240z rev counter (tach)

240z

240z

Club Member
Hi,
I've posted this on other boards, but no one has been able to answer! <img src="http://www.zclub.net/iB_html/non-cgi/emoticons/confused.gif" border="0" valign="absmiddle" alt='???'>

I was wondering what the input signal needs to be for the standard 240z rev count/tach, is it simply a 12v square wave? I'm asking because currently my car hasn't got one hooked up. The haltech engine management system has a tacho out, but is a 5v square wave, so I was wondering whether I could just boost this signal to get it to work with the standard tacho.

If not I will have to go the autometer route...
 
I believe the 240Z and 260Z tachos work on an inductive loop principle, i.e. there is no direct signal input into the unit. If you look on the back, you will see a U-shaped loop of white wire in a clip secured by a small screw which is where the signal is picked up. Most aftermarket ignition systems (e.g. MSD) or engine management systems like the Haltech, don't provide the right sort of signal, so no tacho movement at all. I've always used Autometer tachs in my conversions with good results. On my 280ZX, I was able to strip one down and install it in the dashboard in the position occupied by the standard unit and this is happily triggered by the Emerald management system. On a 240Z V8 I initially cannibalised a similar sized tacho from a Rover which used a conventional direct input from the ignition system and built that into the 240Z pod. Matt Bannister's HSCC racer uses my old RatZ Autometer which sits nicely in the original dash hole and also provides a shift light facility.
 
Thanks Mike - great explanation!

Looks like I'll be getting an autometer tacho then, do you know if its the 5" size? Also can you remove the standard tacho easily without taking out the dash? Also just thought aren't the indicator lights on the tacho, would I just tape them off?

I think I'll get a black face, I do like the white face ones, but I think it might look odd unless you replace all the gauges...

thanks again.
 
The main clocks are easy to take out without disturbing the dash.
2 screws, one in the top (you can visibly see that one), and one underneath (head under dash job).
I seem to remember taking out the speedo first, so as to get my hand round the back of the rev' counter and un-plug the connecter. If you need to disconnect the speedo' cable, you should be able to do it by sticking your hand round the back from underneath the dash. A bit fiddley.
 
Rob - you're stuck in 260Z mode! On the 240Z the main instruments are attached by wing nuts onto support brackets on the rear - very hard to get to and tough to get undone if its the first time. Small, strong hands needed to reach right up under the back of the dashboard (blind&#33<img src="http://www.zclub.net/iB_html/non-cgi/emoticons/wink.gif" border="0" valign="absmiddle" alt=';)'>. They will push out the front once these are undone. Speedo cable needs to be disconnected

Sorry I can't remember what size Autometer gauges were used without access to the cars concerned......

Indicator lamps would have to be reinstated elsewhere.
 
Just heard back from haltech, they suggest to get the standard tach to work that I basically tie the negative sides of each coil together, (with a diode on each one) so I get a signal similar to a regular coil, which should then work with the standard tacho.
Does this sound reasonable? Did the original tach simply attach to the negative coil, or could I put in a later model tach which works on this principal?

Thanks again!
 
The inductive loop of the Z tacho picks up from the +ve side of the coil. Ignition supply runs via the ballast resistor to the loop and then to the coil +ve. The loop area is also supplied with a 10amp fused positive supply which feeds to earth via a variable resistor (trimmer). The operation is unlike any other tachometer that I have worked with. I don't think Haltech's suggested fix will solve the problem.
 
Thanks again mike, still looks like I'll be going down the autometer route then!
 
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