"It is almost scary how plug and play most Toyota parts are, at least in this swap. The tach looks virtually identical to the stock MR2 tach (the mechanical part that attaches to the back of the face). ...we found that the tach slips right in and, thud, no tach adaptor needed." -chall
"You will need the tach overlay for an NA MKII MR2. It has a 180 deg sweep with a 6300 redline. As oppesed to a 180 deg sweep 7000 redline/ 7250 revlimit Turbo gauge. Without it your tach will be completely inaccurate. It fits and has the same font/ look as other MR2 gauges." -Luke
I have also used a 1k ohm resistor and diode hooked to two of the negative sides of the coils and used the stock tach. If I've not updated the link, I will shortly.
I have a '94 Camry V6 instrument cluster that I purchased about a year ago to use in the swap, but never did. This tach is almost identical in design to the MR2 N/A tach, using an almost identical circuit board, so it was actually pretty easy (for me) to determine which resistors to change to convert the tach.
There are two design changes to be made:
1. Obviously the frequency input is 50% higher for the V6, so the amplifier gain must be reduced to correctly scale the needle swing.
2. The input voltage to the MR2 tach is much higher because it comes from the igniter, picking off of the "low side" of the ignition coil instead of the ECU signal (12V pulsed).
Changing this resistor will make the tach more sensitive on the input so that it can be used with the lower 12V signal from the ECU:
The MR2 tach resistor is a 43K (Yellow, Orange, Orange). The Camry tach used a 24K resistor, but 22k is more common and is close enough. With this change, you can wire the OEM V6 ECU "Tach" output wire directly to the MR2 body "Tach" wire.
Changing this resistor sets the gain of the amp that drives the needle as the RPM changes (changing the frequency of the input pulses derived from the coil drivers):
Notice that this resistor is also 43K. Reducing the scale on Dave's tach required a 3.9K resistor, which is a considerable drop in value, but still well with typical design limits.
If you mod your tach like this, the 3.9K value MAY be correct. You will note that it is a larger 1/4w type than the rest of the resistors on the board (except for the Input Voltage resistor), as it is THE calibration resistor that sets the needle swing and it is made to be hand-inserted during factory calibration of the tach.
It might be a good idea to put a 5K multiturn potentiometer in place of the gain resistor to do your own calibration. Note that it will require an accurate handheld tach or a bench type frequency generator (like I have) to get this done correctly.
Also, you may have to remove the needle and carefully place it back on the shaft at a known "RPM" to have the tach read both ends of the scale correctly, but this is only done once the sweep has been calibrated. (I used 1000 and 6000 RPM frequencies; 50Hz and 300Hz respectively.)
And, one last caveat. If you keep your A/C you probably can't use this mod at all. The A/C Amplifier module uses the same tach lead from the back of the MR2 to detect that the engine is running and then allows the A/C compressor to turn on. I haven't verified this, BUT, it is likely that the A/C Amplifier input will NOT be sensitive enough to detect the presence of the lower voltage tach signal, and therefore won't turn on the compressor.
Also, this means you can re-scale a Turbo tach and use it for your V6 as well, just change the face to an N/A face - Indiglo, for example.