How to tell if a slack adjuster is manual or automatic?

Practical Methods for Differentiating Between Manual and Automatic Slack Adjusters

1. Examine the External Structure

Manual Adjuster Markings
Hexagonal Adjustment Hole: Exposed 15mm/19mm hexagonal hole on the side or rear (usually with a dust cover).
Lock Nut: A 36mm large nut next to the adjustment hole (used to fix the worm gear position).
Automatic Adjuster Features
Sealed Housing: Entirely enclosed casting, no exposed adjustment hole (only a small reset hole may be present at the rear).
Reset Interface: 8mm/10mm dedicated hole with a rubber plug (used only when changing brake pads).

2. Listen to the Operating Sounds

Automatic Type Unique Sound: A crisp "click" sound (pawl engagement sound) is emitted each time the brake is released.
Manual Type No Sound: No mechanical operating sound during braking (unless a malfunction produces abnormal noise).

3. Measure the Push Rod Travel

Manual Type Shows Significant Changes: After 30,000 kilometers, the push rod displacement increases significantly when braking (adjustment is required if it exceeds 40mm). Automatic type maintains constant travel: The push rod stroke remains consistent after wear (change ≤5mm within the design life).

4. Check for maintenance traces

Manual type evidence: Tool scratches are visible on the edge of the adjustment hole; new and old paint marks are visible on the locking nut.
Automatic type characteristics: No adjustment marks on the outer casing; no disassembly marks on the rubber plug of the reset hole.

5. Verification in special scenarios

Operation Manual adjuster response Automatic adjuster response
Reversing and emergency braking No abnormality May emit a "clicking" protective sound (to prevent over-adjustment)
Top wheel spinning freely Tire rotates freely Slight resistance (preload of the self-adjusting mechanism)
After water cooling, the Slack may decrease (thermal expansion and contraction) No change in automatic Slack compensation

6. Handling easily confused situations

Manual models disguised as automatic: Some manual adjusters are disguised with plastic covers; prying open the cover reveals the adjustment hole.
Automatic type with reset function: Reset hole ≠ adjustment hole—only allows insertion of a special tool to rotate within 90°.