I’ve read a funny quote some days ago, which stated that “the most expensive car you could buy, is a cheap Mercedes”. In actuality, this isn’t untrue, no matter how painful it might be to accept for those of us who have a fascination for affordable Benzes. Among those troubles that you may encounter, if it either belongs to a new or used model, is seeing how your Mercedes won’t start but lights turn on.
While I do not doubt that most will heartily agree with Mercedes’ status as a premier automaker, they do have one tiny issue… Their cars break, badly. That’s not uncommon, of course. However, the high-tech, overengineered, opulence and performant nature of every Mercedes means that when they do break down, they’re endlessly costly to repair. Hence, why you could pick up old Mercs for pennies.
And when those many moving parts eventually wear down, you have faults such as a Mercedes won’t start but lights turn on. This is certainly a frustrating issue, whether you’ve spent a couple of hundred bucks for that rusty Merc at the local scrapyard. Or, having spent 6-digits on a brand new AMG. So, here are some handy pointers of what to do when your Mercedes won’t start but lights turn on…
Steps In Diagnosing Mercedes Won’t Start But Lights Turn On
First, let’s clarify the meaning behind your Mercedes won’t start but lights turn on. Essentially, it’s an issue where upon turning your key in the ignition – or pressing on the push-button starter, depending on what model you have – the car won’t start. Specifically, the engine would refuse to crank and turn over. However, you also notice that the interior and exterior lights are still functioning properly.
You may even have access to your interior accessories, such as the radio, power windows, and air conditioning, all working fine. Yet, the engine won’t turn on. This is what we meant by noticing Mercedes won’t start but lights turn on. So, what can we do about this? For a start, let’s try to diagnose the problem, step-by-step. Only then, could we have a better idea and understanding of what’s going on with your car.
We should point out first and foremost, to not leave the lights or interior accessories turned on. Since the engine isn’t turning over, it won’t be able to power the alternator. As a result, your 12V battery is left without any continuous charging by the engine. Should you keep the lights and accessories on for too long, the battery will soon deplete in its entirety. So, only turn the lights on for diagnostics.
With that in mind, here are a couple of quick diagnoses (and troubleshooting) steps that you should undertake. This would help you to better try and wrap your head around why your Mercedes won’t start but lights turn on…
Step 1: Check Your Fuel Level
It’s beyond surprising how often it is to encounter folks who keep on driving but never pay attention to how much fuel they have. Granted, it can be understandable, especially if you’ve had a busy day at work, and haven’t been able to keep track of your last refill. Thus, they keep on driving, until the fuel tank practically runs dry, and turns into vapor rather than a liquid.
Naturally, your vehicle won’t start if there’s no fuel to crank the engine with, even if the rest of your Mercedes is perfectly fine. So, the first thing you can do, if you encounter how your Mercedes won’t start but lights turn on, is to check the dashboard. Look at the fuel gauge, and note how much fuel you have. Bear in mind that having just a tiny bit of fuel might still not be enough to crank it.
Step 2: Note Down Any Warning Lights
While you’re there, it’s also a good idea to see if there are any warning lights spat out on the dash. As the lights are still turning on and off perfectly fine, it denotes that there’s some electricity in your car, at least. Should this be the case, then your car’s ECU – its primary computer brain – should be able to log any problems that it found. These are then stored as error codes.
They’ll then flash up warning lights on the dashboard, telling you what part of the car is broken, or needs attention. It’s a helpful way to possibly pinpoint the underlying cause of why your Mercedes won’t start but lights turn on from the get-go. For example, it might show:
- Check Engine Light – Informing you of issues with the engine, emissions system, fuel system, or any other would-be powertrain problem. It could trigger with problems as simple as the gas cap being a tad loose. Or, include other potential faults with the transmission, and so on.
- Battery Light – Tells you that there’s a problem with the battery, charging system, or electrics as a whole. Although your lights might be working, the charge in the battery is low enough, to a point where it’s insufficient to crank the engine.
- Gas Level Light – Related to our earlier point on your fuel level being low, this warning light may also appear if you don’t have any or adequate fuel in the system.
- Glow Plug Light – If you have an older diesel-powered Mercedes, this light may appear. Unlike gas cars that use spark plugs, diesel relies on glow plugs. They create heat in the combustion chamber, allowing your diesel engine to power up. This light appears when the glow plugs aren’t hot enough, preventing your car from starting.
Step 2.5 (Optional): Analyse The Error Codes
While warning lights are useful, a single blinking light can only tell you so much. Hence, why the car’s ECU keeps track of any problems and stores them as error codes that you can analyze later. If the lights on your dashboard are too vague, you could plug in an OBD scanner or reader device into your car, to download the error codes. These diagnostic trouble codes (DTC) are fantastic guides.
They can make you aware precisely of what’s wrong with your car, with detailed error messages. You could then Google these, or pass them over to a local mechanic, to see what’s causing them to appear. It will require you to have an OBD scanner/reader on hand. You could find cheaper ones for several tens of dollars. Or, a pricier unit that runs over a hundred dollars or more.
While unnecessary, the more expensive diagnostic readers have additional features. For example, it can show you every error message in full, and describe what’s going on. In most cars, you can plug the OBD scanner into a port under the dashboard, on the driver’s side. The same applies to Mercedes, as well. Clamber underneath the dashboard, and look just above the pedals.
It may be located right next to the hood release. Nonetheless, some Mercedes models instead have their OBD ports installed on the passenger side. An OBD port will be quite large, as it adopts a 16-pin connector. So, look for a thick and chunky panel. This is then likely covered with a plastic flap, which you can crack open. Thus, revealing the OBD jack that you can connect your reader/scanner into.
Step 3: Take A Peek Under The Hood
If checking the dashboard warning lights haven’t unveiled any clues to you as to why your Mercedes won’t start but lights turn on, you can now move under the hood. Pop the hood open first, and then inspect what’s going on there. The first place you should look is the battery. Remember, your engine needs the battery to provide a high enough charge, which sparks and ignites the engine to life.
While your lights turning on should be indicative that there’s charge in the battery still, it might not be enough to crank up the engine. The bare minimum to crank the engine up for a regular passenger vehicle is around 12V (and 400A). Any lower than that, say 10 to 11-volts, might still be sufficient to power your lights and accessories. However, it would be unreliable to make the engine turn over.
Once you’ve popped open the hood, head over to the battery. Now check to make sure that the:
- Battery is mounted correctly into the engine bay and hasn’t been knocked loose.
- Cables are clamped and connected properly and tightly onto the battery terminals.
- Terminals don’t have any corrosion or noticeable oxidation on them. This may signal that the battery is dying and needs a replacement, or if the charge is too low.
- (Optional) Check the battery’s voltage using a multimeter, if you have on lying around. Test the car battery to make sure that it has enough voltage to crank the engine. We’ve written up an in-depth guide on how to test your car’s battery with a multimeter, so check it out to learn more.
Step 4: Cycle The Engine To Kick-Start It
If the battery appears to be in good shape, you can proceed with cycling the engine. This is a process where you’ll attempt to crank it, and possibly force the engine to turn over. You can begin by:
- Slotting the key into the ignition, and turn the key from the OFF/LOCK position to the START position. Repeat this process 10 times, by slowly going back and forth. Don’t rush this step, as you could cause further damage to your car’s ignition.
- After repeating this 10 times, you can press the brake pedal, and move your gear selector into Park or Neutral. Now, attempt to start the engine, as you normally would. If it doesn’t work in Neutral, try to put your gearbox in Park, and try again.
- Keep repeating the abovementioned steps until your Mercedes sputters back to life. When your key is at the START position, be sure to hold it there for a couple of seconds, at least until the engine tries to crank and turn over.
- If practicing this for 10 or so times still doesn’t work, we’d recommend stopping, and exploring any one of the options below. Forcing your engine to cycle and start for too much may damage the ignition, in addition to the starter and engine.
What’s Causing Your Mercedes Won’t Start But Lights Turn On
If the quick diagnosis and troubleshooting steps from earlier haven’t provided any answers, the cause of why your Mercedes won’t start but lights turn on lies deeper. Here are some of the possible reasons why the engine refuses to crank:
1. Mercedes Won’t Start But Lights Turn On – Battery’s Nearly Dead
When you’re trying to crank the engine, are you able to hear a “click” or “clicking” sound? That noise comes from your starter, whose clicking is its attempt to induce enough electricity to crank over your engine. If you hear that click, it points to the fact that the starter is functioning well, but your battery might not be able to supply enough of a charge for it to work. Thus, your engine remains dead.
Since your lights are still illuminating, the battery isn’t completely dead and has a bit of charge left in it. However, that charge is insufficient to crank the engine, which requires quite a lot of voltage and amperage to start. Expanding on our diagnosis earlier, a car battery needs roughly 12.2V to 12.8V, as it varies from one car to the next. In addition, it requires 400A, but sometimes upwards of 600A.
The requirements change significantly based on what type of Mercedes you’re driving. A regular car won’t need more than the minimum end of that approximation. Models with smaller engines may at times get away with just 100A or 250A. Meanwhile, those big Mercedes SUVs, trucks, and off-roaders with large displacement engines could ask for nearly 1,000A or more.
Over time, any car battery would start to lose charge, before failing altogether. Alternatively, it might be due to you having accidentally left something turned on overnight, draining the charge. Even as a battery might be nearly dead and won’t be enough to start the engine, the rest of your car might still function. The headlights and taillights, radio, dashboard lights, or power windows are easier to run.
2. Mercedes Won’t Start But Lights Turn On – Fuses Are Blown
Your car has a myriad of different electrical components and accessories. That’s why, just like in your house, a car has fuses. If there’s a massive surge in electrical current, the fuses will blow. In doing so, it cuts the flow of electricity across the circuit and prevents whatever part of the car it’s powering from being damaged. That can happen if there’s too much current bursting through.
If that blown fuse happens to be necessary for powering the engine, then, of course, a blown fuse will prevent your Mercedes from starting. Although, the lights and accessories may not be affected by it. You can try to discern the fuse box’s location from reading the owner’s manual. Typically, it would be placed within the engine bay, at the back nearing the firewall, and on the driver’s side.
Pop open the fuse box and inspect the fuses. Then, check the metal wire and pins on the fuses. Could you see any signs of the wire being disconnected? Or, are there perhaps significant burn marks on the fuses themselves? If so, then your fuses are blown. You should also be diligent about the fuses that you’re inspecting. Focus on fuses that relate to the engine, such as the starter relay, or the ignition switch.
3. Mercedes Won’t Start But Lights Turn On – Faulty Starter
Ah, but as you’re trying to start it, could you hear that aforementioned “click” or “clicking sound”? If not, then this means that starter is at fault this time. Your car’s starter consists of two components, a motor, and the solenoid. The motor is the key part of the starter, which spins up the engine to give it a bit of momentum before it can start. Meanwhile, a solenoid connects the battery to that motor.
The motor is connected to the flywheel and the engine. As you twist the key in the ignition, this motor cranks the engine to get it to cycle. All the while, your ignition attempts to light up the engine, as fuel and air start to burn. In layman’s terms, the starter gets the engine moving, before it could then run and operate under its own power. The latter being internal combustion, ignition of the fuel, and air.
Even if the battery has enough charge, a faulty starter would prevent the engine from turning over. It can be caused by either a broken motor or solenoid. If you notice the lights being a tad bit dimmer than usual, then there may be other points of failure within the starter. Primarily, the starter relay, or the starter control switch. Otherwise, dimmer lights may signal a low charge in the battery.
4. Mercedes Won’t Start But Lights Turn On – Fuelling Problems
At the very base of it, you might have an empty gas tank. But to expand more on the fuel, it might be possible that the fuelling system on your Mercedes has gone awry. If there’s an issue preventing fuel from flowing into the engine, then your engine will not crank. There are a few scenarios where your fuelling system could be at fault, and preventing your car from turning over:
- Frozen Fuel Line – If you live somewhere very chilly (below-freezing temperatures), the fuel line could freeze. Consequently, it’ll prevent fuel from flowing from the tank and into the engine. You’d have no other option but to wait for it to thaw out.
- Incorrect Gas Gauge – Fuel gauges could, at times, read the incorrect gas levels in your car. You might see the needle ticking up, even when the tank is bone dry. Or, the sensors reading the gas gauge may have been poorly calibrated.
- Clogged Fuel Filter – The fuel filter exists primarily to catch contaminants, sediment, dirt, as well as a myriad of debris flowing around in the fuel tank. It prevents them from flowing into the engine. As a result, the filter can get clogged over time, which inhibits and blocks fuel from circulating, as well.
- Defective Spark Plugs – Without a spark from the spark plugs, your engine wouldn’t be able to crank at all. Typically, only one or a few spark plugs would fail at a time, resulting in your engine at least sputtering. It’s rare to have all the spark plugs not turned on, but it’s possible.
5. Mercedes Won’t Start But Lights Turn On – Ignition Switch
As you turn your key, the ignition switch is responsible for authenticating your keys and thus starting the engine. Should the ignition switch fail, among the symptoms, you’ll notice is when your Mercedes won’t start but lights turn on. Ignition switches are made to be robust but can fail due to wear and tear. One way to diagnose the ignition switch, if it’s the cause, is by removing the key.
Normally, this should power down your vehicle. But if you still see the dashboard lights staying on, it might point towards a faulty ignition switch. Sometimes, twisting the key in the ignition would fail to switch on any lights or accessories, to boot. Other times, your car may only start at a certain gear, or you’ll see flickering with the lights. Once again, this is due to a failing ignition switch, or blown fuses.
6. Mercedes Won’t Start But Lights Turn On – Drive Authorization System
For most Mercedes models made in the last ten or so years, they’re equipped by default with its Drive Authorisation System (DAS). This is essentially an anti-theft system, which validates your keys used to start the car. Every single Mercedes key fob contains a chip imprinted with a serial and rolling code to communicate with DAS. Although, there are situations in which DAS prevents the car from starting.
Normally, this is to deter theft, as you’ve inserted the wrong key or are trying to bypass the ignition. That said, it can also activate due to other miscellaneous faults with the car. Some of these problems that might prevent your Mercedes’ DAS to unlock the ignition for you include:
- Brake Light Switch – It’s a common item to fail on most Mercedes models, and when it does break, it prevents DAS from authenticating your keys. Unfortunately, you can’t test this by pressing on the brake pedal, just to see if the brake lights turn on.
- Gear Selector – The gear selector module is another part of some Mercedes models that can fail over time. You can diagnose this by looking at the instrument cluster, while also moving the shifter into Park (P), Reverse (R), and Neutral (N), respectively. If the shifts don’t register on the dashboard, then it’s a sign that your gear selector module is broken.
- Transponder – Or, it could also be a problem with your Mercedes SmartKey fob. The transponder chip built into the key fob might fail to be verified with DAS. This prevents you from unlocking your car’s ignition, and won’t let you start the car.
- Electric Ignition System – Otherwise known as Mercedes’ EIS, DAS will not let you access the ignition if the battery is fully or partially discharged. Therefore, locking you out of starting up the engine.
That then rounds up our look at why your Mercedes won’t start but lights turn on. In all, the common suspects of what’s causing your Mercedes-branded vehicle to not crank over is related to electrics. It’s either a case of a failing or nearly-dead battery. Or, it might be a series of blown fuses, while a faulty starter and ignition switch could also be it. Then, we’ll have to get to Mercedes’ DAS anti-theft tech.
While it keeps your car safe, it can be fairly sensitive to what is considered to be stealing. Issues with some other, seemingly unrelated, components could prompt it to lock you out of the ignition. With that in mind, it’s a good idea to try and start up your Mercedes with a spare key, just to see if the fob is what’s behind it all. But first, make sure your battery’s charge is up to par, or if it needs changing.