W126/R107 idle speed fluctuating? Check & replace the idle speed actuator

“Idle hunts”, “engine stalls at the traffic light”, “RPM jumps at standstill” – with these search terms, many owners of a W126 S-Class or an R107 SL with V8 eventually end up at the idle speed actuator. The component sits in the intake tract and regulates the volume of air that flows past the closed throttle valve at idle. If it sticks or fails electrically, the control unit can no longer hold the idle speed. In this guide we show how to narrow down the problem, what you can check yourself and which part number fits your car.

Symptoms

Symptom	Likely cause
Idle speed fluctuates evenly up and down	Idle speed actuator sticking due to deposits
Engine stalls when coasting or when shifting into gear	Idle speed actuator no longer opens sufficiently
Idle speed permanently far too high	Actuator stuck open or air leak in the intake tract
Idle speed drops when the A/C is switched on	Actuator no longer responds to the control
Idle hunts, and in addition displays flicker or relays click	Another cause: overvoltage protection relay – see Guide: check OVP relay (ÜSR)
Engine is hard to start when warm, idle rough afterwards	Another cause: fuel pressure – see Guide: KE-Jetronic fuel pressure regulator

Causes

The idle speed actuator works in an area where oil vapor from the crankcase ventilation and fine dirt from the intake air meet. Over the decades, this builds up a sticky coating in the air passage and on the moving slide. The actuator becomes sluggish, reacts with a delay, and the engine speed starts to oscillate because the control system constantly corrects.

Then there are the electrics: the winding and connections inside the actuator age from heat and vibration, and connector contacts corrode. If the actuator fails electrically, it stays stuck in one position – the idle speed is then permanently either too low or too high. Brittle hoses and seals around the actuator also cause air leaks, which make the problem worse.

Testing it yourself

1. Visual check for air leaks: Inspect the hoses at the idle speed actuator and in the intake tract for cracks and loose clamps. This costs nothing and rules out a common cause.
2. Check the connector: With the ignition off, pull off the connector and check the contacts for corrosion and a firm fit.
3. Feel the function: With the engine running, place your hand on the housing. A working actuator can be felt operating. If you feel nothing, an electrical fault or a stuck slide is likely.
4. Clean: With the actuator removed, deposits in the air passage can be dissolved with throttle body cleaner. If the idle speed is smoother afterwards, contamination was the cause.
5. Swap test with a tested part: If the problem persists after steps 1 to 4, replacing the actuator with a tested one is the surest way to narrow it down.

Solution

If the idle speed actuator is electrically defective or still sticks despite cleaning, replace it with a tested genuine VDO part. Make sure to check the part number of your installed actuator: for the M 116 and M 117 V8 engines with KE-Jetronic it is A0001411625 at a price of 149,00 €. For the 500 V8 up to 1985 there is the older design with clamping ring, A0001411425 (VDO 408.202), at a price of 199,00 €. Both parts are genuine used parts whose condition is described honestly; the photos show the part actually delivered. You will find further injection system components in our category Fuel system & injection.

Compatible vehicles

Part number	Model series	Models per product data
A0001411625	W126	420, 500, 560 SEL / SEC (V8 M 116 / M 117, KE-Jetronic)
A0001411625	R107	SL (V8 M 116 / M 117, KE-Jetronic)
A0001411625	G-Class	500 GE
A0001411425	W126	500 SE, 500 SEL, 500 SEC (up to 1985)
A0001411425	R107	500 SL (up to 1985)

What counts is always the part number on your installed actuator. If you are unsure, tell us your VIN or part number – we will check the fit before you buy. You will find further parts for the S-Class in the category W126.

FAQ

How do I recognize a defective idle speed actuator on the W126 or R107?

Typical signs are a fluctuating or too-low idle speed, stalling when disengaging the clutch or shifting into gear, and an idle speed that drops when the A/C is switched on. If the actuator no longer works at all, no fine movement can be felt at the housing with the engine running.

Can I clean a dirty idle speed actuator?

Deposits from the crankcase ventilation can often be dissolved with throttle body cleaner if the slide is merely sticking. If the electrics inside the actuator itself are defective, cleaning will not help – then replacing it with a tested genuine part is the solution.

What is the difference between A0001411625 and A0001411425?

A0001411425 is the older design with clamping ring for the 500 V8 up to 1985. A0001411625 is the VDO idle speed actuator for the M 116 and M 117 V8 engines with KE-Jetronic. What counts is the part number on your installed actuator.

Can a hunting idle also have another cause?

Yes. Besides the idle speed actuator, air leaks at hoses and seals, a faulty overvoltage protection relay (OVP relay) or problems in the fuel system are possible. That is why you should do the inexpensive checks first.

Does anything need to be adjusted after replacing the idle speed actuator?

The actuator itself only needs to be installed and connected correctly. If the idle speed is still wrong afterwards, the basic setting of the injection system should be checked by a specialist, because a new actuator does not compensate for a misadjusted system.

KFZTeile-Thot is a specialist business for tested and refurbished Mercedes parts with its own test benches. Over 1,000 positive eBay reviews show that our customers can rely on our descriptions and testing of the parts. Do you have questions about fit or the problem? Call us at +49 179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form.