2007 BMW Z4 Key Fob Battery Replacement: Step-by-Step Guide

Is your 2007 BMW Z4 key fob starting to feel a bit sluggish? You might be experiencing the common issue of a weakening key fob battery. Like any battery, the one in your BMW Z4 key fob has a limited lifespan. If you’re noticing decreased range when locking or unlocking your doors, or if the system is intermittent, a battery replacement is likely in order. Fortunately, replacing the battery in your BMW Z4 key fob is a straightforward DIY task that can save you a trip to the dealership and the associated costs. This guide will walk you through each step, providing insights into opening your key fob, diagnosing potential issues, and successfully replacing your battery.

Part 1: Safely Opening Your BMW Z4 Key Fob

The first step in replacing your BMW Z4 key fob battery is to carefully open the key fob casing. This process requires patience and a delicate touch to avoid damaging the plastic housing.

1. Tools You’ll Need:

   • Scalpel or thin, sharp blade
   • Small flat-head screwdriver or plastic pry tool
   • Spoon or similar smooth, prying object (optional)

2. Starting the Separation: Begin at the rear end of the key fob, where the key ring attachment is located. Using your scalpel or sharp blade, gently insert the tip into the seam between the two halves of the key fob casing.

3. Carefully Cutting Along the Seam: Make multiple shallow passes along the seam, applying gentle pressure. Avoid forcing the blade too deeply, as this could damage the internal components or the casing itself. The goal is to gradually weaken the adhesive or clips holding the two halves together.

4. Using Pry Tools to Separate: Once you’ve made several passes with the blade, switch to a plastic pry tool or a small flat-head screwdriver. Insert the pry tool into the seam you’ve created and gently twist to begin separating the halves. You can also use a spoon, as the original author did, for a wider, gentler prying action.

5. Patience is Key: Work your way around the key fob, patiently prying and cutting as needed. Take your time to prevent slipping and damaging the case or, more importantly, injuring yourself. Remember, slow and steady wins the race.

6. Complete Separation: Continue working around the seam until the two halves of the key fob are completely separated. You should now have access to the internal components, including the battery.

Alt text: Close-up showing the initial steps of opening a BMW Z4 key fob using a scalpel and cutter to carefully separate the two halves along the seam.

Alt text: Image demonstrating the use of a plastic pick and a spoon to further separate the two halves of the BMW Z4 key fob after initial cutting, emphasizing gentle prying.

Part 2: Diagnosing Battery and Button Functionality

With the key fob open, you can now assess the condition of the battery and the functionality of the buttons.

1. Battery Voltage Check: Use a multimeter to measure the voltage of the battery. A healthy CR2032 battery should read approximately 3V. The author measured 3.00V initially, which seems acceptable, but further testing revealed an issue under load.

2. Voltage Drop Under Load: To simulate the key fob in operation, measure the battery voltage while pressing each button. If the voltage drops significantly (as the author noted, down to 2.65-2.7V), it indicates the battery is struggling to supply sufficient current and is likely failing, even if the resting voltage seems normal. This voltage drop under load is a key indicator of a battery that needs replacement.

3. Button Resistance Test: To check the buttons, use the resistance (Ohms) setting on your multimeter. Measure the resistance across the terminals of each button:

   • Button Not Pressed: The resistance should be very high or infinite (open circuit).
   • Button Pressed: The resistance should drop to near zero (closed circuit), indicating the button is making proper contact.

4. Identifying a Faulty Button: In the original author’s case, they noticed the lock button didn’t cause the same voltage drop as the other buttons. Further resistance testing confirmed the lock button was not functioning. Shorting the button terminals while measuring voltage drop confirmed the button itself was the issue, not a problem elsewhere in the key fob.

Part 3: Understanding Button Variations and Ordering Parts

The author encountered an interesting issue: a variation in button types within their BMW Z4 key fob. This highlights a potential complication when ordering replacement parts.

1. Button Type Discrepancy: Standard replacement kits often include three identical “long” buttons with legs at each end. However, the author’s key fob had a mix: two long buttons and one square button (the lock button) with legs at each corner.

2. Diamond Key vs. Other BMW Keys: The author observed that “diamond” style BMW keys typically use long buttons, while other BMW keys may use square buttons. The mixed configuration in their key fob seemed unusual.

3. Glue on Charging Coil: Another unusual observation was glue on the charging coil inside the key fob, which the author hadn’t seen in online pictures of other BMW key fobs. This could be a model-specific variation or an anomaly.

4. Ordering the Correct Parts: Due to the button variation, the author wisely decided against a standard button kit. Instead, they opted to order:

   • Two CR2032 batteries: Essential for addressing the primary issue of battery weakness.
   • A Surface Mounted Button Kit: This kit contained both long and square button types, ensuring they would have the correct replacement button regardless of the specific type needed. Purchasing a kit provides versatility for future repairs as well.

Alt text: Diagram illustrating the two types of surface mount buttons found in some BMW key fobs, highlighting the difference between the long buttons with two legs (blue circles) and the square button with four legs (red square), as seen in the 2007 BMW Z4 key fob.

Part 4: Battery Replacement and Potential Button Repair (Next Steps)

The final steps involve physically replacing the battery and, if necessary, replacing the faulty button. The author was awaiting parts delivery to complete this stage.

1. Battery Replacement: Once you have the new CR2032 battery, carefully remove the old battery from the key fob. Take note of the battery orientation (+ and – sides) before removing the old one to ensure you insert the new battery correctly. Slide the new battery into place.

2. Button Replacement (If Needed): If you’ve diagnosed a faulty button, surface mount soldering skills are required to replace it. This involves desoldering the old button and soldering in the new surface mount button. A fine-tipped soldering iron is recommended for this delicate work.

3. Reassembly: After replacing the battery and any faulty buttons, carefully snap the two halves of the key fob casing back together. Ensure they are securely closed. You may need to apply slight pressure to fully close the case.

4. Testing: Test the key fob functions (lock, unlock, trunk release) to ensure the battery replacement has resolved the issue and that all buttons are working correctly.

Conclusion

Replacing the battery in your 2007 BMW Z4 key fob is a manageable DIY project. By following these steps for opening the fob, diagnosing the issue, and replacing the battery, you can restore your key fob’s functionality and avoid unnecessary dealership expenses. Pay attention to details like button types to ensure you order the correct replacement parts if button repair is needed. With a bit of patience and care, you can successfully complete this repair and keep your BMW Z4 key fob working reliably.