Several fellow mariners have reported occasional erratic readings on their tachometers, where the needle jumps around unpredictably. I recently encountered a similar issue on my boat and conducted some research into the cause.
Many high-output alternators with smart regulators, as well as numerous solar panel controllers, charge batteries using Pulse Width Modulation (PWM). Since the tachometer also relies on PWM signals to measure the alternator (and thus the engine) RPM, these two PWM sources can sometimes interfere with each other, resulting in sporadic tachometer readings.
A simple solution is to disconnect the solar panel when the alternator is running, preventing the controller from generating PWM signals. This can be done by installing a switch on the positive wire running from the solar panel to the controller. Alternatively, a relay can be set up on this wire. Wired to open when the engine is running and close when it’s off, this relay ensures the solar panel’s controller won’t interfere with the alternator’s operation.
Installing a switch or relay also has another advantage. The smart regulator monitors the battery’s charge level and adjusts the alternator’s output accordingly. If the solar panel is adding power to the system, the regulator may read a higher net charge than the battery alone and put the alternator into float mode prematurely. This could limit the performance of your high-output alternator, reducing its effectiveness.