my bosch hammer only works in drill mode, hammer function does nothing at all.

Diagnosis

The failure of the hammer function on the Bosch GBH2-28L typically stems from a mechanical disconnection between the mode selector dial and the internal striking mechanism. When the tool remains in "drill only" mode despite the dial being turned, the internal selector fork has either slipped out of its groove, snapped, or the selector dial itself has stripped its internal splines. This means the motor is still spinning the chuck, but the piston is not being engaged to create the reciprocating hammering action. You can confirm this by listening to the tool. In a healthy hammer-drill mode, you will hear a distinct, rapid rhythmic "clacking" or thumping sound. If the tool sounds identical to a standard handheld drill while set to hammer mode, the selector fork is not pushing the gear into the hammering circuit. If the selector dial feels "mushy" or rotates without a firm, audible click into the different positions, the internal locking pin or the nylon fork is definitely the culprit.

How to Fix It

To resolve this, you must access the gear housing to inspect and replace the selector components.

1. Disconnect the tool from all power sources. Remove the side handle and the depth gauge.
2. Remove the four screws securing the plastic motor housing. Carefully pull the housing away from the gear box, ensuring you do not pinch the trigger wires.
3. Locate the mode selector dial on the exterior of the gear housing. Remove the retaining screw and pull the dial out to expose the selector shaft.
4. Inspect the white nylon selector fork (Part #1 607 001 005). Check for hairline fractures or a complete snap at the pivot point. If the fork is broken, the selector dial will spin, but the gear will remain in the "drill" position.
5. If the fork is broken, remove the retaining clip and slide out the damaged nylon fork. Install a new genuine Bosch replacement fork, ensuring it is seated deeply into the selector gear groove.
6. Apply a small amount of high-temperature molybdenum grease to the fork and the sliding gear interface to ensure smooth transitions.
7. Reassemble the housing, ensuring the selector dial clicks firmly into the "Hammer Drill" and "Hammer Only" positions before tightening the final screws.

If That Didn't Work

• Check the Piston O-Rings: If the selector fork is intact but there is still no hammering action, the internal piston O-rings may have perished or leaked. If the air cushion is lost, the piston cannot build the pressure required to strike the bit. You will need to disassemble the cylinder and replace the seal kit.
• Inspect the Striker Plate: In rare cases, the metal striker plate that transmits the force from the piston to the SDS-plus chuck can wear down or crack. If the plate is sheared, the piston will move, but the energy will not transfer to the bit. This requires a full gear-box teardown.
• Verify the Lubrication Level: If the tool has been run dry or the oil has leaked out, the hammer mechanism can seize. Check the oil window or drain a small amount of oil. If the oil is black and contains metal shavings, the internal hammer gears are stripped and the gear head must be replaced as a complete unit.
• Check the Selector Dial Splines: Inspect the plastic teeth on the inside of the mode selector knob. If these are rounded off, the knob will turn, but it won't actually move the shaft that operates the fork. Replace the plastic dial if the teeth are worn.

Heads Up

• Grease Maintenance: Every 50-100 hours of heavy use, check the gear grease. Using the tool in hammer mode without proper lubrication will rapidly wear down the nylon selector fork and the piston seals.
• Avoid "Dry" Hammering: Never run the tool in hammer mode without a bit installed for extended periods, as this can cause the striker to bounce unevenly and damage the selector mechanism.
• Bit Compatibility: Ensure you are using genuine SDS-plus bits. Using oversized or non-standard bits can put lateral pressure on the chuck, which can bend the internal selector rod over time.
• Mode Switching: Always bring the tool to a complete stop before switching between Drill and Hammer modes to prevent shearing the teeth off the selector gear.