tach sensor test?

Jul 8, 2011
49
Catalina 30Mk III Oyster Bay
I have a Catalina 30 Mk II with a Universal M 25XP engine. Recently I noticed my tach gauge will slow down even though I can hear the engine at a constant rpm - tach display comes back up to speed if I increase engine speed. Any thoughts on testing tach sensor, tach cable or tach instrument to isolate problem?
 
May 20, 2016
3,014
Catalina 36 MK1 94 Everett, WA
you can get6 a cheap optical tach at harbor freight for about $20-30. On the back of the tach is an adjustment screw - you can fine tune the tach so it agrees with the optical. The way your tach works is it measures the AC frequency coming off the alternator (before it gets rectified into DC) this frequency then gets multiplied by a magic factor that includes number of alternator fields, pully to flywheel rations - to come up with RPM's.

Les
 
Dec 19, 2006
5,809
Hunter 36 Punta Gorda
I always hear from people who have trouble with the tack acting funny at different times
and happen to me and checked everything and even tried a new tach and guess what it still
acting up but not always so I tried changing the alternator and worked fine for a while.
I gave up and most times it works OK but does act up every now and than.
The older GM's have a sensor on the back of the bell housing and you can check that with a ohm meter but my YM has the wire coming off the alternator and had a rebuilder check and he said it was OK.
Good Luck
Nick
 
Aug 22, 2016
1
Catalina 30 MKII Dallas, TX
When my tach starts acting funny, first thing I do is replace the alternator belt. Typical symptoms are strange readings at low RPM with better readings at higher RPMs.
 
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