I have an m-35 universal diesel. Recently the tach occasionally drops to zero when I go above idle speed. I have metered it and the batteries still seems to be charging even with the tach showing 0 rpms. Suggestions where to check wiring?
I'd check all connections first. If nothing obvious, try a new gauge.
I had this happen but it turned out the whole alt failed (no charge either). I suspect there is a way to check the ac pulses from the back of the alternator with a volt meter to confirm if it is alt or tach related.
Sail Jerry: The tach get it signal from the alternator!! You can check the wires at the back of the Tach instrument, but first really look at the white wire connection to the alternator!!!
A few thoughts
Thanks for the ideas. With our first two days of rain in NYC in months it's a good time to work below decks. A day of wire tracing!
The connections to check are
Signal: AC post on the alt and S post on the tach
Power and negative supply wires to the tach (which could be daisy-chained to other gauges. Power will originate from the key switch.
The negative originates in the black harness wire in the engine compartment - where it's terminated depends - could be on a buss, the bell housing, the exhaust manifold stud. It runs thru the gummy bear plugs if they are still there. The tach S wire does not.
The techwiki contains a Teleflex troubleshooting guide for the engine gauges.
I had this problem and it was caused by poor contact in the range selector adjustment on the back of the tach. Fixed it by repeatedly turning the selector between settings (note where you started - I think they are identified with letters). This seemed to clean the contacts in the switch and it has been working great for two years.
Jerry : Try some Radioshack "Contact Cleaner & Lubricant" it really works great on electrical connections like the wire connection (Balmar especially) for the tach connection on the alternator!!
A thought
Thanks for the extra ideas. I was out for the past 5 days, including sailing back home in the remnants of Ian through the Long Island sound. The tech worked great 90 to 95% of the time. Still looking through all my connections.
Jerry : I the tach works "most" of the time - that confirms that your problem is most likely a loose/dirty connection !!
A thought