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However, my devious mind tells me that probably the only difference is a solder bridge or jumper on a circuit board inside the tach. You'll probably mess up the case getting it open, but I would do it anyway and see what I find, just to satisfy my curiosity.
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I couldn't find anything that looked like a way to change any settings, but I found this Tach Mod Page
and later found a reference to it at YJ Powertrain Swap
where it is mentioned that this mod will work on a YJ tach.
Now on to the tach surgery.
The back showing drain holes and the holes for the lights. No concern here for being able to keep it water proof since it never was.
The front bezel is crimped on the housing and is removed with a screwdriver.
Got the bezel off and verything can be reused except the gasket, but a little silicone will work fine.
Remove the two nuts and screw holding the wire clamp from the back.
Ease the guts out.
The red and black wires lead from the circuit board to the connections on the meter. Nothing has been done to the tach yet.
According to the Toyota Tach Page, 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(red and black wires).
Nothing to loose and an accurate working tach to gain after a 302 swap. I'll give it a try.
Went to radio Shack today and they did not have the 5K pot, but they had a 10K Cerment pot. I installed it according to the Toyota Tach directions. I have not done the 302 swap yet so I have not checked the tach on a V-8, but the pot can be adjusted to lower the tach reading down to zero at all engine speeds I checked.
Here is how I did it.
The schematic for the pot.
The pot. Note that pins 2 and 3 (closest two) were used although pins 1 and 2 could have been used. Also, the unused pin has been removed.
Solder the wire to from the pot to the black meter lead to the board before glueing because the pot wiring lays on the board above the wiring to the meter. Then solder the other wire to the red meter lead. Glue the pot so it is parrellel to the assembly mounting studs.
Now you can make a guess and drill a hole in the housing to allow you to turn the pot after reassembling the tach. You could also loop one of the wires running from the pot to the meter outside the tach so you could cut that wire if you should ever want to use the tach on (or swap back to) a 4 or 6 cylinder.
Reassemble and enjoy....I hope.
Edit 3-21-10: After the mod you will have to calibrate the tach. All you have to do is hook a known accurate tach to the engine and start the engine. Turn the screw in the pot until the two tachs match. The higher RPM you use the more accurate the tach will be. Keep safe engine operating limits in mind though. I am still running the original tach that I modified and have had no problems with it. I receive PM's every so often wanting to know if it will work on Chevy swaps. I don't know if it work on an engine not running a dura spark ignition. My guess is that it will work, but try it at your own risk. If you try it and it does work, please post here and let us know.