Ok so hopefully this post will help anyone looking at sussing out the tacho (in my case a 1993 Hiace RZH113) issue using a 5V square wave RPM signal from an ECU. Ive been driving it around for 9 months now with no working tacho and thought I’d do some homework and get it sorted.
Background to all this shizz: The original 2RZ that came in the hice would have had a feed from the coil to the tachometer supplying it with a 12v voltage spike causing the tacho to correctly display the RPM.
Thats all changed though, Ive got another 4 cylinders (4+4=V8) and a Spitronics aftermarket ECU outputting a different type of signal better suited to newer tachometers / aftermarket units.
Problem solved : A Tach adapter is needed here! You have options too… You can Google the Megasquirt tacho adapter schematic and follow that through to make your own or you can look around and see what is for sale. I spent ages looking at different brands that do tach adapters but be aware that different tach adapers convert slightly DIFFERENT signals and this may not be clear when browsing the product on the web…. I came across 2 options I would and have willingly spent $ on… MSD tach adapter 8920 and the Procomp Electronics adapter, both retails for around US$70. I went with the Procomp, piss easy wo wire up taking about 15min to do a clean job… Ground , IGN , 5v feed and Tach out… easy!
Tach works BUT next problem was the tacho reading 2X what the engine was doing… Why? because the ECU is sending out 1 pulse per spark, now the engine has 2 coil packs instead of 1 and so the tach reads double the pulses.. Well thats what I believe anyway..
Problem solved: Now this worked for me , your Hiace / Hilux or whatever may be totally different but there is info out there regarding calibration using a potentiometer if your factory tach doesn’t already have one on board, if thats a negative for you go check this out…..For me though, I went to Jaycar and bought a standard 5K POT just incase but to my delight after removing the cluster and undoing the 3 small screws from behind and removing the tach unit i saw a small white POT used for calibration. See below…
Cool now that was done all I had to do was plug it back in, start the engine with the PC and tuning software going, look at the PC to see my RPM and twist the POT until I got the correct RPM on the dash, checked it through the rev range and bang , done!
Hope this has helped