambda control and ABS, terminal 87L the idle control unit, and terminal 30a provides filtered permanent positive for the fault memory. For this, the 9-pin version uses two separately switched relay stages with a 10 A fuse each.

After decades, the relay usually fails for one of two reasons: condensation under the cover and constant temperature cycling make a solder joint crack, or a fuse blows on a voltage spike. In both cases the faults often show up only when warm.

Testing it yourself

1. Visual and moisture check. The relay sits on the right behind the battery under the black cover, next to the ABS and KE control units. Verdigris or moisture are first indications.
2. Check both fuses. The 9-pin version has two 10 A fuses, one per relay stage. If only one has blown, only some of the consumers fail. If a new fuse blows again immediately, have the alternator and regulator checked first.
3. Count the pins and read the part number. Nine contacts and two fuses mean 9-pin. If the housing reads A 000 540 52 45 or A 000 540 67 45 with a blue 15 A fuse, it is the later HFM generation – then this part does not fit.
4. Clean the contacts, seat the relay firmly. Clean the contacts and reseat. If the fault returns after a short time, that points to a cracked solder joint.
5. Cross-check when warm. Drive until warm and watch for surging idle, ABS light and starting behavior. If these symptoms occur together, the OVP relay (ÜSR) is the obvious candidate.

Solution

With a cracked solder joint or a damp circuit board, replacing it with a tested genuine part is the clean way. For the 9-pin version, that is the Part number A 201 540 37 45 (2015403745): used genuine part, tested for function, both fuses intact, 59,00 €. The plug-in relay is directly interchangeable and replaces the 5- and 7-pin predecessors A 201 540 08 45, A 201 540 30 45, A 201 540 32 45 and A 201 540 38 45 upward – not the other way round. If your relay has only seven contacts and one fuse, you will find the matching variant in the guide W124/W201 surging idle? Check 7-pin OVP relay.

Not to be confused with the HFM generation for M104/M111 from approx. 1993 (A 000 540 52 45 / 67 45, blue 15 A fuse). You will find further relays and control units for these model series in the category Mercedes relays & control units.

Compatible vehicles

Fitment for part number 2015403745 (9-pin, two 10 A fuses) according to product data:

Model series	Models
W201 (190)	190, 190 E, 190 E 2.3, 190 E 2.6, 190 E 2.3-16, 190 E 2.5-16, 190 E 2.5-16 Evolution, 190 D, 190 D 2.5, 190 D 2.5 Turbo Diesel
W124 sedan	200 E, 230 E, 260 E, 260 E long, 300 E, 300 E-24, 200 D, 250 D Turbo Diesel, 300 D, 300 D Turbo Diesel, 260 E 4MATIC, 300 E 4MATIC, 300 D Turbo Diesel 4MATIC
W124 Coupé, Convertible, Estate (T-Modell)	230 CE, 300 CE, 300 CE-24, 200 CE / E 200, 200 T, 230 TE, 300 TE, 200 TD, 250 TD, 250 TD Turbo Diesel, 300 TD, 300 TD Turbo Diesel, 300 TE 4MATIC, 300 TD Turbo Diesel 4MATIC
W124 chassis	230 E, 250 D
W126 / C126	260 SE, 300 SE, 300 SEL, 420 SE, 420 SEL, 500 SE, 500 SEL, 560 SE, 560 SEL, 350 SD, 350 SDL, 420 SEC, 500 SEC, 560 SEC
R107 SL	560 SL
R129 SL	300 SL, 300 SL-24, 500 SL / SL 500
W202 C-Class	C 180, C 200, C 180 T, C 200 T, C 200 Kompressor T, C 200 Diesel
W461 / W463 G-Class	230 GE, 290 GD, 200 GE, 230 GE / G 230, 300 GE / G 300, G 320 / G 36 AMG, 250 GD, 300 GD / G 300 Diesel, 350 GD Turbo / G 350 Turbo Diesel
Commercial vehicles	T1 Bremer Transporter 207 D to 410 D, Vario 512 D, T2/LN2 709 D and 711 D

Does not fit: W124 400 E and 500 E – these models have no separate overvoltage protection relay; the function is integrated in the base module.

FAQ

How do I know that I need the 9-pin version 2015403745?

Count the contacts and fuses on the removed relay: nine contacts and two 10 A fuses mean 2015403745. The 7-pin numbers have only one fuse.

Can the 9-pin version replace a 7-pin relay?

Yes. The 2015403745 replaces the 5- and 7-pin predecessors upward, because surplus pins find no mating contact in the old socket. The other way round it does not work.

Does the overvoltage protection relay 2015403745 fit the R129 and the G-Class?

Yes, the R129 300 SL, 300 SL-24 and 500 SL are covered, as well as numerous models of the G-Class W461 and W463.

What happens if I install the wrong OVP relay (ÜSR) generation?

The HFM generation A 000 540 52 45 / A 000 540 67 45 (blue 15 A fuse) is not interchangeable. If installed incorrectly, the engine cannot be switched off and the transmission no longer shifts.

How much does the tested overvoltage protection relay 2015403745 cost?

59,00 € for the used genuine part, tested for function. Free shipping within Germany, 60-day return policy.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with our own test benches. Relays go over the test bench before sale; over 1,000 positive eBay reviews speak for this approach. Unsure about pin count or part number? Send us your VIN and model year, and we will check the fit free of charge: +49 179 8991069 (Mon–Fri 8 am–6 pm) or via our contact form.