Re: Tach filter/erratic/unresponsive/etc. pics/video 56k death!
After doing a bit of research as far as the tach signal goes, I did the math (based upon the crankshaft rotates at ½ the speed of the cam, or the other way around... I'm a little tired, but the math I did works out) and it comes out to 62.5Hz
(1,000/8=125, 125/2=62.5) = 1,000rpm on the tach. From that you can create a chart of what frequencies you will use to verify gauge accuracy.
- 1,000rpm = 62.5Hz
- 2,000rpm = 125Hz
- 3,000rpm = 187.5Hz
- 4,000rpm = 250Hz
- 5,000rpm = 312.5Hz
- 6,000rpm = 375Hz
I had already disconnected the original 'resistor' from the resistor pack on the gauge, and started with the 199.8k-ohms showed in the previous post. I found out that this was a little too low... posts in other threads mentioned capacitor leakage (mainly on a couple threads on thirdgen.org), but I'm not too concerned about that currently. I found a 10k resistor and put that in series with the 200k in resistors I already had, which cut the error in about half. I wound up using a total of 220k-ohms to get an accurate reading on the tach. I won't know for sure if this is 100% accurate until I can check it on an actual running engine, but it should get me in the ballpark.
Ignition waveform on an oscilloscope I stumbled on:
200k-ohms
1,000
2,000
3,000
4,000
5,000
6,000
220k-ohms
1,000
2,000
3,000
4,000
5,000
6,000
You can see at 200k-ohms that it's about 100rpm off per 1,000rpm (pretty clear by the time it reaches the frequency for 6k on the tach). My assumption so far would be that 165k-ohms could be used for a v6, and 110k-ohms for a 4-cylinder.
Here's a little clip of the tach running after I found the correct resistor.
http://www.youtube.com/watch?v=lvup_fKo6Qs
So as a rule of thumb here, the higher the resistance, the higher the tach will read. If your tach pegs immediately when you start the engine (but sits at 0rpm when it's off), then chances are the resistor replaced here is 'open', aka the resistor isn't making contact. Either the solder could have had a bad connection and broken (in which case it can be re-soldered), or the resistor is bad and needs to be replaced.