73 Tach TroubleShooting

[KenFirch]

The tach in my 73 hasn't worked since I bought it back in December, and I'd like to fix it.  Distributor still has points (for now) and it looks like the wiring to the coil is correct.  I did try to take a look under the dash when I had the seats out and it looked like it was connected on that end, but hard to get a good look at.  So, what's the best way to determine if it's a wiring issue or the tach itself?   If the tach, what years are compatible replacements.   Are 73 tachs known for failing?   Really not looking forward to tearing apart the dash to replace the tach, but that may be what it takes.  Any suggestions would be appreciated.

 

Thanks,

Ken

 

[Zed Head]

Body Electrical has a wiring diagram that will show you how the tach works.  The coil can't get power unless somebody wired around the tach,  The chapter also describes how to remove the tach,  No need to tear apart anything.  The tachs do go bad.

 

http://www.nicoclub.com/FSM/240z/

[SteveJ]

If you have a bad tach, a friend of mine (and member here ) got the tach for his 260Z rebuilt here: http://www.bobsspeedometer.com/1/120/index.asp

He was pleased with the work.

[Captain Obvious]

Here's a snipped from the FSM on which I've made some notes (in red). Might help to determine if it's a wiring issue or a tach issue?

 

 

[KenFirch]

If I look at the Body Electrical diagram (thanks Zed Head) it shows the coil wiring as Black to the negative and Black/White to the positive.  My picture shows the opposite.   Would it even run with the coil leads switched?    

 

Edit:  It does run fine.

Edited June 3, 2015 by sfm6s524

[SteveJ]

If you have the polarity swapped on the coil, it will cut down on its efficiency, but it would still work.

[KenFirch]

I swapped the coil wires and it ran real crappy, and the tach still didn't work. Looks like it's time to find a replacement tach or send mine off to get fixed.