Testing Yanmar Dynamo Style Alternator

Yanmar uses a "dynamo" style alternator on many of their compact tractors. These nifty little units generate electricity without any brushes so there is zero maintenance and the units are extremely reliable. Basically, if they are turning, they are working. 

It is common for people to replace these alternators when they are having a charging problem when, in fact, that is rarely ever the problem. Almost always, charging problems are caused by a bad regulator. That is the aluminum rectangle box, usually with fins, mounted on the engine side of the firewall. These regulators are the weak link of the system and typically give up their magic smoke after a jump start or starting the tractor while on a battery charger. 

The dynamo can be tested pretty easily. Just unplug the dynamo from the wiring harness and hook a volt meter to the two wires coming out of it. Switch your volt meter to AC (like a house) not DC (like your car) and give it a spin. You can usually spin it by hand or use a drill to spin it if the alternator is off the tractor or just start the engine if it is still attached. 

The dynamo outputs AC and then that is converted to DC by the rectification process in the regulator.  If you see AC voltage on your meter then it is working. Measurements are typically around 14 to 40 VAC depending on RPM and alternator/dynamo model but generally, any reading at all means it is doing its job.  

Mechanic Tip:
Dynamo style alternators have a distinctly rough "lumpy" feel when being turned by hand. This is normal and is just the magnets inside doing their thing.