Replacing the alternator in your Mercedes-Benz GL550 might seem daunting, but with the right tools and a step-by-step guide, it’s a task you can confidently tackle at home. This guide provides a detailed walkthrough for replacing your GL550’s alternator, ensuring your vehicle’s electrical system is back to providing the correct voltage output and keeping your battery properly charged. While this procedure focuses on the physical replacement, understanding the importance of correct voltage output from your alternator is crucial for the longevity and performance of your Mercedes-Benz. A malfunctioning alternator can lead to a multitude of electrical issues, stemming from undercharged batteries to complete system failures. This guide will not only help you replace the component but also ensure you understand the critical role of a properly functioning alternator in maintaining your vehicle’s electrical health.
Before we begin, it’s important to gather all the necessary tools. Having everything prepared will make the process smoother and more efficient.
Tools Needed:
- 10mm socket
- 13mm socket
- 17mm socket
- E-12 socket (External Torx)
- T27 Torx bit (T30 may also be applicable to some models)
- 6-inch extension
- Swivel joint for sockets
- Flathead screwdriver or pliers (for removing panel clips)
- Jack and jack stands (optional, for increased under-car clearance)
Step 1: Preparation and Safety
Begin by positioning your passenger seat fully forward and upright to maximize your workspace. Crucially, disconnect your car battery before proceeding with any work on the alternator. This is a vital safety precaution to prevent electrical shock and damage to your vehicle’s electrical system. Ensure your vehicle is parked on a level surface and, if you require more room underneath, use a jack and jack stands to safely elevate the front of your car. If you have the AIRMATIC suspension system, raising it to its highest setting can also provide additional clearance.
Step 2: Optional Radiator Fan Assembly Removal (For Easier Access)
While not strictly necessary, removing the radiator fan assembly can significantly improve access to the alternator, especially during installation. If you are working alone, or anticipate difficulty maneuvering the new alternator into position, creating extra space is highly recommended.
How to Remove the Radiator Fan Assembly:
- Remove Vertical Fins: Locate the two vertical plastic fins flanking the fan assembly. Each fin is secured by two push-pin clips. Gently pop up the center pin of each clip using a flathead screwdriver or trim removal tool, and then pull the fins free. Set these aside.
- Unbolt the Fan Shroud: Identify the seven 10mm bolts and one T27 (or possibly T30) Torx bolt securing the fan shroud. There are three 10mm bolts on the left side, two securing the center section near the hood latch, and two 10mm bolts plus the Torx bolt on the right side. Remove all these bolts.
- Detach Top Clips: The fan shroud is also held in place by clips along the top edge that engage with the radiator support. You may need to gently squeeze the tabs of these clips while carefully wiggling the panel upwards to release it. Move the weather stripping forward to get it out of the way.
- Disconnect Fan Bolts and Remove Fan: The fan itself is secured by two 10mm bolts at the top corners of the fan housing. Remove these bolts. To provide clearance for removal, reach down to the cooler lines and gently push the rubber aside slightly to free up the mounting tab on the lower passenger side of the fan. Carefully wiggle the fan assembly upwards and out of the engine bay. With the fan removed, you will have significantly more room to access and maneuver the alternator.
Alt text: Radiator fan assembly of a Mercedes-Benz GL550 removed to provide increased access to the alternator location during replacement procedure.
Step 3: Alternator Removal
- Belt Removal: Locate the tensioner pulley. Using a 17mm socket on the tensioner (it might be partially hidden by the serpentine belt), apply counter-clockwise pressure to relieve tension on the belt. While holding the tensioner, slip the serpentine belt off the alternator pulley. It’s a good idea to take a photo or sketch a diagram of the belt routing before removal to ensure correct reinstallation. Removing the engine cover and passenger-side air intake duct can improve visibility and access during this step.
- Alternator Bolts: The alternator is secured by four bolts: two at the top and two at the bottom. These bolts are E-12 External Torx bolts. It is highly recommended to use the correct E-12 socket for these to prevent stripping the bolt heads. The top two bolts are generally accessible from above using a 3/8″ ratchet and the E-12 socket. Note that the ground strap and a wiring harness clamp are often secured under these bolts. Once the bolts are removed, set them aside along with the ground strap and wiring clamp.
- Access Bottom Bolts (From Underneath): To access the bottom alternator bolts, you’ll need to remove the under-engine tray. This tray is typically held by nine 10mm bolts. There are four bolts along the rear edge and the rest are located along the middle and outer edges. Remove all these 10mm bolts, and the tray will drop down at the back and slide rearward for removal.
- Remove Bottom Bolts: With the under tray removed, you can access the lower alternator bolts from underneath the vehicle. Using a 6-inch extension and a swivel joint attached to your ratchet and E-12 socket can make reaching these bolts easier. Remove the two lower E-12 bolts. Once these are removed, the alternator is now free from its mounting.
- Disconnect Wiring: Return to the top of the engine bay. Rotate the alternator to gain access to the rear. Locate the 13mm nut securing the positive battery cable to the alternator. If you haven’t disconnected the battery yet, do it now! A plastic cap may cover the nut; remove this with a small flathead screwdriver. Unbolt the 13mm nut and detach the positive cable. There will also be a smaller connector, typically a push-clip style plug, for the alternator’s control wiring. Disconnect this plug.
- Remove Alternator: Carefully maneuver the alternator out of its mounting location. With the radiator fan removed (or with careful maneuvering), the alternator can usually be removed from the passenger side of the engine bay.
Alt text: Technician removing the old alternator from a Mercedes-Benz GL550 engine bay during a replacement procedure, highlighting the limited space.
Step 4: Alternator Installation
Installation can be more challenging, especially if you are working alone. Having a helper to assist with aligning and holding the alternator in place is highly recommended.
- Connect Wiring (Before Bolting In): Before physically bolting the new alternator into place, it is significantly easier to reconnect the wiring. Orient the new alternator with the pulley facing downwards. Reconnect the positive battery cable to the alternator’s stud and tighten the 13mm nut securely. Reattach the push-clip style plug for the control wiring.
- Mount the Alternator: Carefully maneuver the alternator back into its mounting position. If you removed the radiator fan assembly, you will have ample space. If not, take your time and carefully position the alternator.
- Start Bolts: This is where having a helper is invaluable. The challenge is aligning the alternator and starting the bolts, particularly the top ones. If working alone, try to get at least one or two threads started on the top bolts to hold the alternator in place before attempting to start the bottom bolts from underneath. If you have assistance, have your helper hold the alternator in position from below while you start the top bolts.
- Tighten Bolts: Once all four E-12 bolts are started, tighten them gradually and evenly. Ensure the alternator sits flush against its mounting points. Be cautious not to overtighten these bolts. Remember to reattach the ground strap and wiring clamp under the top bolts.
- Belt Reinstallation: With the alternator securely mounted, use your 17mm socket on the tensioner again to relieve tension and reinstall the serpentine belt, following the routing diagram you created earlier or the diagram located in your vehicle’s engine bay. Ensure the belt is properly seated in all pulley grooves.
Important Warning: Alternator Orientation and Fitment
It is critical to ensure the new alternator is installed with the correct orientation. As noted in the original experience, some aftermarket alternators, even for the GL550, might have slightly offset mounting tabs. Incorrect orientation can lead to the alternator casing pressing against the engine block, potentially causing damage to the alternator housing and leading to premature failure. Before fully tightening the mounting bolts, double-check that the alternator sits flush and that the bolts thread in smoothly. If you encounter significant resistance while tightening the bolts, stop immediately and re-examine the alternator’s orientation and fitment. The bolts should tighten smoothly with minimal resistance until they are almost fully seated.
Step 5: Post-Installation Checks and Reassembly
- Reconnect Battery and Test Voltage Output: Reconnect your car battery. Start your Mercedes-Benz GL550. Using a multimeter, check the voltage at the battery terminals with the engine running. A healthy alternator should produce a voltage output between 13.5 and 14.5 volts. This confirms your new alternator is correctly adjusting the voltage output and charging the battery.
- Reinstall Radiator Fan Assembly (If Removed): If you removed the radiator fan assembly, carefully lower it back into position, ensuring the alignment tabs on each side are correctly positioned and clear of hoses. Reinstall the two 10mm bolts at the top corners of the fan. Reposition the fan shroud, tucking it under the hood latch mount and aligning the clips over the radiator tabs. Reinstall the seven 10mm bolts and the T27 (or T30) Torx bolt to secure the fan shroud. Reattach the vertical plastic fins and push the pins back in to lock them.
- Reinstall Under-Engine Tray: Underneath the vehicle, reinstall the under-engine tray. Slide the front edge of the tray up and into position under the bumper area, and then reinstall the nine 10mm bolts to secure it.
- Reinstall Air Intake Duct and Engine Cover: If you removed the passenger-side air intake duct and engine cover, reinstall them.
- Final Check: Double-check all connections, belt routing, and tool removal before considering the job complete.
Alt text: Newly installed alternator securely positioned in the engine bay of a Mercedes-Benz GL550, highlighting the completed replacement.
Conclusion
Replacing your Mercedes-Benz GL550 alternator is a manageable DIY task with patience and careful attention to detail. By following these steps and taking the necessary safety precautions, you can restore your vehicle’s charging system and ensure the correct voltage output alternator GL550 Mercedes Benz requires for optimal performance. Remember to always prioritize safety, double-check your work, and consult a professional mechanic if you are unsure about any part of the process. Regular maintenance and addressing issues like alternator failure promptly will keep your Mercedes-Benz GL550 running smoothly for years to come.
