Tach wiring issue

[Tristan Smith]

Ok so I have been trying to wire up an aftermarket Autometer Tach to the 300zx (1991 z32). So far nothing works. The car is a coil-less system with a coil pack on each spark plug. In theory I should be able to take the signal coming out the ecu and use that to trigger the tach. But that doesn't work. I have put a multimeter on the ecu tach signal output, and when in AC mode, it shows an increase in voltage as the engine revs, indicating that there is some sort of signal occurring. My question is this then, one, what type of voltage is need to run the tach? Two, are Nissans (300zxs) of that era low voltage systems, and would a tach signal amplifier help? I know the tach is good because I have used two different ones (one brand new) and gotten the same result. I have checked and rechecked, all the other wires and they are connected correctly. Third, I even tried using an Autometer Tach Adapter. but fgollowing their instructions for what looks like my system, netted me nothing also.

If there are any electrical engineers out there, please help. I will be drinking heavily so as to keep myself from throwing something heavy and jagged at my newly painted car. Thanks.

[Streetwise guy]

I doubt the ECU is actually the final stage of the signal for the coils. The amperage required to run a coil is way higher than an ECU is designed to handle. It will drive an amplifier, possibly mounted in the coils. I shall research and get back to you.

[bonespec]

From Autometer's website

Tachometer Signal Wire Installation
for 3.0L engine(s)
Pin #7, yellow wire with a red stripe at the ECM, or same color wire a the tachometer test connector on the EFI harness, in the left side of the engine compartment. 3-pulse

pass a cold on please.....

[Tristan Smith]

Thanks Bonespec,

But I have been trying to hook up the tach to that wire. Either I am not getting enough of a signal (or the wrong type of signal) off of that, or as I read somewhere, some of the Nissans use a low voltage signal that may be too low to trigger the tach. It's strange though I haven't been able to find anyone who's actually put in an aftermarket tach in a zx that has written, one way or another, anything about the process on the web.

Information super highway my ass! ha

[Greg Amy]

Tristan, on the possibility that your ignition system is similar to the one in my ('91-93) NX2000...I had the same issue when I was building that car. None of the ECU wires, nor the tach signal behind the dash, would work with my Stack dash or with the DL-1. To resolve this I had to tap directly into the power transistor wires, and find the one that had the tach output. had to do the exact same thing for my street Nx, which needed a tach output signal for the aftermarket MSD solenoid I used to switch over the VVL camshafts...

Note the NX does not use coil packs, it has a single coil, but that may lead you down the right road.

[Streetwise guy]

http://www.300zx-twinturbo.com/cgi-bin/manual.cgi

Not so good a diagram, but what you want to do is trigger from any of the coils, on the wire running to the power transistor unit. There will be two wires at each coil, one of which is powered all the time from the coil relay, the other is the pulsed ground from the power transistor unit. Key on engine off, unplugged coil, one wire will be powered, the other dead. Trigger from the dead one.

The signal from the ecu is probably a digital 5 volt low amp signal. Hooking the tach directly to that would probably just result in a misfire.

Your tach will accept a didtributorless signal, as in one pulse per two engine revolutions, I hope.

[Tristan Smith]

Thanks, Bonespec, Streetwise and Greg.

This is what I suspected, so now I have to do my homework and see what will work. I appreciate your help.

[DavidM]

Don't know if you got it fixed Tristan. A quick search of "tach install" over on twinturbo.net shows people using the wire sited in the installation instructions. There's even a shift light install in the tech section using that wire. Maybe post over there and see if anybody has any ideas.

There is a power transitor unit (PTU) wire like Greg said. There's a little loop of wire that sticks out near the electronics on top of the motor. This is where a lot of people clip inductive pick-ups for timing lights. That's the PTU wire. You could try splicing into it and see what happens.

David

[Tristan Smith]

Hey David,

I put an oscilloscope on the ecu tach output wire the other night. No signal whatsoever. So I think that there is an issue with the ECU. I am going to try to track that down in the next week or so, and see if I can get my hands on another ecu to see if that is the case. It would be nice to get this issue cleared up.

[DavidM]

Weird. I know on the 240 the signal that feeds the tach is the same signal that feeds the coil. The stock tach on my 240 went bye-bye before Barber so I was just looking at the wiring diagrams to put in another one.

I haven't looked at the diagrams for the 300 to see if it's the same deal. I'd think the car wouldn't run if that was the case, though, if you're not seeing anything on that wire.

Good luck.

David

[Tristan Smith]

The ZX has a coil on plug set up with a power transistor unit. It get's trigered by the crank angle sensor. The ecu (I believe) then sends that signal on a seperate wire to the tach (the yellow and red one just like the 240sx). The car runs fine, but so signal at that wire.