3VZ-E Tachometer Modification

Note: I've noticed that some people have tried to make this modification to their tachometer for purposes other than those intended, and that, not surprisingly, it hasn't worked. I've sucessfully gotten a stock toyota V6 tach from an 88 4runner to work with the 7MGTE tach signal, and that's it. I suspect that it will work on other model year tachs as well, but I've not tested that.

If you have a 7MGE, you don't need to do this, just get a standard V6 truck / 4runner tach.

Background

After swapping the 7M-GTE into my 4runner, my tachometer no longer worked, although it did with the 7M-GE. I considered purchasing an aftermarket tach, but I like the clean stock look; I did not want to mount a 5" diameter tach on top of my dash. I researched to see if I could find any information on this sort of a modification on the web, but had no luck. Next I searched for wiring diagrams for each of the tachometer circuit boards thinking that I might be able to build some go-between circuitry to modify the 7M-GTE ECU signal to be what the 3VZ-E tachometer needed to operate properly. That search also left me empty handed, so I took a closer look at the 3VZ-E tach circuit board.

 

The Modification

Because my tachometer had been connected to the 7M-GTE ECU for some time and the needle had never moved a bit, I deduced that either:

  1. The 7M-GTE ECU tach signal is higher voltage than the 3VZ-E signal, or
  2. The 7M-GTE ECU signal is lower voltage than the 3VZ-E signal - too low to ever move the needle, even at high revs.

If case 1 were true my tach would have been fried, and nothing I did to it could harm it anymore. To investigate this, I removed the tach and connected it to a DC power supply, slowly increasing the voltage. Sure enough, the needle moved. So it appeared that the tach needed higher voltage than it was getting in order to function at all.

The circuit boards on these tach's have many resistors, and several capacitors. I have an image file of a 7M-GTE board, so I compared it to mine and discovered that they are quite a bit different. The resistors and capacitors have substantially different values, and since I am not an electrical engineer, I didn't want to try to draw up circuit diagrams for each and compare the input to output for the two. I also noticed that there was a potentiometer on the back of my tach, for calibration I presume (pictured at left). I marked the original position of the potentiometer and then connected the tach to the instrument cluster with some jumper wires so that I could get to the back of it easily. I started the engine and slowly turned the pot lock to lock, and the needle never budged. Next I tried revving the motor with the pot at each end of adjustment, and still got nothing. Since the pot adjustment was not sufficient in reducing the input resistance to the tach, I looked more closely at the circuit board. I noticed that the input signal to the tach went directly to a 33,000 Ohm resistor. Reducing that resistance would increase the voltage that the rest of the circuit on the tach saw, and possibly make the tach function properly throughout the rpm range.

So I began reducing this input resistance gradually. Two resistors in parallel have a lower net resistance than either of the two individual resistors do, so with the engine idling I began testing the tachometer by putting varied resistances in parallel with the 33k Ohm one until the needle deflected. I played it safe at first by putting high resistance in parallel with the existing resistor, and gradually lowered the resistance so as not to fry the tach. I noticed needle movement when I had a 15k Ohm resistor in parallel with the 33k Ohm one. The total resistance created by this combination is 10.3k Ohms, but that is not a magic number by any means, and the potentiometer on the tach PCB has more than enough adjustment to make up for 300 Ohms if needed. I removed the existing resistor and soldered in a single 10k Ohm one.

The next step was to validate that it was accurate. This brought up some interesting things about the 7M-GTE ignition system. I was aware that it uses a distributorless ignition system utilizing a cam position sensor for timing, but I learned that it is a wasted spark system. There are three coils that feed the six cylinders, paired up such that when the spark charge is sent one of the cylinders is on its compression stroke and the other is on its exhaust stroke. The wasted spark introduced an issue when connecting a magnetic pickup tach to one of the plug wires, because each spark plug fires twice for every combustion cycle. The result is that the rpm displayed by the diagnostic tach is exactly twice the actual rpm. One other problem is that the stock wires are not shielded very well, or the shielding has deteriorated on mine. When I attached the pickup to the #5 cylinder spark plug wire the reading on the tach was very erratic because of this. The number one cylinder wire was isolated enough that the interference was not a problem, though this required removing the coil pack cover.

Finally, I was able to calibrate the tach now that the diagnostic tach was working properly. I just had to cut the displayed rpm on the tach in half to know the actual engine rpm. With the stock tach connected to the instrument cluster via jumper wires as pictured at the top of the page, I keyed the engine in at 2,000 rpm intervals on the diagnostic tach, and adjusted the potentiometer on my tach with a screwdriver until it matched it. The highest I could go was to 5,000 rpm actual, because the diagnostic tach went to 10,000. I knew that I could make the tach accurate at a given rpm, but after altering the circuit board I was not sure that it would be accurate through the entire range. After matching the tach displays at 5,000 rpm, I ran it the engine through 1,000 rpm increments and compared the displays. The precision with which my tach matched the diagnostic one was very impressive. A friend of mine with quite a bit of engine experience commented that it was much more accurate than a typical stock tachometer. So it was settled, the modification to the tach was a success.