Why is the mirror not heating up when the rear windshield heating is turned on in a Mercedes S?
3 Answers
When the rear windshield heating is turned on in a Mercedes S and the mirror does not heat up, it is because the rear windshield heating function is integrated with the mirror heating function. Therefore, after turning on the switch, wait for a while and touch the outer mirror with your hand to see if there is a warm sensation. If there is, it indicates that the circuit to the mirror is functioning properly. How the mirror heating works: The principle of mirror heating is quite simple. It involves adding an electric heating film behind the mirror lens, utilizing the principle of resistance heating. The temperature can reach 35-60 degrees Celsius, which is more noticeable in winter or foggy conditions, while the effect is less obvious at normal temperatures. Generally, mirror heating does not turn off automatically, so remember to turn it off when not in use to avoid wasting battery power. The heating power of the mirror is not high, but it is not recommended to keep it on for extended periods to prevent damage to the heating element. Automatic mirror heating shut-off function: Some models may come with an automatic shut-off function for mirror heating, typically set for a certain duration (e.g., turning off after 15 minutes of heating) or shutting off when a certain temperature is reached (to maintain a temperature range). This design is more user-friendly, preventing the waste of battery power if the owner forgets to turn off the heating function.
I've encountered this issue with the Mercedes S-Class where the rear windshield heating works but the mirror heating doesn't, several times before. Normally, the rear windshield heating and mirror heating are linked together, controlled by one switch for both functions. If the mirrors aren't heating up, it's likely an issue with a branch of the heating circuit. Possible causes include a blown fuse, especially when the defogger is used frequently in rainy conditions, leading to excessive current that can blow the fuse; or aging or broken heating elements in the mirrors themselves—Mercedes luxury car mirrors are made with fine materials but are prone to wear; another possibility is poor electrical contact, such as moisture entering the connector causing a short circuit. Safety shouldn't be overlooked, as poor visibility in the mirrors during rain increases the risk of accidents. I recommend first checking the fuse box for the corresponding heating fuse. If it's intact, take it directly to a 4S shop for professional diagnostic tools—don't attempt to disassemble it yourself to avoid damaging the car. Regular maintenance should include cleaning the mirror area to prevent dirt buildup, which can affect heating efficiency.
I also encountered a situation where the rearview mirror heating failed while driving a Mercedes-Benz. In such cases, you need to troubleshoot step by step. First, confirm that you have turned on the rear defroster switch. The rearview mirror heating in Mercedes-Benz usually activates automatically and doesn't require separate operation. If the switch is functioning normally but the rearview mirror remains cold, check the fuse box in the engine compartment or door panel. Look for the fuse related to the rear defogger function to see if it's blown, and try replacing it with a new one. If the fuse is fine, the issue might be a faulty relay in the rearview mirror module or loose/aged wiring, especially in older vehicles. Good visibility is crucial for safe driving in rainy or foggy conditions, so it's advisable to address the issue promptly to reduce the risk of rear-end collisions. During regular maintenance, remind the technician to inspect the electrical system to prevent such problems.
