Hi everyone, I did a search and found tons of posts about this and saved the tach filter schematic. I just want to make sure I have built this right before I hook it up and chace frying something. I installed a new tach circuit board on my 78 and my tach still has the same problem of reading twice as high as it should, (tach reads 1600 @ 800rpm, 3000 @ 1500rpm and so on). The way I have built this is as follows: distributor into the 5.6k resistor, then one leg of the .1uf capacitor and the 10k resistor all spliced into each other. The other leg of the .1uF going to ground. The the 10k resistor goes to the 100pF cap. and a wire to the tach all at this splice, then the other leg of the 100pF cap to ground? Is this even remotely correct?

Thanks,
---Chris

 

I can't remember the values, but .1 mfd, and 100pf seem correct...as I recall...

the wiring you describe seems correct to my thought pattern....

'give it a go' and see what happens, it's NOT about to harm anything...

GENE

 

Yup -all sounds good. Good luck

I think your problem is beyond a tach filter.
The filter should remove fast transient edges and high voltage spikes.

Your problem of twice the frequency should not be effected by the filter.
Think of it like this ... if the filter removed the 1600rpm signals @ 800,
then the same filter would not allow the 1600rpm signals to the tach at 1600.
I hope I'm wrong, but I don't think the filter is your cure.

Do you run any aftermarket parts - like an MSD ??

 

haha....I forget, good memory there Dave, yes, using the stock tach output with an MSD unit will mess up the tach but good, you MUST use the MSD box output, and nothing else...

GENE

 

Thanks for the responses, I went out and hooked up the filter I made, and your right, it didnt fix a thing, but I know it is supposed to be there, and since I just replaced the tach board, I didnt want to fry it without having a filter. What I did to fix the problem was adjust the little pot. on the side of the new tach board. Just was a weird coincidence I guess that the orig. tach board was reading twice as high, and then the new one I installed was reading twice as high.

Thanks,
--Chris

 

Good to see you figured it out fast.
Yes - you are correct that the filter can only help
and perhaps save your new board from early death.

 

All aftermarket tach boards have one trim pot - and it's to adjust calibration at redline.

if you can't do a full redline calibration - then the higher the rpm you can run (and then adjusting the pot) the better calibrated you will be over the entire RPM range.

But before you do that, you need to do a ZERO calibration - and this does not involve the pot. You power up the tach with the engine not running. The electronic tachs needle should settle on zero. If it doesn't... the way to calibrate ZERO is to pull off the pointer and re-install it to where it's at the zero.

Only after this should you try to calibrate for rpm.

 

Hi there.
New here.
I just repaired the Tach in my 76 Corvette which was stuck at 3500 RPM.
Here is my take on the forums comments on Tachs, filters and double pulses.


The first thing I did before ripping out my tach was scope the Tach signal coming out of the distributor. I did not find a nice clean square wave but found a signal that normally sits at about 10V. It pulses down to 0 volts, goes back to 10 for a bit and then jumps up to 25V.
IF this signal is fed into a different tach circuit with an A.C. coupled input, it could easily be measured as two distinct pulse which would double the RPM reading.

The filter circuit tossed around wouldn't change this.

The filter circuit does little to protect the tach signal input as there was already resistors and caps on the input of my tach. The value of the resistors were in the Kohm range. High enough to limit current down to mA.

The filter circuit will shave off spikes, probably to eliminate false tripping, not to protect from frying.

The input protection on my tach consisted of a single 1W zener diode which is woefully inadeqzute for protection in an automotive circuit. Zeners blow short and then blow traces (as mine did) I replaced the zener with an LM2940CS-12 and a 100uF cap. Works fine.

When tested, the working tach responded at half the RPM I expected when I fed in a square wave, hence paragraph 1 above.

Kilroy1024

 

Now I just installed a MSD 6al in my vette and the Tach worked fine for a few days then it started to get bouncy. Now its totally useless because it bounces every where. I ran it out of the tach out put from the box. What needs to be done to fix this problem? Thanks guys.

 

Bouncing around how?
Between 0 and a good value?
Any access to a scope and or signal generator?
If you put in a good square wave, you shoud get a reading.
What year Vette?