If you need to jump-start a BMW, be extremely cautious and follow the proper procedure to avoid blowing fuses or bricking ECUs. Damage to the BMW electronics while jumpstarting the engine is extremely rare, but it does occur. When jump-starting a BMW, the most important thing to remember is to reduce the battery voltage difference between a dead BMW and a donor vehicle before starting the engine.
I jump-started a lot of BMWs in the past 20 years and by following what I learned when I started working I never ever had a single issue. In this article I’ll try to explain how BMW’s main battery system works, what are the risks of jump-starting a BMW, and of course, the correct jump-start procedure.
Let’s get started.
How Can a Jump Start Birck ECU
In most cases, jump-starting a BMW will go smoothly. However, things don’t always go as planned, and you end up with a blown jump start terminal fuse, alternator, or, worse, one or more bricked ECUs. And I don’t mean by switching the positive and negative lead, which will almost always brick something. So, it is critical to follow the jump-start procedure to avoid costly repairs.
Besides the E Series FRM module that will die if you just look at it in the wrong way, I’ve dealt with a few bricked modules after a jump start that would otherwise work for the life of a car. That is, instrument cluster (KOMBI), Front Electronic Module (FEM), Dynamic Stability Control (DSC), and maybe a few others I can’t recall. Besides being crazy expensive, even if you source a used one, they are not just plug-and-play.
So, if you want to brick your BMW’s electrical system, here’s how to do it.
First, you want a big, chunky, and rapid voltage spike. And by big spike, I mean at least some 150 to 200 Volts, as most modules have built-in overvoltage protection that we need to overcome. I don’t have the exact numbers, but some 25V to 50V will do nothing here.
That’s why you need to empty your BMW’s battery as much as possible. Then go out, start a donor vehicle, raise RPMs and have someone connect the jumper cables as fast as possible. Now, this is crucial – you want that the donor’s alternator feels how much is your battery discharged so it can send over as much electrical juice as possible.
Now, if your BMW’s electrical system comes to life it’s not over yet. As the voltage drops below about 10V your BMW will shut off all control modules. Now that the voltage spike from a donor alternator woke up your BMWs electronics but didn’t manage to brick them, you need to switch on as many electrical consumers as possible. That is, start with the rear window defroster, then fast hit those seat heating buttons, AC, and windshield defrosting while your right hand is at climate controls.
At the same time, with your left hand, turn on the headlights, high beams, and steering wheel heating. Beware that you need to be fast, as voltage spikes only occur when creating an instant load.
If you are a fresh BMW owner it will take some practice.
Yes, voltage spikes can really kill your BMW computers. When you connect the jumper cables from a working donor car to your BMW jumper points, the donor alternator can produce a voltage spike of 100V to 200V for up to half a second, and when this happens you don’t want to be the ECU.
The primary determinant of the voltage spike’s height is the voltage difference between two connected 12V electrical systems.
Now, if you, on the other hand, want to jump-start your BMW without any drama and just drive away, you need to take care of the voltage balance between your BMW and your current donor.
Balancing The Voltage
Before starting first turn off all electrical consumers in both cars. If you’re BMW is completely dead (no ignition and dead instrument cluster) you can only switch off the lights. To balance the voltage as much as possible, you should connect the jumper cables with both engines off. Yours is off anyway, but just to be clear. First, connect the red positive to both cars and then the black negative.
On BMW you must always use jump-start terminals, so don’t try to be smart and connect the negative clamp to the engine or some other metal you found suitable. If the donor car is not a BMW, you should make sure you’re using the right points, so if you are not sure, consult the owner’s manual.
Besides that, the terminals should be clean, and depending on where you live and how old is your BMW this might not be the case for a negative jump-start point. So, make sure there’s good contact between the cable clamp and the jump-start point.
With the jump cables properly connected start the donor car’s engine (obviously) and let it idle for a few minutes. This will initiate the increased charging from a donor’s alternator.
If your BMW was completely dead – the instrument cluster in dark – now is the time to switch on the ignition and turn off all electrical consumers.
As the alternators produce only about 30-40% of their rated capacity at idle, you’ll need to increase the RPMs to about 1.5k for the Diesel and about 2-2,5k for a gasoline-powered engine.
Keep the increased idle for a few minutes and try to start your BMW. If the engine does not start, wait a few more minutes and repeat.
Good. Now that your BMW has started it’s not over yet. After your BMW’s engine come to life leave both cars idle for at least FIVE minutes before disconnection jump starts cables.
Also, beware that the donor’s electrical system is at risk just as yours as it works at its maximum capacity.
You Need To Have Proper Jump Start Cables
By proper, I mean thick and heavy cables with enough cross-section to let the current flow.
You should consider the following five factors when choosing a set of jumper cables: wire gauge, insulation, cable length, clamps, and the amperage rating.
But first, let’s define a few words before we move on. You might have heard the term “booster cables” used to describe jumper cables at one point. They can both be used interchangeably because these are just two different names for the same thing.
Wire gauge refers to how thick the wires are. This will be designated by a gauge value number between 1 and 12. The lower the number, the thicker the cables will be. Thicker wires are more effective because they allow more electricity to pass through them.
A 6-gauge or 4-gauge set of cables will be perfect for most vehicles. If you have a car with a particularly large engine, you may want to look for a 2-gauge set of cables. Cables with a gauge rating of 1 are really only needed for heavy-duty vehicles such as diesel trucks. One other thing to keep in mind is that the lower the gauge value, the more expensive the cables will be.
Best Jumper Cables for BMW
When it comes to buying jumper cables, you want a set of cables that can actually start your BMW with an empty battery. Therefore, the maximum amperage should be the primary consideration while selecting appropriate jumper cables. The quickest way to determine how much amperage you require is to consult the battery label on your BMW. If the battery has an 800A rating, you should utilize cables with an 800A or higher rating.
For example, if your battery is rated at 900A, you can’t go wrong with a set of quality cables rated at 1000A.
How to Jump Start a BMW
- Switch off all electrical consumers in both cars (lights, heating, AC…)
- Switch off the engine of the donor car
- Attach the red cable to the positive terminals of both cars
- Attach the black cable to the negative terminals of both cars
- Start the engine of the donor car and let it idle for a few minutes
- Increase the engine speed of the donor’s car and keep it for a few minutes
- Start the BMW with the discharged battery as usual
- If the first attempt fails, wait a few more minutes
- After your car starts, wait at least five minutes with both cars idling
- Disconnect jumper cables in reverse order
