tach needle goes from 3500 and reads to 5500 when rpms arent even close to tach reading. never goes to zero, even when motor is not running and key is on power position. happened all at once. any idea what could have caused this? also, if you think this is a circuit board problem, could you tell me how much trouble it is to get the tach out of dash? thanks

 

The electric tachs, as in the 77, never go to zero, even with the engine off. Your problem could be the tach filter. Its the little thing that looks like an old condenser, mounted on your intake manifold near the distributor. Wiring connects it to the Tach terminal on the dist cap. Its cheap and easy to replace.

 

Dont it go to zero when you turn the key on? Cold enough for ya here in Jersey today? My tach wasnt working correct also and I had to replace the board. Getting the tach out of the dash is ez, getting the dash out of the car wasnt very fun. Not high tech here, just time consuming and its a little tight back there to say the least. Mine turned out to be the board, and being Willcox Corvette put a how to video on line showing how to replace it and set the pointer, I got my board from them. I hope its just the filter as mentioned above. I dont remember if I pulled the column or just got it low enough to remove the dash. I removed my seat also for extra room while on my back. Label as many wires as you can. When putting the dash back be careful while snugging up on the column, as you want to be sure the top of the tach housing has clearance and it wont hit anything, because the plastic mounting tabs on it will snap. Heres a shot of the back of your dash if it helps. You can see the blue tach, maybe ya can follow those wires and look for problems. Good luck.

 

john65, about how long did it take you to take the dash out, do you have to take the top dash pad that the speakers are mounted to out or just the part that the speed and tack are mounted to? was it worth doing or more trouble than it was worth? thanks can you let me know step by step how to do it and what year was your car?

 

Change curciut board on tach.

 

No, the top dash stays in.
To me its worth doing, driving along and seeing that tach not working would bother me. I have a 77 also. Do you have an AIM (Assembly Instruction Manual)and wiring diagrams? They sure do come in handy. The good news is theres no rush so you can take your time with it. First off, disconnect your battery, and its probably best to remove the steering column completley from the car for this. There are 3 or 4 wire connections coming off the harness that go to the column that must first be unhooked. If I remember correctly, they can only go back on 1 way so thats ez. Under the dash trim are 2 bolts that support the column along with 2 more at the lower end of it, then of course it unbolts at the rag joint. With that out of the car the real fun begins. In no paticular order, start removing the connections, 1 by 1 mark each wire and where it pluged into back there. Remove all wires, lights, speedo cable, tach wires, grounds, and unplug the headlight switch. 2 vac hoses go to it also. Make a note of which port they plug into on the light switch. For the most part the dash is held in by screws along the top front and both sides. Remove the dash and you will see how the tach is fastened. Its easily removed. At this point I suggest you check out the Willcox website. Their video will show you how the tach must first be set to zero. Its done with 12volts and a ground and its very ez. With the dash out its now a good time to replace any bulbs or faulty wiring. Going back in is just the opposite. This is all doable, just takes a little time. Good luck.

 

thanks for all your help, where can i get the AIM and wiring diagrams you mentioned? i bought one book from somewhere and it is absolutely useless, would like to get a good one.

 

Willcox, Zip, and Corvette Central just to name a few. Check on ebay also.