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Technical Forum (Read Only) => Instrumentation => Topic started by: bd on October 30, 2012, 07:50:41 PM

Title: Common causes of "bouncy" speedometer needles
Post by: bd on October 30, 2012, 07:50:41 PM

Nine (9) of the more common causes of "bouncy" mechanical speedometer needles and their solutions, in the approximate order of occurrence:

• DRY SPEEDOMETER CABLE
  Remove the driver side A/C duct from below the steering column if the truck has A/C.  Reach up behind the cluster and disconnect the cable from the back of the speedometer head by depressing the cable's spring steel retaining tab toward the cluster with your left thumb while simultaneously pulling the cable away from the cluster with your left hand.  Maneuver the cable down below the dashboard.  Use needlenose pliers to grasp the end of the cable and pull it completely out of its casing.  Wipe the cable clean with a lint-free rag and inspect the cable for obvious wear, broken strands, and kinks.  If the cable is obviously kinked or damaged in any way replace the entire cable and casing assembly.  Liberally smear the cable with lightweight lithium grease as you feed it back into its casing making sure that the cable doesn't pick up any grit or debris from the ground or cab floor in the process.  Then reattach the cable to the back of the speedometer in the reverse steps of removal.  If the vehicle has multi-segment cables or factory Cruisemaster cruise control, apply a similar lubrication process to each cable section.
  
  
• BENT, KINKED, MELTED OR OTHERWISE DAMAGED CABLE CASING AND/OR CABLE
  Even subtle kinks in the sheath will cause a bouncy needle.  If either the cable or its casing is kinked, replace the complete cable assembly.  If the cable casing is melted, replace the complete cable assembly and reroute it to avoid a recurrence.
  
  
• THE “SHAFT” OF THE DRIVEN GEAR IS EXCESSIVELY WORN
  This is most common with plastic gears.  Remove the driven gear and its support bushing from the transmission case.  With the driven gear still in its bushing, wiggle the gear side-to-side.  Next, remove the driven gear from its support bushing and inspect the shaft of the gear for excessive roughness and concentric grooves.  If there is greater than ~0.006" of side-play or if the shaft of the gear is rough or exhibits concentric grooves, replace the driven gear, the gear seal, and the support bushing o-ring.  Inspect the support bushing per the next heading.  Coincidentally, a concentric groove in the area of the shaft seal seat will result in nuisance fluid leaks.
  
  
• THE “SUPPORT BUSHING” FOR THE DRIVEN GEAR IS EXCESSIVELY WORN
  This is most common with aluminum bushings in automatic transmissions.  Remove the driven gear and support bushing from the transmission case.  Slide the gear out of the bushing and inspect the gear shaft hole running through the center of the bushing for non-concentric (oblong or oval) wear.  Replace the support bushing, bushing o-ring, driven gear, and gear seal if the gear shaft through-hole is worn even slightly oblong.  Be alert when purchasing a replacement bushing for automatic transmissions since bushing offset is dependent on the number of teeth on the driven gear, which affects driven gear diameter.
  
  
• WORN OUT PLASTIC DRIVE AND/OR DRIVEN GEAR TEETH
  Generally, the driven gear will show the greatest wear.  Although not always necessary, it is generally best to replace both gears, the driven gear seal, and the support bushing o-ring.  Carefully inspect the support bushing for wear and replace as needed; see the previous heading.
  
  
• STRIPPED CABLE OR DRIVE KEY SOCKET IN THE END OF THE DRIVEN GEAR
  Replace the driven gear, gear seal, and support bushing o-ring.  Verify adequate cable (or drive key) penetration into the end of the driven gear as well as the back of the speedometer head.  Verify free rotation of the driven gear in its support bushing and inspect the support bushing for excess wear.
  
  
• WORN OUT SUPPORT BUSHING INSIDE THE SPEEDOMETER HEAD
  Have the speedometer head overhauled by a specialist or replace the complete speedometer head assembly.
  
  
• DRY OR WORN OUT RATIO ADAPTER BETWEEN THE CABLE AND TRANSMISSION
  Rebuild and lubricate or replace the ratio adapter.  Inspect the drive key and replace as needed.
  
  
• SOME AFTERMARKET CABLES ARE NOT CORRECTLY SIZED OR FORMED TO PROPERLY FIT THE FACTORY SPEEDOMETER HEAD
  Replace the cable with an original AC Delco cable (if available) that has a molded plastic tip to engage the speedometer head bushing.  This solution especially applies if the "bouncy needle" was coincident with installing an aftermarket cable.