usdmholden

Alot of this is FYI kind of information, ignore whatever you want.

So I built the VATS bypass circuit often referenced on LS1tech and found linked here:
http://temp.corvetteforum.net/c3/70lt1/vats.shtml

Used the following values
2200 ohm resistor
3300 ohm resistor
10000 ohm resistor
1 microfarad capacitor

All components on the corvetteforum schematic NOT listed appear to be circuit protection and are mostly irrelevant to the circuits frequency generation.

The circuit in the corvetteforum link above matches nearly identically to the circuit example for the astable operation time delay circuit in fig 3 as provided by National Semiconductor in the datasheet for the LM555 timer component, which I used in my build.
https://www.national.com/ds/LM/LM555.pdf

Using the National Semi circuit you can see that
Ra = 3300 ohm
Rb = 12200 ohm
C = 1e-6 farad

Plugging and chugging the numbers in National Semis formulas you get a theoretical output frequency of 52.1Hz and a t1/t2 ratio of 1.27 using the corvetteforum schematic exactly as is.

Interestingly enough, if you swap the 2200 and 3300 ohm resistors so the new Ra=2200 and the new Rb=13300, then you get a theoretical output frequency of 50.1Hz and a t1/t2 ratio of 1.17, both slightly better than schematic as shown. Ideally t1/t2=1, but probably doesn't make much difference in the real world, and you may never get to theoretical perfection anyway.

I measured my build on the oscilloscope at 45.2Hz using components with 20% tolerance. Tighter tolerances would probably improve the frequency generation.

Now that that's all said and done, how much slop is there in the signal which the ECM expects to receive? I understand the nominal frequency which the ECM expects is 50Hz, but if it's actually given 40Hz or 60Hz, will it still work? Has anyone tried it?

usdmholden

Forgot in the first post.

Drove the circuit with 12.0V from a programmable power supply.

Current measurement was .02A.

Measured peak to peak output waveform voltage was about 11V.

Power consumption was .02*12 = 0.24W.

Given the 0.24W total consumption, on the frequency control side of the circuit, you can use 1/4W resistors and have plenty of safety margin.

Chrome_Rust

iTrader: (4)

hmm..... Interesting.

I am sure no on here has tried this. I want too, but not near that point in my build. Thanks for this information