The wiring diagram shows the electronic tach in a 75 is grounded through circut 150. Anyone have any idea where the ground terminates ? Thanks

 

Thru the ground straps that are plugged into the back of the board. Or is that not the Q?

 

The connection at the back of the tach seems to be fine. I'm wondering where 150 grounds under the dash or frame.

This past week the tach just 'stuck' at 3000 rpm. On key up and key down it 'twitches'. Tach filter has been disconnected for over two years as it had failed and was causing erratic readings. As the 75 tach was the only one with 3 transformers I'm hoping the problem is an in-effective ground.( Additionally two weeks ago the power windows blew a fuse - replaced the fuse and now they're OK - but coincidentally the power windows also ground thru 150.

Any help would be appreciated

 

Not to totally rule out the ground issue, but your tach is displaying symptoms of the driver chip being fried.
If there was a lack of ground, then the needle would not "twitch" as you describe it - I assume this happens when you turn the key to run, but then that's all the life you get out of it?
The lack of tach filter has probably finally cooked the driver chip due to the voltage spiking from the ignition.
Sounds very much like the board itself, rather than the connections - exactly the same thing happened to mine - twitching or re-zeroing, but no movement! Replacing the chip worked for me, but you can get a replacement board from various places.
I can understand you'd want to try and rule out everything first before tearing the dash out to replace the board, but i think the fact that the needle twitches back to zero at key-on would be proof enough that your ground is ok.
Good luck with it!



EDIT - doh, didn't read you quite right, sorry - the needle's not re-zeroing, but just twitching at the 3000 position? Well, I'd still suspect the chip, but as far as grounds go, I would have thought it shares it's ground with the dash lights and warning bulbs, which I think all ground down to the drivers side footwell ground point. If they work, you could assume the ground is ok. Unless it's not connecting to ground somewhere closer to the tach itself, but of course to check that you're still gonna need to pull the cluster and look at the connections up on there.

 

Thanks. Either way it looks like the dash has to come out. Not my favorite job.

 

Hmm, bit of a stressful job that. Done it myself a few times, but from what I hear it's worse for the older vettes. Best of luck with it!