Anyone out there who knows electrical part of the K321 engine for the Bobcat 371? Please help. Thanks

erwin · Jan 19, 2010

I finally got my machine running like new with new piston, rings, carburetor and even a brand new gas tank and pump. Now, I want to install a front and rear light to make it complete. I first determined that my machine has a 15 amp regulated battery charging system. Then I did the following measurements. 1. With the engine not running, the stator winding measures 0.4 ohme. Both terminals of the winding measure infinity to the ground. 2. With the engine idling, battery connected and rectifier (3-wire) unplugged, the AC output of the stator measures 4.8V. The highest measured AC output as I rev up the speed is 15V. Since I do not have a way to measure the rpm, I did not dare to have the throttle fully open. The shop service manual calls for 28V at 3600rpm. 3. When the positive terminal of the battery is disconnected while the engine is running, it dies. This tells me that there is no or not enough electricity to power the ignition system. Base on those measurements, can one tell if I have a faulty rectifier or magneto?

OldMachinist · Jan 19, 2010

The engine governor should control the RPM's to its max. So open the throttle wide open and see what the AC output is.

eyyu · Jan 20, 2010

OldMachinist said:
The engine governor should control the RPM's to its max. So open the throttle wide open and see what the AC output is.

Thanks, OldMachinist. I'll try that. Mainwhile, I'll do some in-depth reading on college electronics and figure out what DC output should I expect from a 4.8V AC input in a bridge rectifier. Erwin

OldMachinist · Jan 20, 2010

eyyu said:
Thanks, OldMachinist. I'll try that. Mainwhile, I'll do some in-depth reading on college electronics and figure out what DC output should I expect from a 4.8V AC input in a bridge rectifier. Erwin

Measure the voltage of the battery with engine off and then with it running and the rectifier connected what DC volts do you get at the B+ terminal of the rectifier. If it's more than the battery voltage it is charging. Depending on where the wire from the B+ terminal on the rectifier is connected it may die when the battery is disconnected. Try running a jumper wire from the rectifier B+ terminal to the + side of the coil then disconnect the battery and see what happens.

eyyu · Jan 20, 2010

OldMachinist said:
Measure the voltage of the battery with engine off and then with it running and the rectifier connected what DC volts do you get at the B+ terminal of the rectifier. If it's more than the battery voltage it is charging. Depending on where the wire from the B+ terminal on the rectifier is connected it may die when the battery is disconnected. Try running a jumper wire from the rectifier B+ terminal to the + side of the coil then disconnect the battery and see what happens.

Thanks OldMachinist, I'll try the jumper wire tonight. I got the feeling that the 4.8V AC output from the stator coil is not good enough. I kind of hoped that it's enough and it's just a bad rectifier (that would be a easy fix). I did not find a formula online which can tell me the exact expected DC output from the 4.8V AC input. However, a friend from our instrumentation shop told me this morning that it will be something like 7V instead of doubled voltage. I'm going to put a known good rectifier (a bridge kind) there, I can easily rig up one tonight, then I'll know for sure what the coresponding DC output is and by connecting the DC output directly to the + terminal of the ignition coil, I'll know if the magneto is good. I'll keep you posted. I know by the measurements that the stator winding is good. If it's not putting out enough AC output as specified by the service manual or even enough to run the ignition, something could be wrong with the magnets on the inside of the flywheel. Erwin