Tachometer Schematic Anywhere???

[FastWoman]

Hi all,

I'm new to the forum. I just bought a nice little '78 280Z. It's wonderful to own an S30 again. Back in my younger days I owned a '75, and it was the best car I ever owned. My ex made me sell it. :ogre: It's been 15 years, but I'm back behind the wheel of one of these beautiful machines.

Like another recent poster, I have an erratic tach. It doesn't go completely wonky, pegging both ends of the scales and such. Rather, it seems to read about half the correct RPMs, sometimes drifting up and down a bit -- sort of wandering around. Aside from that, the tach seems to be in nice condition.

The electronics geek in me recognizes that I don't have to spend hundreds for a new/used/reconditioned replacement tach. The meter movement is basically sound, so all that should be required is repairing/rebuilding the circuit board. It's probably some Schmidt trigger design -- something with a one-shot circuit. Basic stuff.

My question is this: With so many of these tachs failing, has anyone shared a schematic of the circuit with their fellow Z owners on the Internet? Helpful links? Or perhaps someone makes a replacement PC board? Any leads would be greatly appreciated.

On another matter, does anyone know where I could find a new fusible link block? One of mine has a broken mount.

Thanks!

Peace,

Sarah

[moritz55]

Welcome to this valuable site and thanks for inquiring on a stock Tach Repair approach. I've not be able to locate any schematics for a 240z Stock positive trigger tach for my Sept 1972 built car. There are several styles of Tach Hookups and I'm sure your 280z is a - negative trigger tach off the coil.

Notice my attachments on how the 240z Tach plugs into the harness and the schematic of how it's tied into the ignition circuit. Mine is a 4 wire hookup BUT with 2 of the wires being an inductive loop to complete the ignition circuit (not functional to the tach itself) - so really for the 240z Tach it only has 2 wires - Orange (positive trigger) and Black (Gnd).

Here's the thread from several weeks ago on this subject also. Referenced in this is a member who may have explored repairing a stock tach several years ago - "ESCANLON".

http://www.classiczcars.com/forums/showthread.php?t=34680

Many thanks ...

Edited July 7, 2009 by moritz55

[zcarnut]

I posted the schematic for the 240Z tachs here at the Classic Zcar forum, but it will not help you. The 280Z (and 260Z) tachs are electrically different than the 240Z one. They used a custom integrated circuit to drive the meter movement made by Hitachi that was never made available as a replacement part. Whenever I suspect that a 280Z tachometer is bad I just replace the entire tach with a good used one.

[FastWoman]

Thanks, guys!

Mark, I've emailed EScanlon to see if he found any solutions. Thanks for the referral. Yes, my tach is a negative wire hookup. According to my service manual, it has a +12 supply, gnd, and the signal off of the coil ground. I presume the noise picked up off of the coil ground triggers one-shot pulses. Yes, the 240Z tach does seem to be a completely different beast.

Steve, I think you probably gave me the info I needed. I'm surprised the circuit used a custom IC. Nevertheless, the tach movement would be a galvanometer just like any other galvanometer (right??), and it should be possible to construct a replacement circuit board to drive the thing (should it not?). Hopefully I'll hear back from EScanlon pretty soon with those answers.

How often do these things go bad? In your opinion, would there be any market for a replacement PC board? Perhaps I'm just cheap, but it seems excessive to throw a few hundred dollars at a junkyard part that might also soon fail. The most common failure mode would seem to be the board, which by some reports seems to go funky with higher system voltages. (My system charges at 15.1V. That seems awfully high, but I remember my old '75 car also having a 14.5-15V spec range.)

Thanks for your help!

Peace,

Sarah

Edited July 7, 2009 by FastWoman

[FastWoman]

Any thoughts as to whether one of these could be used (probably with modified output)???

http://rt-eng.com/mediawiki/index.php/RTE_67-74_Tach_kit

The firing frequency for a 6 cyl at 8000 RPM would be the same as that for an 8 cyl at 6000 RPM (the upper end of the scale these tach boards are designed for). The board is actually supposed to handle higher RPMs too.

[zcarnut]

The Hitachi IC was similar in function to the National LM2907:

http://www.national.com/an/AN/AN-162.pdf

If you are dead set on re-using your original tach with a re-designed PCB, then I would start with the LM2907 chip.

[FastWoman]

Thanks, Steve. The LM2907 seems like a good starting point. However, all things considered (including my time), the mopar 67-74 tach board I linked to looks very promising. I've confirmed with the designer that it is very broadly adjustable to fit a variety of applications. Its diameter is just under 3", so I'm guessing it will fit. (I haven't pulled and disassembled my tach yet.) Can anyone confirm whether a 3" dia board will fit in the can?

Thanks!

[wa5ngp]

Sounds like you might be a hardware hacker, I am one of those, and not too ashamed to take the cheap route if I learn something in the process. It it were me I'd probably replace the whole board with a circuit that uses a 555 timer used in monostable mode. Then just tweak the R/C values until you get the right readings on the meter. I suspect that you are correct that the tach meter itself is nothing more than a ma meter, maybe 0-1ma and I'm sure you can figure that out pretty quickly with some resistors and a voltage source.

However, before you canabalize it totally when you take it appart look for poor connections or any kind of corrosion.

have fun

don

Edited July 15, 2009 by wa5ngp

[FastWoman]

Thanks, Don. Yes, I'm a hacker. ;-) I went ahead and ordered the Mopar board, though. Considering the savings in time, the $75 price tag is relatively easy to swallow. If necessary, I can mount it on the exterior of the can in its own clamshell project box, but hopefully it will go inside.

[Zed Head]

FastWoman's first ever post? Maybe? Wow.

I picked up a wrecking yard tach the other day, because it was there, spares are good to have, and my current tach is not perfect. Took my 76 tach halfway apart to see what's in there, but got stuck on the glued-on PCB's on the housing and decided to re-visit after some web time. And found this old thread. Are there alternative internals for the old coil negative triggered tachs? I have a 78 tach in my 76 because the 76 would hang at high RPM after it got warm. The 78 tach is offset by 200 RPM so not ideal either.

The wrecking yard tach might be a 75, it's font is weird and primitive looking. It would be nice to rebuild the 76 tach and put a good, solid and visibly correct unit back in.