tachometer needle bouncing

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gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
At idle, the tach needle bounces quite a bit. The victron 601 will show a small negative i number. Then the tach stops bouncing & the i number jumps into the 30s or 40s (100 amp alt). This repeats regularly.
Under power, the i number was pretty steady in the high 70s with state of charge in the 60% range. When the SofC reached 80% I noticed the voltmeter needle pulsing and the i number was jumping from 30 to 40 but the tach was steady.

Wiggling wires did not reveal any shorts. I did change the regulator plug. I have 2 regulators and have the same result with both regulators.

Help!
Thnx
 
A

alexco38

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Sep 25, 2008
1,096
CS 30 Toronto
If your motor is a diesel (and most gas), then more than likely there is a blown rectifier diode in the alternator. The Tach counts the number of alternating current (AC) cycles from the alternator.

The 6 diodes are arranged in a three phased full wave configuration. If one or 2 diodes are dead, the AC pulse will be missing at least 1/3rd. The needle usually fluctuates a tiny bit but the weight of the dial arm smooth it out. That's why at high speed, the needle seems to be steady.

Bring your alternator to any automotive electric shop and have them check it out. It is not too expensive to replace the diodes.

It's possible you switched the battery selector while the engine is running. That action usually blow the diodes.
 
kloudie1

kloudie1

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Nov 6, 2006
10,080
Hunter 34 Mandeville Louisiana
Has been discussed on forum in the past .. If the alternator is making current, the tach works. When the batts get full and the regulator cuts off the juice, the tach will stop working.. under a slight load, it will cycle on/off and the tach needle will jump.. Check belt to be sure it is tight, as Ron says. I think since the alternator is making 30/40 amperes, it is fine..
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
Belt just tightened.

Don't do the rotary switch thing.

This just recently started happening
 
A

alexco38

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Sep 25, 2008
1,096
CS 30 Toronto
needle on tach

kloudie1 said:
Has been discussed on forum in the past .. If the alternator is making current, the tach works. When the batts get full and the regulator cuts off the juice, the tach will stop working.. under a slight load, it will cycle on/off and the tach needle will jump.. Check belt to be sure it is tight, as Ron says. I think since the alternator is making 30/40 amperes, it is fine..
Click to expand
Does that mean whenever the batts get full the needle on all boat will jump? If thats the case my batts were never full as the needle never jump.

BTW. When the batts are full the terminal voltage is still 14.4v. Just the absorbsion rate i.e current drops.

If you have 30/40a going into the batts, your batts are discharged. Its not how much your alternator put out. It is how much the batts can take that determines the current.
 
kloudie1

kloudie1

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Nov 6, 2006
10,080
Hunter 34 Mandeville Louisiana
Depends on what kind of tach you have .. Some read the alternator pulses.. some read a sending unit that counts the flywheel teeth or other gear teeth. The on/off thing mostly has been reported on boats with large alternators and external regulators..
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
The alternator output current has noting to do with the alternator voltage. The output current is controlled by the batteries not the regulator. The regulator only controls the voltage. So unless the engine running voltage is the battery resting voltage (never seen this in 20+ years BTW) the alternator is putting out a signal for the tack to read.
The only explanation for a bouncing tack needle is the diode pack has a bad diode which causes the pulse timing to be off or low or the tack is bad. Since it only happens at low RPMs I'm leaning toward the diode pack. Pretty easy to test with a multimeter and battery
For the record a single bad diode will not effect the alternator output except at max current. But the batteries control the current so unless the batts are completely discharged the alternator is not putting out full output (53 amps??? stock??) or what you indicated of 20-30 amps. ie a 53 amp stock alternator can put out 20-30 amps even with a bad diode or 2.
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
It's a 100 amp alt. I have seen output in the 90's in the past. Today only about 70 something with SofC in the 68% range. So output is lower than capable
 
jerry

jerry

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Jun 9, 2004
64
Catalina 320 500 Stockton, Mo.
Same thing happed to me

The exact same thing happened to me last fall. Everything seemed OK with the wiring to altinator, but when I went to check battery water level found a loose post. Tightened it up and working perfect since.

Worth a check.
jerry
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
Checked AC voltage of the alternator. Should be 1 volt. Read 27 volts when the needle was bouncing, 30 volts when charging. Looks like a bad diode. Spare goes in tonite.
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
Update:

Finally had the time to dive into this further. Took the alt to Battery Shack in Marathon. Alt tested fine. All diodes good. No high AC readings.

He discovered I had a mismatch in pulleys! At a glance 3/8 isn't much different than 1/2". Changed the pulley on the alt and reinstalled the alt. Seems to work fine now. So far, no bouncing tach.

I had the belt very tight but evidently the wrong size pulley was causing slippage. It did not slip for several hundred hours.

Hope that is the last of this, on to my engine drive frig problems!
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
No signs of wear on the belt and I checked tightness first thing when I started having the tach issue
 
G

gettinthere

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Nov 26, 2008
1,970
Endeavour 42 Cruisin
Back to this issue.
Fresh belt, VERY tight. Alternator checked OK at shop. Tach still bouncing. When the tach bounces, victron shows no charge, then tach needle stops bouncing and victron shows decent amps charging. This comes and goes roughly every few minutes. Happens with 2 different voltage regulators.
 
Bill Roosa

Bill Roosa

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Jun 6, 2006
6,990
currently boatless wishing Harrington Harbor North, MD
Kinda sounds like one of the diodes has gone south.
When the regulator turns on the alternator it has enough voltage to smooth out the single drop when that diode is active but when the regulator has the rotor turned off and you are just reading residual output from self excitation (did that once and highly recommend it BTW) it is enough to be readable as a drop in RPM.
Is the needle bouncing up or downward? that is to say when it is reading steady then goes to bouncing is the bounce a lower or higher RPM?
 
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