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How to recalibrate your rev counter after an engine swap

Discuss modifications on your 1979 to late 1998 SFA 4x4 Hilux here.
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krazy_kameleon
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How to recalibrate your rev counter after an engine swap

Postby krazy_kameleon » Tue May 15, 2012 11:37 am

I found this on the web, thought it might come in useful...
Rick
Toyota Tach Mod
Recalibrating a Stock Tach for an Engine Swap

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The following info details how I modified my 1985 Toyota truck stock tachometer to correctly display the engine speed of my swapped in Chevy V6.
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.

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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.

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STEP BY STEP PROCEDURE (as done on a 1985 cluster):

Remove the two screws securing these two brackets.
tachmod1.jpg
tachmod1.jpg (15.24 KiB) Viewed 4246 times


Push to release all tabs on top and bottom of front cover.
tachmod2.jpg
tachmod2.jpg (15.39 KiB) Viewed 4246 times


Remove these five screws.
tachmod3.jpg
tachmod3.jpg (19.86 KiB) Viewed 4246 times


Pull the tach gauge from the cluster.
tachmod4.jpg
tachmod4.jpg (15.62 KiB) Viewed 4246 times


Wire the potentiometer across the M- and M+ terminals....polarity doesn't matter.
tachmod5.jpg
tachmod5.jpg (14.06 KiB) Viewed 4246 times


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."

Thanks for reading!
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Copyright (c) 1995, 1999, 2002, 2004 by Jay Kopycinski, All Rights Reserved.
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Re: How to recalibrate your rev counter after an engine swap

Postby 4x4megaworldpta » Tue May 15, 2012 11:53 am

Will this also be possible with a diesel Tacho. Mine still have to be done in maybe such a way.
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Re: How to recalibrate your rev counter after an engine swap

Postby krazy_kameleon » Tue May 15, 2012 2:14 pm

I dont think so Louis as the diesel generates a square wave signal not the pulse that is generated by the coil,
I think your only solution is to fit the diesel cluster (or at least the tacho part) I may be ableto help you out there as I have converted back to petrol and have a new cluster.

I'm sure the electronics boffs will be able to confirm this or not
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Re: How to recalibrate your rev counter after an engine swap

Postby 4x4megaworldpta » Tue May 15, 2012 2:23 pm

Well mine was diesel, I just want to have the tacho work again. Any Ideas?
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Re: How to recalibrate your rev counter after an engine swap

Postby krazy_kameleon » Tue May 15, 2012 2:32 pm

Ok so correct me if I am wrong

you had a diesel bakkie which is now running a petrol engine and you have the petrol tach in the cluster?
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Re: How to recalibrate your rev counter after an engine swap

Postby 4x4megaworldpta » Tue May 15, 2012 2:36 pm

It was diesel, still have the Diesel Tacho. Now it is 4Y Efi Petrol. :lol:
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Re: How to recalibrate your rev counter after an engine swap

Postby krazy_kameleon » Tue May 15, 2012 2:56 pm

OK, now I got it.

You will need to swap out the diesel tach for a petrol unit as the diesel tach can"t "understand" the signal generated by the coil.
after that its just a matter of tracing the M+ and M- wires from the cluster plug into the engine bay and hooking them to your coil.
M+ to positive coil output and M- to negitive coil output.

or just bypass the standard harness and run your own 2 wires...

Louis I have sent you a pm with regards to efi on a 22r
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Re: How to recalibrate your rev counter after an engine swap

Postby MOFASA » Tue May 15, 2012 4:21 pm

Ok maybe you guys can help me now...... :think:

I have a diesel motor and i dont know if the clocks have been changed but the bakkie came out with a 2L petrol engine..... :problem:
My rev counter has power to cos as you turn on the ignition the needle jumps up a fraction and then goes down..... :roll:
But i have no idea where to start tracing the fault of it not working.....
Any one had the same problem :?: :?: :?:
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Re: How to recalibrate your rev counter after an engine swap

Postby krazy_kameleon » Tue May 15, 2012 4:23 pm

Johno where is the red line on your rev counter?
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Re: How to recalibrate your rev counter after an engine swap

Postby MOFASA » Tue May 15, 2012 5:44 pm

krazy_kameleon wrote:Johno where is the red line on your rev counter?


It goes into a white line at 5500 and then a yellow line (the red) at 6000
So it may be a diesel rev counter :think:
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Re: How to recalibrate your rev counter after an engine swap

Postby zepplin » Tue May 15, 2012 6:35 pm

Mine worked just fine by adjusting the built in potentiometer behind the tacho. This was on a petrol 4 cyl to petrol 6 cyl swap.
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Re: How to recalibrate your rev counter after an engine swap

Postby 519franco » Tue May 15, 2012 8:27 pm

do you solder the red and blue togeter with another wire
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Re: How to recalibrate your rev counter after an engine swap

Postby Thabogrobler » Sun May 05, 2013 9:14 pm

Errrr....My SFA used to be 4Y, now 2L2T.

I can swop out the pertol Tach for a diesel one. Where will I pick up the wires/signal for the new, diesel tach?
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