assassin10000
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I'm trying to figure out how to modify the tachometer circuit in my car to accept the signal from an aftermarket standalone ECU. Unfortunately I DON'T have enough electronics knowledge/background to figure this out first time through on my own 
. Help would be much appreciated.
If you want to see my original thread about this on the car forum where I originally was seeking an answer, its here:
Using AE86 Tachometer & Aftermarket ECU 12v output? - Toymods Car Club
From what I understand, the original circuit is triggered off the positive voltage spike that occurs when the ignition coil fires. The 'input' of the tachometer circuit is connected to the negative side of the coil. I believe the spike could be up to 20-40v. The circuit as I understand it, is a frequency to voltage type, moving the tachometer motor to display RPM on a scale depending on the voltage derived from the input frequency (galvanometer?).
The aftermarket ECU has a 12v output, and is NOT enough to trigger the tachometer circuit. I tried it.
I pulled the circuit board itself and another guy was kind enough to draw up a diagram for me. Here's a few pics, identifying the components, the physical circuit board and the circuit diagram.
Now, I've found out that this circuit board was used on several different engine/ecu's with BOTH 4 & 6 cylinder engines, which would have different 'frequency' counts due to the number of cylinders fired per rotation. And with both 'high' (20-40v) and low (12v) signals, depending on if it had a distributor with a single coil or coil packs.
I also found this, which outlines what other guys have done to modify the tach input signal:
**broken link removed**
Anyways, here's what I'm thinking needs to be done.
A) based on the other tachometers I've run across change:
C2 to '472' (4700pf)
C4 to 33uF 25V
R4 to 15K
R8 to 39K
B) follow the PDF and change
C2 to '472' (4700pf)
R4 to 24K
R8 to 68K
Now, to quote myself here's what I 'think' the breakdown of differences are after looking at several different tachometer circuits. And this is what I based the changes for option 'A' on. Feel free to skip the 'quoted' part below this if you are able to understand the circuit diagram and can help me with what needs to be changed.
. Help would be much appreciated.If you want to see my original thread about this on the car forum where I originally was seeking an answer, its here:
Using AE86 Tachometer & Aftermarket ECU 12v output? - Toymods Car Club
From what I understand, the original circuit is triggered off the positive voltage spike that occurs when the ignition coil fires. The 'input' of the tachometer circuit is connected to the negative side of the coil. I believe the spike could be up to 20-40v. The circuit as I understand it, is a frequency to voltage type, moving the tachometer motor to display RPM on a scale depending on the voltage derived from the input frequency (galvanometer?).
The aftermarket ECU has a 12v output, and is NOT enough to trigger the tachometer circuit. I tried it.
I pulled the circuit board itself and another guy was kind enough to draw up a diagram for me. Here's a few pics, identifying the components, the physical circuit board and the circuit diagram.
Now, I've found out that this circuit board was used on several different engine/ecu's with BOTH 4 & 6 cylinder engines, which would have different 'frequency' counts due to the number of cylinders fired per rotation. And with both 'high' (20-40v) and low (12v) signals, depending on if it had a distributor with a single coil or coil packs.
I also found this, which outlines what other guys have done to modify the tach input signal:
**broken link removed**
Anyways, here's what I'm thinking needs to be done.
A) based on the other tachometers I've run across change:
C2 to '472' (4700pf)
C4 to 33uF 25V
R4 to 15K
R8 to 39K
B) follow the PDF and change
C2 to '472' (4700pf)
R4 to 24K
R8 to 68K
Now, to quote myself here's what I 'think' the breakdown of differences are after looking at several different tachometer circuits. And this is what I based the changes for option 'A' on. Feel free to skip the 'quoted' part below this if you are able to understand the circuit diagram and can help me with what needs to be changed.
