Tachometer

[AnsleyS]

Can you tell me whether the tachometer on the Lehman 120 feeds off the alternator or the engine. Mine is jumping all around.

Thanks

 

[Larry M]

I believe it’s driven off the tach. The 135 is. Check to see if the belt is slipping and physically inspect the alternator and it’s wiring. Also check the wires at the back of the tach. If it continues, I’d pull the alternator and take it to an alternator shop for testing. They should be able to bench test it while you wait. Good luck.

 

[rgano]

Can you tell me whether the tachometer on the Lehman 120 feeds off the alternator or the engine. Mine is jumping all around.

Thanks

My two Lehman 120s had a cam-operated sending unit on the aft right (looking at engines from astern) side of the engine close to the area where the engine mounted fuel filters were. Check the connections at the tach and the engine.

 

[syjos]

Lehman's can have either a tach receiving pulses from the alternator or a tach receiving a signal from a sending unit mounted on the right side as Rgano described.

The tach using alternator pulses gets the signal via grey wire from the alternator.

I'd check the items Larry described first.

If your tach is driven from the alternator, there is an adjustment screw on the back of the tach to adjust the reading. Some times, turning that adjustment screw back and forth may solve erratic tach readings. That screw is used to adjust the tachs RPM reading for various alternators, number of cylinders etc so make sure to note the idle RPM so you can reset that after you are done. You can try spraying electrical contac cleaner at the adjustment screw while you wiggle it back and forth. There is a potentiometer in there that could be dirty.

On tachs with senders, the senders are matched to the RPM of the mechanical tach adapters shaft rotation so there are no adjustments or pots to get dirty. The 120s tach adapter turns 1/2 of engine rotation counter clockwise. Try what Rgano suggested, you could have loose or corroded connections. Strip back insulation to make sure the wire is bright and shiny.

 

[rgano]

I would add to syjos' post that even though I had the sending units on my engines, my tachs had the adjustment screw on the back. With that said, I noted that moving the adjustment screw with another watching the dial resulted in a limited range of needle movement which was not always linear, but it did improve things somewhat. With four tachs in the boat, I remember wishing that digital tach conversion was not so expensive. With one tach now and a need for more accurate tach reading on a planing boat, I quickly changed to digital after I bought this vessel.

 

[WayneDana2]

I had the same issue on my 120. Turned out to be the spade connector on the alt. It was loose and dirty. Replaced the connector and no issues since. Coulda just cleaned and crimped it a little, but had a stash of spade connectors already onboard.

 

[syjos]

I would add to syjos' post that even though I had the sending units on my engines, my tachs had the adjustment screw on the back. With that said, I noted that moving the adjustment screw with another watching the dial resulted in a limited range of needle movement which was not always linear, but it did improve things somewhat. With four tachs in the boat, I remember wishing that digital tach conversion was not so expensive. With one tach now and a need for more accurate tach reading on a planing boat, I quickly changed to digital after I bought this vessel.

I installed the Aetna digital tach which is accurate plus or minus 1 revolution but kept the analog Motorola tach for it's hour meter function.

The digital tach is good for setting the exact cruise RPM.

 

[SteveK]

Mechanical to electric sender is located left centre below fuel filter, above oil cooler. Unless of course it comes off alternator

View attachment lehman.pdf