Having problems with the tach on my CD31. After motoring for an hour or so, the tach begins to fluctuate between engine speed 2400 RPM and zero. May continue in this fashion for several minutes or longer then return to normal operation or remain in the fluctuating mode. Tach is very HOT to the touch. Appears that it may be temperature related. Could be time for a new unit. Any thoughts, thanks.
Capt. Dick
Fluctuating Tach.
Moderator: Jim Walsh
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Dick Turner
- Posts: 20
- Joined: Feb 7th, '05, 10:31
- Location: CD 31 "Mystical" Hull #32 New Bern, NC Berthed at Pecan Grove Marina, Oriental, NC
Re: Fluctuating Tach.
Hey Dick,
I'm upriver from you at Northwest Creek. Funny you should mention the tach, because the one on our 28 has been doing the same thing for the past year. It seems to happen just as the engine is warming up. For the first few minutes the needle jumps up and down, then stops at about 1500, when it's really only running just above idle at 700 rpm. A few minutes later, the needle will have settled at the correct reading. The tach works fine once we're under way, and the engine (Volvo MD-7B) runs great. Good luck and keep me (us) posted on what you find out!
Richard
I'm upriver from you at Northwest Creek. Funny you should mention the tach, because the one on our 28 has been doing the same thing for the past year. It seems to happen just as the engine is warming up. For the first few minutes the needle jumps up and down, then stops at about 1500, when it's really only running just above idle at 700 rpm. A few minutes later, the needle will have settled at the correct reading. The tach works fine once we're under way, and the engine (Volvo MD-7B) runs great. Good luck and keep me (us) posted on what you find out!
Richard
Re: Fluctuating Tach.
On some engines, the tach is linked to the alternator. The tach will display zero when the batteries are fully charged (maybe after an hour of motoring).
From your description, it sounds like the charging circuit of the alternator is cycling on and off. Next time this happens, try turning on some high current devices to kick the alternator back into continuous charging mode and see if the tach returns to normal.
From your description, it sounds like the charging circuit of the alternator is cycling on and off. Next time this happens, try turning on some high current devices to kick the alternator back into continuous charging mode and see if the tach returns to normal.
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Steve Laume
- Posts: 4127
- Joined: Feb 13th, '05, 20:40
- Location: Raven1984 Cape Dory 30C Hull #309Noank, CT
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Re: Fluctuating Tach.
I had the same sort of problems with the Tach on Raven. I changed it out for a new unit and all has been well ever since. It just didn't seem worth messing with a 20 some year old instrument.
If it is getting hot, I would definitely get it out of there. Heat build up is not normal and could result in bigger problems than a fluctuation in the reading. I new unit is not all that expensive. It will give you an accurate reading and will not be a short circuit or fire risk.
One thing you might do first if you really want to keep the old Tach, is to check all the connections from the Alt to the panel. This includes the two trailer type plugs in the wiring harness. These things are a definite problem area and something I eliminated. There is really no good reason to have them in the circuit and many reasons to get rid of them.
A short in the Alt circuit is not something to be taken lightly, Steve.
If it is getting hot, I would definitely get it out of there. Heat build up is not normal and could result in bigger problems than a fluctuation in the reading. I new unit is not all that expensive. It will give you an accurate reading and will not be a short circuit or fire risk.
One thing you might do first if you really want to keep the old Tach, is to check all the connections from the Alt to the panel. This includes the two trailer type plugs in the wiring harness. These things are a definite problem area and something I eliminated. There is really no good reason to have them in the circuit and many reasons to get rid of them.
A short in the Alt circuit is not something to be taken lightly, Steve.
Re: Fluctuating Tach.
Verify that the heat is not the result of sunlight. An exposed instrument can get hot quickly from radiation heat transfer.