I am helping a friend trouble shoot the tach in his '78. The tach works, but there is a difference between actual versus car tach, that is proportional to RPM:

ACTUAL RPM CAR tach readout
900 1300
1500 2200
2000 2800
2500 3500

Found a non-original capacitor that was mounted on the intake manifold and had been connected in series to the tach input lead (white wire) at the distributor instead of the power lead (pink wire) as the GM electrical diagram shows. The diagram also shows a capacitor in parallel with the circuit in the assembly that grounds to the case. Concerned that the installed capacitor may have damaged the electronic driver board by not being the correct part and installed in the wrong circuit. The capacitor has 17K ohm reading at the insulated ends (these two terminals have the leads that connect to the circuit soldered to them) and no continuity between the case and insulated terminals.

Question is: Does the tach need to be calibrated to correct the situation since it stills works, or does the driver board need to be replaced?

 

We just had a discussion of this for, I believe a couple of weeks ago...there the owner replaced the capacitor but typically its estimated that 90% of the time board needs replacing...it is 35 years old!

Double check that schematic...pink is power and connects directly to the distributor. Capacitor routes through white.

Original boards left the factory calibrated with no mechanism to recalibrate...they were etched onto a chip. Today they come pre-calibrated unless you pick up the Wilcox "custom" board that permits calibration.

 

Thank you very much for the help!

 

The 'tach filter', mounted on the intake manifold, is not just a capacitor. It is a filter network of two capacitors and two resistors...of specific values...that cleans-up the tach signal before being processed by the circuit board. Below is a website with that info:

http://temp.corvetteforum.net/c4/1mo...achfilterj.jpg

 

That was mine that was fixed with the filter a few weeks ago. I had the same issues. Make sure to solder the connections!

 

Maybe a quick mention. Capacitor values are in microfarads(uf), and its the resistors measured in ohms. If you're getting any kind of ohm reading(except a quick pulse) on a cap, then there's a problem with it. No expert on the 78' tach board, but the mentioned adjustible Wilcox job, sounds like the ticket.

 

Thanks for the info on the filter circuit. We are not sure if the one on the car is OK, or if it's a problem. A couple of questions:

1) If we replace the tach driver board, and the filter (capacitor/resistor) is bad, could it potentially damage the new board?

2) Can we test the new driver board hooked up to the tach, without the filter in the circuit? Should we expect to get an accurate RPM reading, and is there any potential for damage to the driver board?

3) Does anyone still make the OE filter, or do we need to make one if it needs to be replaced?

 

By pass the filter and see what happens. I haven't had a tach filter for 11 years, no problems yet. I recently installed an Autometer column tach when I went to the track and found the reading on my original was off by 200 rpm @ 70mph.

Wilcox has some help also: http://forums.corvetteforum.com/c3-t...-accurate.html