My VW Golf mk2 (1989) came without a tach (rev counter) when I got it. It never had any installed. It had a big clock in place of tach instead. I knew some models were equipped with tach so I bought one from ebay and it fits in place of clock. It is supposed to be a working unit. Don't know the actual model it came from but it looks simple enough to have a fight with. I have tried to figure out its inner workings but am running out of hair.
I have aftermarket tach installed and working nicely. I've put a plastic ring to motor endshaft and use hall effect sensor to read magnets embedded to that plastic ring. Works well. So there is a system to give pulses to tach already.
I used 8V voltage regulator to give smooth power to hall effect sensor. As a result hall effect sensor gave ~8V signal. Aftermarket tach didn't have problems reading this. This same signal wired to correct terminal (I suppose) on original tach doesn't make it move. I tried removing regulator and giving it 12V pulses instead. No luck.
I powered meter needle bi-metal directly through a 100 ohm resistor to see if it responds. Yes, it moves nicely. Mechanism seems very delicate. There is no servo motor moving the needle.
Now this original tach has a small PCB on it. I removed it and figured out its schematic. I believe input power terminals are correct (+12V and GND connections) but what kind of signal do I need to feed at it? Original 1.6l petrol 4 cylinder didn't have anything special electronics and any tach signal must have come from distributor. 2 signals per revolution? I wish I had a proper signal generator and lab power supply. Now I've removed hall effect sensor from my car and am using cordless drill to spin that plastic ring in front of sensor. I can read pulsing voltage and have tried to feed it to two different terminals.
I've also tried searching datasheet for SN29736P chip. No luck yet. I strongly believe it has controllable sinking output.
First image shows PCB from both sides. Right side of image is mirrored for easier comparison. Red wire goes to suspected +12V terminal. Black wire (GND) goes to pin 1 (hardly visible). Blue wire is my current signal wire. Second image is crude schematic with resistor values measured. I didn't remove resistors to measure them or didn't take a look at color codes. So there might be something wrong on that schematic. R9 represents needles bimetal resistance.
Any help and/or guidance is highly appreciated.