Reference Manual

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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.

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:

• 1/4" socket
• Soldering Iron
• Rosin Core Solder

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:

• The first step is to remove the cluster and take it to a suitable work area where you can disassemble it.
• Remove the four 1/4" screws retaining the lens to the cluster assembly.
• Lift off the Lens and the mask, lift out the indicator light housing and the high beam/turn indicator housing. Set these items safely aside, being careful not to lose or damage the filters.
• Remove the two 1/4" screws retaining the speedometer and remove the speedometer from the cluster housing.
• Remove the remaining 1/4" screw retaining the tachometer.
• Three screws on the rear of the tachometer slide into spring clips to make the electrical connections. So remove the tachometer from the cluster housing by pulling firmly on the face plate.
• Place the tachometer face down.

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.

• Solder a short piece of wire between the appropriate traces as shown below.
  Be sure to use rosin core solder, Do Not use acid core solder or flux.

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.

• Place the tachometer in the cluster housing and push down firmly to engage the spring clips.
• Place the speedometer in the housing.
• Reinstall the three 1/4" screws that retain the speedometer and the tachometer. Do Not Overtighten, these screws go into plastic and will easily strip out the plastic if overtightened. Tighten no more than snugly.
• Place the indicator and high beam/turn light housings into place and insure that the filters are in place and properly positioned.
• Place the mask and the lens into place and reinstall all four screws.
• Reinstall the cluster assembly.