What's the correct tach signal? 0-12v or higher?

[Spiff]

Just wondering if the stock 82 square tach will work with a 0-12V signal from my ecu or if it needs a high voltage spike?

[datzenmike]

The 720 tach signal comes from the negative side of the coil. The positive side is 12 volts. The distributor is a switch that applies a ground to the negative side and then removes it to fire the coil. The negative coil terminal will 'see' a square wave of on and off 12 volts. Now between the coil negative terminal and the tach is an in line, 2.2K ohm resister.

 

 

 

So if you have a single coil just connect the 720 tach (and it's 2.2K ohm resister in line) and it should work.

 

It does not need a high voltage spike!!!

[Spiff]

I have four coils and an aftermarket ecu. I never had a tach so I don't think I have that resistor. But I have a tach on the way so I want to make it work with my aftermarket ecu, should I just try connecting it with a 2.2kohm in line?

When we connected an oscilloscope to the coil negative (before I switched to cop setup) the signal coming of the coil negative was about 40-50volts, I had a pertronix flamethrower coil and ignitor though. Not sure if it that has anything to say

[datzenmike]

The tach resistor is taped up, in line, on the harness and looks like a small lump or knuckle. Usually it's on the main harness somewhere behind the left side headlights and the coils. Maybe the earlier 720s up to '83, it was on the battery side behind the headlights.

 

I cut one open once and there is a generic 1 watt 2.2K ohm resistor in there... that all it is.

 

The ECU was 40-50 volt? I don't know how or if this will affect the tach. It relies on the frequency of the pulses not the amplitude.

[DanielC]

It is normal for the voltage on the primary side of the coil to spike way above 12 volts. 

[Spiff]

I'll have to check my wiring.

No the ecu can deliver a 0-12V tach signal.

When I was running fuel only I had problems with the coil negative output frying the crank input on the ecu and when we measured the signal it was peaking at about 50volts. So we had to build a circuit to make it safe for the ecu. Now I'm running crank sensor and coil on plug and I want the "tach out" of the ecu to drive the tachometer so that's why I wondered if it was run by a 0-12V pulse or a much higher pulse like many other older cars

[datzenmike]

Well the primary and secondary coil windings are connected together at the positive coil terminal. When the magnetic field collapses and the high voltage spark is produced it's very possible some of this 'noise' gets into the primary wiring. Oscilloscopes are very sensitive and can read incredible short spikes that really have no effect.

[Charlie69]

I found a tac resitor on one of my 1980 720s located behind the master cylinder on the firewall.

[Spiff]

I did a search today but couldn't find anything, inside or out. So does anyone know the ohm rating of the resistor or could anyone measure it for me?

Or do I even need it since I'm not gonna trigger of the coil negative but the ecu 0-12V signal?

[Spiff]

Nevermind. I found out it's 2.2k ohm

[datzenmike]

I think the stock round or square 720 tach needs the resistor. Try RadioShack can't be more than $0.50 for a pack or 10

[Spiff]

Tried to hook it up today but nothing happened. I hooked it up to the ecu with a 2.2k ohm resistor inline but nothing. So then I tried to use a relay coil to emulate the higher voltage spikes you get from ignition coils but still nothing. 

And finally I tried without the resistor but even then the needle didn't even budge.... 

Is it safe to say my tacho is dead? The guy I bought it from says it was in working order before he shipped it but it's obviously had a rough ride because the needle was broken in three pieces when it arrived...

[datzenmike]

Try connecting to another vehicle's coil negative terminal. You'll need ground and a 12v supply.

[wayno]

Try connecting to another vehicle's coil negative terminal. You'll need ground and a 12v supply.

 

I have done this myself, I tested a 720 square tach on my 521 and it appeared to work properly.