Tachometer Gauge Problem

[johnjjo]

Hello guys, have 1968 R/T
Tachometer gauge doesn't work.... i have 3 of them in the shop all of them doing same thing....
So feed to tachometer gauge clock is there (V12), wire that coming from negative coil was on positive..... so i swapped it to negative, how its supposed to be.... gauge wasn't working before! now its still NOT working and car wouldn't ran and only run if you disconnect wire that goes to coil. but as soon as u plug it back car just dies...
Please HELP
THANK YOU!!!!!

[BLK 68 R/T]

So if the wiring is all stock, running points ignition, I believe it should be as follows. If you are running electronic ignition the tach should still function it just wont be accurate.

Wire from dist. to negative on coil
Blue wire from engine harness to positive on coil
Tach wire on negative on coil
Blue power wire from fuse box to back of tach
and you already mentioned the clock wire being plugged in.

[johnjjo]

So if the wiring is all stock, running points ignition, I believe it should be as follows. If you are running electronic ignition the tach should still function it just wont be accurate.

Wire from dist. to negative on coil
Blue wire from engine harness to positive on coil
Tach wire on negative on coil
Blue power wire from fuse box to back of tach
and you already mentioned the clock wire being plugged in.

Original wires was not working it had a brake somewhere so I rewire, and currently 2 wires goes to tach. one wire goes straight from negative on coil to tach, second wire goes from fuse box to that blue wire that was there before (I just cut it of and wired new one to it with clamp). if one of this wires not going straight to tach and connect somewhere else on there way to tach, that could be my problem, if yes where???
also i ground tach when i connect all that just so u know. if i dont ground it. it dont make no different
Thank you!!!

[BLK 68 R/T]

Can you post a picture of the tach you are trying to hook up? Wires to coil and from the fuse box do not intersect anywhere else they are both direct from point A to point B.

[johnjjo]

Can you post a picture of the tach you are trying to hook up? Wires to coil and from the fuse box do not intersect anywhere else they are both direct from point A to point B.

here is pictures
thank you. for quick response

ALSO thought something wrong with coil, i swapped with another used one, same thing...
on position ON coil
+ read at V7.4
- read at 1.4
on running
+ read at v8.1
- read at v2.4

[BLK 68 R/T]

I'm fairly certain you have the wires crossed on the back of the tach. 12v from fuse box should go to the long stud. Neg coil wire attaches to the tab. Also the metal frame of the tach has to be grounded to the gauge cluster housing in order for it to work.

[johnjjo]

I'm fairly certain you have the wires crossed on the back of the tach. 12v from fuse box should go to the long stud. Neg coil wire attaches to the tab. Also the metal frame of the tach has to be grounded to the gauge cluster housing in order for it to work.

Thank you soooo MUCH!!! it was crossed.... i dont know how bacouse it was connected like that before and on other none working charger was connected same way. Now TACH working !!! running at 1200, but when i turn off engine it wouldnt go back to zero, only if i force it with finger. also how do i know if it showes me correct rpm.
thank you!!!

[justcruisin]

Best way to know if it is accurate is to connect a known rev counter up at the same time and compare it. If you want it to be accurate you need to first sort out the needle rise and fall without it hanging up. The board on the inside of the tach needs to be replaced if you want it to be accurate. I used a digital board from Real Time Engineering, I compared it with another tach and my rev limiter and it is bang on the mark through the whole range, well worth the spend if you want to rely on it and don't want an aftermarket tach on your dash.

[BLK 68 R/T]

Glad you got it working 

I don't know the consequences of hooking up the wires backwards but it's possible that the tach was damaged and that is why the needle will not return to 0 when you shut it off?? I may be way off on that just don't know for sure. It's also possible that it just needs a good cleaning and lubrication.

If you are running electronic ignition you can buy a tach adapter that will correct the signal or you can purchase the electronic board as suggested in the previous post. Both of these options will make the tach read correctly.  If you are using points no adapter is needed.

As for verifying the RPM, like was suggested in the last post, you need a known accurate RPM measuring device to use and compare it to your current tach and see if there is a difference.

Good luck!