Tach Filter

[ssvett]

On a 75, just what exactly, does the tach filter do? And how do you know if it is working? Here is my problem, Tach at idle reads 1500RPM's, the car is only running about 700 RPM's...........What's up with this...............Thanks.......Steve

 

[resto75]

tach

I hope someone knows because mind reads 1300 at idle , but a external tach says 800 rpm. I just ignore it.

 

[yorkbeachbum]

I do know that 75 was first year of electronic tach. With the engine off, the tach should read 1000 RPMs. Maybe it needs some sort of calibration?


Yorkbeachbum
:confused

 

[C5Phil]

ssvett,

A Tach Filter is intended to remove noise from the signal line that might make your tach read high. Most of this noise is small impulses, high frequency compared to the actual tach signal. So a Tach Filter is really a low pass filter electronically speaking. You can construct one with a 1k-2k resistor and a .01uf-.033uf cap. You can purchase these components at Radio Shack. I have attached a small schematic, if it does not make sense let me know.

Regards,
C5Phil

 

[resto75]

I think I heard that theres some type of electronic card inside the tach and that it can be calibraited, or something like that. I guess if you have it rebuilt they do all that.

 

[Roy M]

I had these same problems when I purchased my 79. Replaced the tach's printed circuit board and everything fine. Got it from Chicago Corvette for approximately $65. This was a couple of years ago so the price may be higher now.

 

[ssvett]

Many thanks guys, I apriciate the input. And C5Phil, thanks for the electronic refresher course, you brought back memories of power generation school in the Army. You used a couple of words that I never thought I would here again However, it now makes sense what it does, and how it does it, what don't make sense is why they ever changed to it Thanks again, this will give me a couple of things to throw at it in the AM...........Steve

 

[Dave's75Ray]

Steve, Here is the response I received from Eckler's concerning calibration:

"ONCE BOARD IS INSTALLED, TACH NEEDLE NEEDS TO BE RE-CALIBRATED. TURN IGNITION ON WITHOUT STARTING CAR AND RESET NEEDLE TO ZERO BY REMOVING AND RE-INSTALLING IT. POSSIBLE PROBLEMS AND SOLUTIONS:

TACK DOES NOT WORK WITH ENGINE RUNNING/TACK STAYS AT 1000 TO 1500 RPM WITH KEY IN OFF POSITION-THIS IS NORMAL. TURN KEY TO THE ON POSITION,ENGINE NOT RUNNING & TACK SHOULD SHOULD READ ZERO.THIS IS NORMAL. WITH THE ENGINE RUNNING IF THE TACK READS ZERO THE PROBLEM IS EITHER IN THE DISTRIBUTOR OR THE WIRE COMING..."

This is all I received from Eckler's, I hope it helps.

Dave