The Toyota tachometer accepts a pulsed ignition signal from the coil. This signal is converted into a proportional DC signal that drives an analog current meter in the dash to display engine rpm. Its reading can be scaled to correctly display the correct engine rpm by adding a calibrated resistive shunt to the meter input.
A 5k ohm trim potentiometer is placed across the input contacts to the tachometer meter.
This is done directly at the meter posts inside the instrument cluster.....see details below.
For this purpose, it is best to use a small 10 turn potentiometer to provide easier calibration of the tachometer. Trim pots as shown here are small, accurate, and inexpensive, but most any high turn count potentiometer can be used. These can be purchased at any electronics parts store.
Solder wires to two of the leads. Look at the small schematic on the potentiometer. You want to solder one wire to the middle terminal (usually labeled #2) and the other wire to either of the remaining two terminals (usually labeled #1 and #3). Then find a way to add it to the wiring at the back of the meter. This must be done on the instrument cluster at the meter post themselves.
The figure here shows schematically how this is done. Either polarity will work fine. Once installed, you will be able to recalibrate, or scale, the tachometer reading by adjusting the potentiometer. By running the engine and using another accurate tachometer as a reference, adjust the potentiometer until the engine rpm readings are both the same. Once done, you will have a factory tachometer that reads accurately for your V6 or V8 engine.
Remove the two screws securing these two brackets.
Push to release all tabs on top and bottom of front cover.
Remove these five screws.
Pull the tach gauge from the cluster.
Wire the potentiometer across the M- and M+ terminals....polarity doesn't matter.
Philippe Maier adds this about modifying newer trucks......
"Jay Kopycinski documented his 4.3 conversion and described how to convert the tach on his vehicle. He describes using a ten turn pot (5k ohm) on the tach posts. I went to perform this modification on my 1990 4Runner SR5 which now has a V8.
I found that there was already a small white potentiometer on the circuit board of the tach. I took an original reading (42.5k ohm) then reinstalled the tach into the instrument cluster. I removed the speedo to gain access to the philips style screw on the little white pot on the board (it points towards the speedo). Then I reconnected the instrument cluster (temporarily), started the vehicle, and adjusted the pot until the factory toy tach matched the actual RPMs at different key RPMS (ie. 2000, 2500, 3000, ....). Bingo! No modifications required!
Works very well. The final reading on the little white pot was
around 31.1 k ohms. My guess is that you should probably be able to make
this adjustment on most post-1990 vehicles (have received confirmation this
works on a 1995 model). Time to do this adjustment, about 30-60 minutes.
Sure beats having that other tach mounted to my steering column !
And just as accurate."