W140 rear window heater not working? Check the control unit
Short answer: If the heated rear window in the Mercedes W140 stays cold – often along with a failed hazard warning system or turn signal unit – the shared control unit with part number 1408203026 is often behind it. First check the fuse, switch, and ground. If the fault remains, replacing it with a tested genuine part helps: 169,00 €, available in stock.
In winter, the S-Class rear window fogs up within a few minutes. You press the switch for the rear window heater – and nothing happens: the window stays cold. Anyone who then first suspects the heating element is usually looking in the wrong place. In the W140, the turn signal unit, hazard warning, and heated rear window are all tied to a shared control unit. If this module fails, the affected functions disappear either together or one after another – and that exact pattern is the most important clue to the cause.
Symptoms
First work out which functions are affected. That narrows down the cause faster than any measurement:
| Symptom | Likely cause |
|---|---|
| Rear window heater not working, indicator light in the switch stays dark | Control unit not switching the output through, or not getting power itself |
| Rear window only gets lukewarm, or switches off after a short time | Contact resistance at the plug slot, or a tired switching contact in the control unit |
| Hazard warning dead, left and right turn signals still work | Hazard warning branch in the control unit has failed |
| Rear window heater and Hazard warning fail together | Classic picture of a faulty control unit 1408203026 |
| Not to be confused with: Only turn signals and hazard warning affected, rear window heats fine | Then the separate turn signal control unit is more likely – see W140 turn signals & hazard warning not working |
| Not to be confused with: Only a narrow strip of the window stays fogged | Broken heating element in the glass – the control unit is then working correctly |
Causes
The module isn't just a signal device, it also switches load. Of the three functions, the rear window heater draws by far the highest current, and it runs on nearly every drive in the cold season. After three decades, that adds up to a very high number of switching cycles.
In workshop practice we repeatedly see the same tell-tale signs for this: burnt switching contacts inside, dried-out electrolytic capacitors on the board, and – especially common – fine cracks at the solder joints of the load-carrying components. Such hairline cracks explain the typical in-between stage where the heater sometimes works and sometimes doesn't, often depending on temperature or road jolts. Add to that corrosion at the plug contacts where moisture has sat in the electrical box for years.
Important for diagnosis: such a module rarely dies all at once. If the hazard warning has been acting up for weeks and the rear window heater now fails too, the matter is usually clear.
Testing it yourself
Work through the points in order – they're sorted by effort and cost, with the control unit deliberately last.
- Measure the fuse, don't just look at it. Pull the fuse for the rear window heater per your vehicle's fuse chart and check it with a continuity tester. Hairline cracks in the fuse element are barely visible to the naked eye.
- Check the switch. Does the switch click cleanly into place, and does the indicator light come on? If it stays dark, the switch itself may be faulty, or it isn't getting feedback from the control unit.
- Check voltage at the window. With the ignition on and the heater activated, measure directly at the rear window's contact tabs. If battery voltage is present there and the window still stays cold, the fault is in the heating element or its ground connection – not the control unit. If no voltage is present, the search continues toward the control unit.
- Clean the plug slot and ground. Pull the control unit, check the contacts for verdigris and burn marks, dry-clean the plug slot, and plug it firmly back in. At the same time, check the associated ground points. Quite often the fault is already fixed here.
- Cross-check with a known-good control unit. If everything upstream is fine, replace the module on a trial basis. If the rear window heater and hazard warning then work again, the case is clear.
Solution
If the control unit is confirmed as the cause, only replacement helps: the modules are a sealed unit and aren't designed for repair while installed. We rely on tested genuine parts here rather than aftermarket copies.
Matching part from our stock: Genuine control unit for turn signal unit / hazard warning / heated rear window, part number 1408203026, personally tested and functional, for 169,00 €.
Mercedes W140 control unit turn signal unit, hazard warning & heated rear window
Part number: 1408203026
Condition: personally tested, working · genuine part
169,00 €
View control unit 1408203026You'll find more switching units and relays for the S-Class and other model series in the category Mercedes relays & control units, all parts specifically for your model series collected under W140. For older model series with a separate turn signal relay, it's worth checking our guide beforehand W124/W201 turn signals not working – a different part number is used there.
Compatible vehicles
| Model series | Note |
|---|---|
| Mercedes-Benz W140 (S-Class) | Documented use for this part per parts data |
| Other model series (W124, W126, W201 …) | Not documented for this part – different part numbers apply there |
The W140's electrics and equipment were revised several times over its production run, so the plug slots in the electrical box are populated differently accordingly. Always compare the number printed on your module before removal. Not sure? Send us your chassis number and we'll check the number before you order.
FAQ
How can I tell whether the control unit 1408203026 or the rear window itself is faulty?
Fastest via a voltage measurement at the rear window's contact tabs. If battery voltage is present with the heater switched on and the window stays cold, the heating element or its ground connection is at fault. If no voltage arrives there despite the fuse and switch being fine, that points to the control unit.
Do turn signals, hazard warning, and rear window heater always fail together?
No. The module contains several function branches that can fail separately. Often only one function is lost first, frequently the hazard warning, and the rear window heater only joins in weeks later. If the failure affects only the turn signals and hazard warning, also check the separate turn signal control unit.
Can I replace the control unit myself?
Yes. The module is plugged in rather than bolted, no special tools are needed. Disconnect the battery before working on the electrical box, and when plugging it back in, make sure the unit seats cleanly and fully without force.
Does the control unit 1408203026 also fit other Mercedes model series?
For this part, use is documented exclusively for the W140. We can't confirm the fit for other model series – different part numbers are fitted there. If in doubt, contact us with your chassis number.
What does the tested control unit cost, and is it available immediately?
The tested genuine part with part number 1408203026 costs 169.00 €. It's in stock with us and ready to ship immediately. As it's a used genuine part, stock is limited.
KFZTeile-Thot is a specialist for tested and refurbished Mercedes parts with its own test benches. Every control unit, every relay, and every instrument cluster goes through our test station before shipping – that's why we have over 1,000 eBay reviews (almost exclusively positive) behind us. Not sure you have the right part in front of you? Call us on 0179 8991069 (Mon–Fri 8 am–6 pm) or write to us via the contact form – ideally right away with your chassis number and a photo of the removed module.