Throttle Linkage Tension on a Caterpillar D4D 78A

[MikePhua]

On the Caterpillar D4D with 78A‑series serials, there’s a common issue where the throttle lever slips back, causing the engine to slow even though the operator keeps the throttle pulled. This isn’t simply sloppy linkage — it’s a problem in the governor control at the injector pump.

What’s Happening Inside the Injector‑Pump Control
At the end of the throttle linkage, where it meets the injector pump/governor, there’s a small circular housing fastened by four bolts. Inside that housing are three springs and two rollers, seated around a ring. The role of these parts is to provide friction-based “holding” — the springs push the rollers out against the ring’s inner surface, so the linkage stays at the position you set, but still lets you move it when you pull the throttle.
If this assembly is dirty, corroded, or oiled incorrectly, the rollers can slip, and the linkage won’t hold. One experienced user advised: take it apart carefully, clean every component, then roughen up the inner diameter of the ring so the rollers can grip better. Do not oil any part besides the spindle — too much lubrication kills the friction that’s needed.

How to Adjust It

1. Remove the four fasteners on the little round housing at the end of the linkage.
   
2. Extract the internal parts and carefully clean them: springs, rollers, and ring. Make sure the spring holes are free from crud so the springs can operate freely.
   
3. Rough up the ring’s inner surface to improve grip — a bit of grit helps the rollers “bite.”
   
4. Reassemble, making sure the rollers are pressed outward by the springs. When properly assembled, they should hold the throttle in place but still allow controlled movement when you actuate the lever.
   
5. Use a clean gasket when resealing the housing — a worn or wrong gasket can upset the tension. According to one parts breakdown, the correct gasket is often listed as part 9S‑2308.
   

Why This Adjustment Matters

• Without proper tension, the throttle linkage will slip back during operation, making the tractor hard to control.
  
• If over-lubricated, the friction mechanism fails; if under-cleaned, corrosion or debris causes sticking or erratic behavior.
  
• Proper maintenance extends the life of the governor mechanism and helps the operator maintain stable engine speed.
  

Common Pitfalls to Avoid

• Don’t oil the inside of the housing (except the spindle) — lubrication ruins the friction clutch effect.
  
• Be careful removing the housing; these are delicate springs and rollers that must be reassembled correctly.
  
• Inspect and clean the spring cavities to ensure consistent spring force when reassembled.
  
• Double-check the sealing gasket to prevent leaks and maintain the correct spacing for the friction parts.
  

Technical Terms Explained

• Governor: Device on the injector pump that controls fuel delivery based on throttle position.
  
• Rollers & Springs (Inside Housing): Provide a friction clutch mechanism so the throttle holds position without slamming.
  
• Spindle: The rotating shaft inside the housing that links to the throttle cable.
  
• Gasket: Seals the housing; prevents oil or dirt from entering and maintains internal geometry.
  

Conclusion
What seems like a loose or slipping throttle lever on a D4D 78A is often just a dirty or misadjusted internal friction mechanism inside the governor housing. By carefully disassembling, cleaning, slightly roughening the ring, and reassembling without over-lubing, you can restore proper throttle tension. This tweak brings better control and stable engine operation — and it’s something many owners have successfully fixed themselves.