How to calibrate Stock NA Tachometer for V6 Signal

[sengk]

This is a write up on how I modified a stock mr2 NA tachometer to read a v6 engine signal properly.

I read that a 97+ Camry tach is a direct replacement for the mr2 one but I was unable to locate any at the junk yards so this left me with only two options: installing an aftermarket tach or modifying the stock tach. I didn’t want to give cops any more reasons to pull me over and now that I’m older it looks sort of silly to me now so I went with the latter option.


Here are the websites that I found helpful for this mod:

TOYOTA TACH MOD

MR2 Wiki - MKII:3vzfe TACH browse


Requirements:
Non-turbo tach – This mod was done on a non-turbo tach so I don’t know if it will work on a turbo tach, but I don’t see why not.
Soldering iron
Solder
10k or 5k ohm 15-turn trim potentiometer – radio shack $3
22k ohm resistor ¼ or ½ watt 5% tolerance – radio shack $1
Multi-meter
Soldering skills
Hot glue gun and glue stick
Quick splice connector (Scothlock Connectors.)
Some wire
Electrical tape
Wire stripper and cutter



Disclaimer:
This modification could possibly leave your tach non-functional. Do not hold me accountable for what you do to your own car. Proceed at your own risk.

Okay let’s get down to business.

Step 1: Remove the combination meter from your car and remove the front plastic cover to get access to the gauges– please refer to the bgb for this step.

Step 2: Flip the gauge cluster over and remove the three screws holding the tach to the gauge cluster. You should see SE, M+, and M- next to the screws. After you remove the screws the tach gauge will fall out of the front of the gauge cluster.








Step 3: Using your multi-meter set to read 200k ohms find the 43k resistor on the tach circuit board. It’s the biggest one there. Now use your soldering iron and remove the resistor from the circuit board and replace it with one of the 22k resistors you bought. The orientation of the resistor doesn’t matter so don’t worry about it.



Step 4: Using your multi-meter again, find the 7.5k resister. It should be the second largest one compared to the 43k resistor. It might even be the same size, I don’t know because I threw them out already. Use your soldering iron, remove the 7.5k resistor and replace it with the trim potentiometer. The potentiometer has 3 leads. Use the middle one and the one furthest away from that. You will have to bend the pins out some to fit it in the holes in the circuit board. Be careful with the pins because they break off easily. Break the unused lead off of the potentiometer and secure the potentiometer to the circuit board with the hot glue gun while making sure that the leads on the potentiometer aren’t touching any other connections on the circuit board.



Step 5: After calibrating my tach I ended up with a resistance of 3.27k on the potentiometer, so as a base line you should use your multi-meter, set it to 20k ohm, and adjust the potentiometer via the little screw head to 3.27k. This way you wont have to do a lot of adjustments on the car.



Step 6: Don’t install the tach gauge back into the cluster you wont be able to adjust the potentiometer that way. Take the 3 screws that you removed from the gauge cluster to remove the tach in step 2 and tread them half way into the holes on the back of the tach gauge circuit board where it says M+, M-, and SE. Use quick splice connector and some wire to splice into B3 on the B connector of the combination meter wiring harness. This will be your tach signal. You will also need a +12v and ground connection. I removed the connector on the back of the cigarette lighter to get the ground and power connections. Run two wires into this connector and use your multi-meter to test them to see which wire is power and which is ground. If you can’t remember, label them.





***Warning*** the repair manual says that you should never let the tach signal touch ground while the car is running, so be careful!

Step 7: The stock tach gauge has 3 screws/terminal posts: M- (Tach signal), M+ (+12v power), SE (Ground). Your aftermarket gauge should have these same connections plus one extra wire for the backlight. You can hookup the backlight wire to the +12v power wire or leave it unwired. Now just connect both gauges up using the wires that you ran in step 6 and tape all the connections up with electrical tape.



Step 8: Now start the car and hold it at a steady RPM and adjust your stock tach to match the aftermarket one. Make sure you use different RPM points to calibrate. I used 1k, 2k, 3k, and 6k. Keep an eye on both gauges as you rev to see if the stock gauge is more or less doing what the aftermarket one is doing, adjust the potentiometer with a flat head screw drive as necessary.

Step 9: After you are satisfied with the calibration of the stock tack turn off the car and disconnect both tach. Remove the power and ground wiring. You can also remove the tach signal wire if you want but I left it there with the end taped up incase I wanted to install and aftermarket tach in the future. Make sure you tape it up with electrical tape and label it so you know what it is later.

Step 10: reinstall the stock tach back into the gauge cluster and reinstall everything back in your car the reverse way you took it apart.

That’s it. I tried to be as detailed as I could but if I missed anything please let me know.

[canadam]

Awesome write-up.

Sticky ?

[sengk]

Glad I can finally give back to the community.

[Weasy2k]

GREAT work sengk!

[Gimpster]

I was able to source the tach module from my donor car, but I thought I'd post some more useful info. I've used this to recalibrate multiple tachs in various swap projects I've done.

His article is very good, but I thought I'd post some info to make it a little more easy/scientific

Instead of using the car itself to calibrate (free revving etc) you can use an EL CHEAPO battery charger as your signal source. Basically you need to power up the guage and ground it with a 12v power source, like say a PC power supply.

Then on the signal input you use the battery charger, here is a snip from the place I learned how to do this:

PREFERRED SIGNAL SOURCE...an inexpensive 12v battery charger can be used as a deadly accurate signal source for the re-cal of your tach. This signal source is so steady and accurate that there is no need to compare the reading with another V-8 calibrated tach. Battery chargers typically use a rectifier bridge to convert the AC power source into DC. This turns the 60 hertz sine wave input into a 120 hertz ripple wave DC output. The ripple wave output is be read as 3600rpm by the stock rotary calibrated RX-7 tach. For a V-6 application, an RX-7 tach should be adjusted to read this signal as 2400rpm. For a V-8 application, adjust your RX-7 tach to read this ripple signal as 1800rpm. Please note that you must use a 12v battery for a power source, and the battery charger as a signal source only. The ground wires for both the battery and the battery charger should be connected to the ground terminal of the tach.

I realize that says RX7, but the RX7 tach works like a 4cyl tach. So the RPM values are still valid. The cheaper the battery charger the better, some of the baller models don't behave in this manor.

I've used this method on two RX7's, my S10 and a Mustang.

Edit: http://members.tripod.com/~grannys/rx7tachrecal.html

Gotta give credit, where credit is due

[sengk]

That some great info Gimpster. It will save a lot of time during the calibration process.

[conor]

Awesome stuff! Best sticky in forever...

This is going to come in handy on my next project.

[Weasy2k]

thats also a good idea gimpster...this is a great thread

[V6'er]

^ And, it's ALMOST correct.

To truly calibrate a frequency meter, which is what the tach is, you MUST use more than one calibration frequency. Simply using a 120Hz signal is not enough (because you are modifying the frequency response of the meter).

Sengk is very close to hitting it right, but, you have to remove the needle on almost every tach to get the "offset" correct. The pot allows you to adjust "gain" (needle swing) only.

So, to get it really accurate, you need high and a low frequencies like sengk is using with an aftermarket tach, and, you need to remove the needle and put it at the correct spot once the full needle swing is achieved.

I also found that using 15K, not 22k, results in greater reliability. (This resistor sets the input voltage range to the tach chip, which is actually a frequency to voltage converter.)

[DaveMush]

Dork

So that is how you modified my Tach oh so long ago Mitch....

I still have it too.

[sengk]

Originally Posted by V6'er

^ And, it's ALMOST correct.

To truly calibrate a frequency meter, which is what the tach is, you MUST use more than one calibration frequency. Simply using a 120Hz signal is not enough (because you are modifying the frequency response of the meter).

Sengk is very close to hitting it right, but, you have to remove the needle on almost every tach to get the "offset" correct. The pot allows you to adjust "gain" (needle swing) only.

So, to get it really accurate, you need high and a low frequencies like sengk is using with an aftermarket tach, and, you need to remove the needle and put it at the correct spot once the full needle swing is achieved.

I also found that using 15K, not 22k, results in greater reliability. (This resistor sets the input voltage range to the tach chip, which is actually a frequency to voltage converter.)

I actually did remove my tach needle also. I had it off a few times already so When i did the final calibration I held the aftermarket tach at a constant rpm and replaced the needle on the stock tach at the same position. I didn't put this part in the write up because i was unsure whether or not this made a difference. I've also had a problem with bending the metal pin before on a different car while installing gauge faces so I didn't want other people to take more risks than necessary.
Thanks V6'er for clarifying this. It sounds like you have a better understanding on this than I do so please add more info if you can to improve on this.

thanks,
sengk

[Oteck]

mmmm is it that hard to recalibrate the needle? i just left the ignition on acc putting the tach at zero

[V6'er]

^ When I calibrate a tach, it is within about a 1% error. Removing the needle is a requirement to get it right.

I've found many cracked needle shafts, the plastic cracks over time from the tight fit on the shaft. You should glue the split first.

And, Turbo tachs and N/A tachs require different gain resistors. I don't use a pot, I put two resistors in parallel when necessary to get the value as close as possible. This is actually more stable in the long run, but it takes a lot more time to get it right.

[Gimpster]

Every tach is different, but I've had good luck using just the battery charger on the bench. My S10's tach is dead-nuts on, I verified it with the ALDL datalogger. I'm usually too worried about trying to keep the nose/rear of the truck straight to look at the tach though @ WOT.

It's also worth noting that I don't generally strive for NASA / Aerospace quality accuracy

If you can't tell from most of my posts, I'm pretty shadetree.

V6'er, what's the best way to generate multiple frequences on the bench to test? I wonder if something like the Megasquirt Stimulator could be used (since it generates RPM signals to test your Megasquirt with)?

I'm using the Avalon module, and did the turn car to ON -> place the needle on. It's off by about 200rpm no matter what I do (200 too high, verified with OBDII logger). I'm thinking about recalibrating the MR2 module, because it's kind of annoying.

[V6'er]

200 RPM high at any point means your needle sits 200 RPM high. You remove it with a steady signal on the tach and very carefully place it back on and tap it down at the right spot.

I use a frequency/function generator, a frequency counter, and an oscilloscope to calibrate a tach. (When I send it out, it had damned well better not come back.)

The scope lets me know that I'm looking at a good signal with no DC shift, and I also test every tach for it's mininum signal level to lock onto. (Peak to peak signal.)

[sengk]

Originally Posted by Gimpster

Every tach is different, but I've had good luck using just the battery charger on the bench. My S10's tach is dead-nuts on, I verified it with the ALDL datalogger. I'm usually too worried about trying to keep the nose/rear of the truck straight to look at the tach though @ WOT.

It's also worth noting that I don't generally strive for NASA / Aerospace quality accuracy

If you can't tell from most of my posts, I'm pretty shadetree.

V6'er, what's the best way to generate multiple frequences on the bench to test? I wonder if something like the Megasquirt Stimulator could be used (since it generates RPM signals to test your Megasquirt with)?

I'm using the Avalon module, and did the turn car to ON -> place the needle on. It's off by about 200rpm no matter what I do (200 too high, verified with OBDII logger). I'm thinking about recalibrating the MR2 module, because it's kind of annoying.

I'm with Gimpster on the accuracy part. For me this mod was "good enough" because it brought the redline down to where it should be and not off the gauge. The needle at any given rpm is close to where it should be. Thats accurate enough for me as I dont have the fancy equipment to generate a signal to test with. I'm mainly concerned with seeing the redline coming anyway. Before the mod I would have to count in my head after I watched the needle go off the scale and stop moving all together.

V6'er is right though, this method wont be the most accurate.