Yanmar 3GM20F Tachometer Slow

[Jim Oursler]

My 84 Hunter with the referenced engine was reading 2800-3000 RPMs. Yard mechanic tested engine, and said my tach is slow. He got 3400-3600.. So how do I adjust the tach?

 

[Randy]

Tachometer calibration

My tachometer was also reading slow. Actually, I was glad to have found this out--previously, I had been concerned that I had an engine problem. Instead, it was just the inaccurate tach. As for calibrating it, mine was a tough job. My mechanic used a handheld digital tach to read the crankshaft revolutions, and compared the readings with the tach. Prior to adjustment, the tach was low by up to twenty percent. (Thankfully, I had always kept rpms very low, adjusting them more by sound than by the tach indication!) My mechanic requested instructions from Teleflex (maker of the tach). Unfortunately, I didn't have the tach which had the simple screw adjustment on the back. Instead, I have the kind with 14 mini switches which can be set to a zillion different combinations, so the tach can be used with a bunch of different engine/alternator combinations. After following the instructions, he set the correct switch settings for my engine/alternator, and the tach is now accurate, reading about 5% high, which is a nice, built in safety margin. The process took a couple of hours, all told. Good luck in getting yours adjusted!Randy

 

[Gordon Torresen]

Tachometer adjustment

The tach is the same for a 2GM20 or a 3GM30. There is a sender that screws into the flywheel housing that generates a voltage as the flywheel ring gear teeth pass by. The read-out is a meter that that uses this voltage to move a needle.  The gap between the sender and the flywheel teeth is critical as are all the wires and connectors between the sender and the read-out. Make sure the sender is all-the-way screwed in and that your connector pins are clean. I am not ruling out a defective sender or read-out, but check the above stuff first.  The post on calibration with dip switches is probably for a Universal engine that reads AC from the alternator. His problem may be from what I described above about connectors, and a dip switch correction may only be temporary as corrosion continues.