Tachometer erratic

[Monty]

During a 3.5 hour motoring run, I witnessed the tach drop to zero for a second and then return to "normal." This happened in sequence several times and then the tach stayed "normal". We were motoring at 2300 rpm to outrun some weather. This was repeated several times over then next few minutes and then returned to normal levels for the rest of the trip. Prior posts have alluded to faulty connections and rotating the switch in back of the tach to clean contacts; I will address these later this week. The batteries had been on a charger on our mooring so were fully charged as we left. The solar cells wiring had not changed; and it was a very cloudy day. Engine room blower ran the entire time as did all the instruments and cabin fans so there was a constant load on the batteries.

Am looking for thoughts and ideas explaining this erratic behavior other than the above. This is 2007 C350 Mk 1. About 1300 hrs on the engine. Thnk you! Steve

[dcmason@masoncci.com]

This just happened to our boat as well. I removed the panel, changed the switch setting and rechecked the connections. It is now working fine.

[TBOT422]

Monty,
This happens on our boat as well with fully charged batteries. For us, the issue is the solar panel charger. It's top voltage limit is higher than the alternator's top voltage limit. I believe the solar charger charges the batteries up to 14.4 V. The alternator has a limit of 14.2 V. When the battery voltage exceeds 14.2V, the alternator shuts down and therefore the tachometer will go to 0. When this happens on our boat, I simply turn off the solar panels knowing that the batteries are fully charged.
Good Luck

[Monty]

many thanks for all the advice! Steve