Reference Manual

Chapter 6: Electrical/Interior: Erratic/High Tach Fix

The Tachometer in an Oldsmobile Cutlass instrument cluster can, over time, develop a problem causing the tachometer to indicate a higher RPM than actual engine RPM.
Tach Jumper

  1. Instrument Cluster Assembly
  2. Shorting Adapter

A shorting adapter (Jumper) at the rear of the gauge cluster assembly is used to set the tachometer to indicate the RPM of either a 6 or 8 cylinder engine. Over time this jumper will often develop poor contact. The loss of connection will cause the tachometer to indicate a higher RPM than actual engine RPM.

This could appear as an intermittent problem where the tachometer will occasionally jump to a much higher indicated RPM, or a permanent loss of connection in which case the tachometer will constantly indicate a much higher RPM than actual engine RPM.

Reforming the jumper will sometimes work, for awhile. A permanent solution is to solder a jumper wire in place of the jumper and never suffer the problem again.

Tools required:

Time required:

The time required will depend much on how long it takes you to remove the cluster assembly from the dash. The actual disassembly of the cluster and soldering the tach should take no more than an hour.

The Procedure:

Back of Tachometer circuit board
Rear of the Tachometer

To calibrate the tachometer the jumper (8 cylinder jumper pictured top left in the image above) plugs into the top hole for a 6 cylinder, or the bottom hole for an 8 cylinder engine. When installed the jumper should short the solder printed circuit board traces above and below the respectives holes.

Tachometer jumper in place
8 cylinder jumper wire installed in the Tachometer

A straight piece of wire will do the job, but there is a reason why I formed the jumper above in such a strange shape. When trying to solder such a short small piece of wire, it'll have a tendency to move around quite a bit. By bending the wire this way, I just dropped it into the hole and the shape greatly limits it's tendency to move around while I'm soldering it.

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