Tachometer Repair and Upgrade Guide

March 8, 2012

This tachometer repair guide also includes information on tachometer light shift operation, and the steps for installing a tachometer adapter.

A tachometer repair may be necessary if it fails to properly measure and display the RPM level of the engine of your car. There are a variety of mechanical and electrical difficulties that can arise with the tachometer gauge, but generally these problems are simple to solve.

Be sure to turn off the engine before you make any repair. Always work in a well ventilated area on a hard, level surface. Always put on protective eye wear when working on your vehicle.

Check Your Owner's Manual
Check the index of your owner's manual to find and read the section on the tachometer before you begin the repair. This section of the manual should provide you with some instructions about checking and repairing your tachometer.

Check the Fuses
Find the fuse box in your car and look for the fuses for the tachometer. Check to see whether the fuse is in working condition or if it has blown. A blown fuse will look much like a light bulb that has burned out. There may be a black discoloration of the fuse and there will be a break in the metal strip. If the fuse is blown, remove it and replace it with a new one.

Look for Display Failure
A tachometer that is not showing the RPM level may have a damaged LED display unit. Older display units can stop working from usage and will need to be replaced.

Check Wire Connections
Look over the parts inside the tachometer. A tachometer unit or its parts can melt or burn if the wiring is blown. Corroded and damaged wiring will result in erratic display readings and may lead to a failure in the tachometer. Read the owner's manual to check the wiring connection diagram. Compare your tachometer to the wiring diagram and repair or reconnect any loose or damaged wires.

If the calibration is out of sync, the display readings will display as either unusually low or high RPM levels in comparison to the actual working of the engine. To repair this issue, the tachometer needs to be reset. Read the owner's manual for instructions on resetting the gauge.

Replace the Tachometer
If you are still unable to find the cause of any problems with your tachometer, install a working replacement unit. If the replacement unit does not work properly, you will know there is a problem with the wiring leading to the unit.

Repair and Reset the Car ECU
Your ECU may need repair work. Open the fuse panel. Use the fuse diagram to locate the fuse for the engine control unit and remove it. Insert the key and turn the ignition to "On" without cranking the engine. Wait five minutes. Replace the fuse you removed. When the "Check Engine" light blinks, turn off the ignition.

Seek Professional Help
You can remove the tachometer and take it to a repair shop or make an appointment to take your car in for a service call. A professional mechanic can test and replace car gauge issues or simply find any problems and help solve them. You can also contact the manufacturer and speak with someone in the service department for assistance.

Tachometer Upgrades

The classic uses for the tachometer are to inform the driver when to shift gears, setting engine idle, and information on slippage or traction. However, the tach has become more commonplace on newer vehicles because they yield information about the engine's power output, and can be used to increase gas mileage.

When selecting a new tachometer, try to select a model that matches the other instrument gauges in the vehicle. Additionally, if the dash does not have a blank space to add this gauge, another location must be selected. Ensure the location allows for easy gauge reading, and does not obstruct or interfere with the driver's vision or ability to operate the vehicle. Care must be taken when wiring the gauge, because a pinched or shorted wire can cause erratic readings, burn out the ignition module or drive you nuts replacing fuses.

For the wires that must travel into the engine compartment, try to use an existing grommeted opening. If none is available, use a grommet to insulate and protect the wires as they travel through the hole you drilled. Tachometers use four wires. The red wire connects to the 12-volt supply at the fuse box. The black ground wire connects to the vehicle's chassis. The white wire is for illumination, and connects to the dash lights. The green wire is the tach signal, and connects to the coil or ignition terminal on electronic ignition systems.

How to Install a Tachometer Adapter

A tachometer adapter enables a tachometer to perform accurately after executing an engine swap with an engine that has a different number of cylinders. For instance, if you swap a V-6 for a V-8, the tachometer signal needs to be re-calibrated from the V-8 to read accurately for the V-6. Every manufacturer includes specific instructions with the tachometer adapter, but there are similar basic steps for all models.

Test the Adapter
The tachometer adapter connects to a positive 12-volt supply wire. It converts the current it senses to a 12-volt signal. Locate the positive wire from the coil. Use a volt meter to be sure it is using 12 volts.

Mount the Adapter
Mount the tachometer adapter under the hood of your vehicle. Be sure to place it as far away as possible from any heat sources, to prevent any damage caused by overheating. Once in place, mark the mounting holes. Drill the three holes necessary. Use the screws to secure the unit.

Connect the Adapter Wires
Be sure to direct all of the tachometer adapter wires away from heat sources as well. The red wire connects to the ignition switch 12-volt supply. The red wire connects to the positive wire of the coil. The black wire connects to the engine ground and the gray wire connects to the tachometer trigger wire.

How Does a Tachometer Shift Light Work?

A tachometer shift light allows you to set the proper engine speed to change gears. For those relatively new to racing or drag racing with an altered rev limit, a shift light can help to improve performance and acceleration by giving you an easily noticed signal when the engine is at the required speed. It can be seen without focus being shifted from the road.

Role of a Shift Light
Just because an engine can be revved to a certain speed doesn't mean revving to that speed will produce optimal performance. For engines that have altered timing, different cams or forced induction, optimal acceleration may be at a lower than redline speed, or sometimes in the case of modified valve trains, the optimal engine speed may be at a level that is higher than can be registered by the factory tachometer. For these cases, a shift light can tell you out of the corner of your eye when the exact time to change gears is. This level of precision can often be enough to shave tenths of seconds off of a quarter mile time or lap time, which can mean the difference between winning and losing. Best of all, these lights can be easily seen without having to take your eyes off of the road to read a tachometer, which can also reduce potential accidents.

Pill Activated Shift Lights
A common feature found on many aftermarket tachometers is the pill activated shift light. For tachometers to register engine speed, wiring is attached to the speed sensor or from the sensors on the instrument cluster in order to read the voltage signal that corresponds to engine speed. These tachometers have a socket attached to the housing where fuse-like pills of specific voltages can be inserted. When voltage reaches a level that corresponds to the level that activates the pill, the circuit is completed and the shift light is illuminated, signaling the driver that shifting is required.

ECU Activated Shift Lights
Some programmable ECU's may be capable of triggering accessories based on engine sensor input. When the engine's sensors detect the voltage for the desired engine speed, they can trigger activation of an external shift light. Some vehicles may have a shift light installed in the gauge cluster from the factory. By altering the programming of the ECU, different engine speeds may be set. Otherwise, operation can generally be adjusted via one of the dials on the dash. Some models, such as those found in some BMW models, are integrated into the tachometer itself, and gradually change its color as engine speed increases.

Shift lights can be helpful in a variety of competitive environments and can have a dramatic effect on lap times, especially when driving an extensively modified engine. They are commonly available with many aftermarket tachometers or alone and generally take only a few minutes to install.