Tweet
My VDO tach finally died after acting odd for a year or two. They are not cheap, so I took a chance on a FleaBay Chinese knockoff for $25.
It fits the 85mm hole exactly and has nice red lighting, so far so good. What wasn't good was it would sit at 0, sometimes go to 2500, or sometimes go to 3,000. All totally unrelated to what the engine was doing
Before sending the thing all the way back to China, I decided to Google-Fu some repair ideas and found out something interesting - these low RPM tachs that read to 3 or 4 thousand RPMs primarily go on diesel engines and primarily read off the alternator AC tap. That is a much cleaner waveform than what comes off the ignition coil. I could move the connection to the alternator, but a couple of other ideas found online looked promising. VDO themselves suggests using a diode in the line and another person used a 10K resistor. I found a 5K resistor on the boat and tried that and it worked
I'll have to calibrate it to my timing light/digital tach, but for now the suggested ratio setting seems very close, idle is about 800 RPM and full throttle about 2400-2600. You would go nuts using one of these in a car, the needle is damped enough that shifting at a certain RPM would always miss.
It fits the 85mm hole exactly and has nice red lighting, so far so good. What wasn't good was it would sit at 0, sometimes go to 2500, or sometimes go to 3,000. All totally unrelated to what the engine was doing
Before sending the thing all the way back to China, I decided to Google-Fu some repair ideas and found out something interesting - these low RPM tachs that read to 3 or 4 thousand RPMs primarily go on diesel engines and primarily read off the alternator AC tap. That is a much cleaner waveform than what comes off the ignition coil. I could move the connection to the alternator, but a couple of other ideas found online looked promising. VDO themselves suggests using a diode in the line and another person used a 10K resistor. I found a 5K resistor on the boat and tried that and it worked
I'll have to calibrate it to my timing light/digital tach, but for now the suggested ratio setting seems very close, idle is about 800 RPM and full throttle about 2400-2600. You would go nuts using one of these in a car, the needle is damped enough that shifting at a certain RPM would always miss.
Attached Files
Comment