Hello,
After searching the whole forum saw several people experiencing the same as me but no answers to the issue.
Basically I have added one of those big Tach from Aliexpress that seems to be common for this application. I built the transistor board - nothing worked. Then I ditched the whole board altogether and connected RPM wire directly to pin 9 - worked perfect.
Currently sometimes it works, most of the time needle just jumps randomly. Tried to find if I have some electric noise coming from somewhere - all seems fine. The only issue I saw is the signal voltage is below 2.5V. It actually is around 2.39V, tried multiple Arduinos - most of them were in the 2.25 range which is not enough to control the needle.
In the SimHub documentation it is written for this case to check Arduino configuration. What I noticed is that the Tach stops working if I remove the external power supply cable on my USB powered hub - RPM voltage falls below the needed minimum. When powered externally I am having same behavior as before - sometimes it works ok, sometime it just jitters all the time. When Arduino is plugged directly on motherboard - tach does not work at all.
When sending test values from SimHub - test max and all other discrete positions - it works without issue.
Does maybe anybody have any idea? I am using Arduino Pro Micro as the Nanos and Uno I tested gave me lower voltage on Test Maximum on pin 9.
OK, at the end I managed to find the culprit. Of course it was low current/voltage on the data line. Traced it back to the USB charger I am using to power the hub. Apparently either it was doing bad or the transformers were introducing much noise. I am using it to power my wireless keyboard (charge), the KM switch I am using and the powered hub, which controls two of my Arduinos and shifter. I was experiencing some issues with the shifter (it is based on USB gamepad DIY board). Since changed to new USB3 port charger - works for the time being stable as expected.
I also made a quick test - I have an Arduino Uno prepared for another project. When powered only with USB it provides enough power to drive the Tach (no transistor circuit, REV line directly connected on pin 9).