Collecting the parts for a 302 Ford swap into a YJ. The question I have is, "Has anyone found a way to get the factory YJ tach to function correctly after a 6 cylinder to v-8 swap?" It seems to me all that should be required is to recalibrate the tach due to the fact that the motorcraft ignition is being retained. Any ideas on how to recalibrate the tach?

This has been discussed before and the answer was "Can't be done." You might try doing some searches on the subject, but I think the answer was that on the factory tach there isn't a 4-6-8-cylinder switch.

All you can do is tape a new set of numbers to the face, get used to the tach reading 1/3 high, or by a new tach.

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.

If the YJ factory tach hooks to the negative side of the coil then I found a 4-6-8 divider/multiplier box that will covert your 8 cylinder output to a 4 or 6 cylinder output

I was helping someone with a 4.0 conversion and the computer puts out 2 pulses every 1 revolutions like a 4 banger would and we determined he could either find a 4 cylinder tach to use or use one of these converter boxes.

When JU gets back up, I post a link for the converter box. I tried a google search but got 5,000 hits and most were for distributerless converters.

The Tach is basically a big averaging votmeter with a very slow response....the more pulses, the more the needle deflects. If you open it up, you will most likly see a resistor divider bridge....you could most likly trim that out with resistors...but to be honest, I wouldn't bother. By the time you open it up and go through that work, your better off buying a replacement set up for a V8...one of the big problems is getting the meter back together so that it does not let in air and moisture.....once that happens, you end up with a foggy tach every morning....

Here is the site:
http://mysite.verizon.net/vze7erz1/id1.html#tsdmod

Looks pretty simple to hook up.


$40-$50.00 Depending

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By the time you open it up and go through that work, your better off buying a replacement set up for a V8...o

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Any idea who makes a factory style replacement V-8 tach for a YJ?

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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.

UPDATE....



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.

Attached Images

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Finally got a chance to try to calibrate the tach and this mod does work. The tach will be accurate after it is properly adjusted.

Hello


I am French and serves to me as a translator!!!

I found a solution to modify my tachometer thanks to post but I seek the datasheet T2685A 6k which poses problem to me.

If somebody little to help me.

Thank you

@+

Welcome to the board!

I don't see a reference in this thread to that data sheet. What is it and where have you looked?

And when you have time tell us about your project. We're interested in Jeeps around the world.