Rev Counter Behaving Super Weird

[aircobra]

after i fired up the car TONITE , having overcome the timing problem- thanks to multiple legends on the forum, i can report the following

1. the instrument lights work - yes

2. the wiper motor works - yes

3. the temp and pil pressure gauge are good - although i noticed that the oil gauge  it doesn't get past half way - is that bad?

4. this is what happens to the rev counter

when you turn the key, the needle moves to 500 rpm, pauses and then swings all the way to 6,000 - and stays there regardless of the revs

.. looks like the tach is driven by the output current from the alternator? i'll look up the fsm in a sec

 

[aircobra]

sweet fa in any of the fsms - just a groove photo

i noticed in the wiring diagram, the wiring runs thru the coil resister  ( it says 0.5ohm on the pic) . i did by a generic aftermarket coil with the external resistor, so maybe its the wrong resistance

[Linton]

think it goes on the negative side of the coil, not on the resistor. oil pressure half way is ok. only worry if is not reading, it will drop a little as the engine warms up

Edited April 7, 2017 by Linton

[aircobra]

+ bw directly from coil, gw from ign switch ( and that has the resistor across ) and a sep - and + from other places?

[aircobra]

i've now connected all the electrics and its still wierd, for example turning on the blinkers causes the needle to fluctuate. i'm pretty sure there's only one way to connect the tach and its looks like a dead simple device

i know that the 260 tachs have a different casing and connectors, but i was wondering if the insides are the same?

[aircobra]

no leads on this? anyone?

[Six_Shooter]

What year of Z? There were different tachs for different years. 1970 to 1973 used a current triggered tach (the positive lead to the coil passed through a loop on the back of the tach), 1974+ uses a negative triggered tach (trigger connects to the coil negative). The part that muddies this is that many people also like to swap the newer tach into older Zs to go to a points triggered type tach.

Edited August 30, 2017 by Six_Shooter

[aircobra]

its a '71, but as you said, anything could have happened. i do remember seeing this white loop when i had it out

http://i42.tinypic.com/2wg5gl2.jpg

maybe i've got it hooked wrong?

is there anyway to bench test it?

[Six_Shooter]

That is a current triggered tach, so a '70 to '73 tach.

How EXACTLY do you have it connected?

From the picture you posted it looks like you're pulling power from the power for the tach through the loop and then I assume to the coil? To the positive?

A diagram of EXACTLY how you have this connected will be paramount to try to sort this out.

[aircobra]

i used the FSM and it show the tacho connected with a loopback from the ignition switch - green white and black white

the taco black white connects to the coil positive, the coil resistor then gets the other black and white and the green white that loops back from the switch

 i dropped it off at an instrument  ( howards in melbourne) place last week - see what they say

its a pretty simple analogue circuit on the taco - 2 transistors  and a couple of resistors and capacitors

my understanding is that it bascally converts the ignition pulses into a variable current and that moves the needle

[peter t]

Google Smiths Tachs and it will explain how different types work. If you check out my thread " XR700 ignition failure" in this section you can see how my tacho is hooked up to work. My car is a 75 model but I had three tachos from different years so god knows which one found its way back in the car. BTW my wiring is not as per Enzo's or the Z wiring book but it works.

Edited December 6, 2017 by peter t