Onan Charging Systems

I received an e-mail regarding the Onan charging system on the Allis-Chalmers 720. Rather than to respond only to the e-mail I thought I would address the issue here so it would become available for everyone. The first test is to take a voltmeter and measure across the battery terminals with the engine not running you should see about 12 volts. Start the engine run it about 1800 to 2000 RPM and the voltage should come up to about 14 to 14.8 volts DC. If it does the system is working as it is supposed to. If your ammeter doesn’t indicate a charge, it is probably bad. If it still reads 12 volts. Get to the white wire coming out of the rectifier and see if you have 14 to 14.8 volts DC. If you do you probably have a broken connection or an open ammeter between the rectifier output and the battery, as the voltage from the rectifier is not getting to the battery. If isn’t still isn’t charging we will proceed forward. Note: Read this completely before starting as some of the easiest tests are covered later and you may want to do them first.

The CCKB engine with the 20 amp. system uses a stator and a ring of magnets inside the flywheel. These produce an alternating current [AC] power source to charge the battery. [This goes to a rectifier assy. and a regulator which were manufactured by PHELON and to the battery.] The AC must be rectified [converted to DC] to be used. There are 2 black and 1 red wires coming out of the stator. The 2 black wires are the AC source and the red wire controls the stator output.

To resistance check the stator, you should have .5 to .7 [5/10] of an ohm between the black leads (with nothing else connected to the stator).  From 1 of the black leads to the red lead you should have about 1.3 to 1.5 [1+3/10] ohms. From the other black lead to the red lead you should have 1.8 to 2.2 [1+8/10] ohms. Next measure from each black lead to ground. Expect to see .1 [1/10] ohm from each black lead to ground. If these measurements are normal, the next step would be to check the AC output.

To check the AC output, the stator needs to be unplugged and the engine running about 1800 RPM. You should see about 17 or 18 volts AC.  Be sure your meter is set to AC VOLTS.

The output of the stator goes to the rectifier assy. Typically this will have 2 black and 1 white wire. Use an ohmmeter to check the diodes. Check from each black wire to the white wire. One direction they should read fairly low resistance [like 10 ohms] depending on the scale the ohmmeter is on., reversing the meter leads should read a very high resistance. [like maybe 10,000 ohms] If either black lead reads near 0 ohms to the white lead and each way with the meter leads reversed you have a shorted diode. If either one reads very high both directions you have an open diode in the rectifier assy.  [Note: Diodes are like check valves in a water pipe, they allow electric current to flow in one direction and not in the other. By reversing the meter leads you are changing the polarity to the diodes and checking this property.]  If the rectifier checks OK, we will check the regulator.

To test the regulator, which has a black and red lead use an ohmmeter on the r X 10,000 scale and connect one lead to the red wire and the other to the base of the regulator. There should be NO deflection of the meter reverse the meter leads and there should be no deflection. In other words there should be NO leakage from the red lead to the regulator metal base. Next repeat the test using the black lead and the base. In one direction there should be no deflection and when the meter leads are reversed should deflect fully.

The logical sequence of events would be to unplug the rectifier and regulator and do the tests on them first, as this would be the easiest. Note the regulator base Must be grounded to function properly in the unit. If the off the unit tests check out we will proceed to a couple of operating tests.

The 2 black wires from the stator connect to the 2 black leads of the rectifier assy. The white lead from the rectifier goes to the wiring harness to the battery and to the black lead of the regulator assy. The red lead from the regulator goes to the red lead that comes out of the stator.

If the tractor doesn’t charge shut off the engine and disconnect the red lead from the regulator. Tape the leads so they don’t short out. Start the engine, the alternator should charge full output, with the engine revved up should go to or above 14.8 volts if it doesn’t the stator is bad. If it does go to full output do not operate it this way very long or the stator or rectifier may get damaged.

If the tractor is overcharging, check the ground on the regulator base, or for a bad wire on the red lead. If they seem OK, again disconnect the red lead and tape the regulator red lead and connect a temporary ground to the RED lead coming out of the stator. Start the engine rev up, using the tractor ammeter or an ammeter in series with the lead to the battery, the unit should not put out over 4 Amps DC.  If it does, it indicates the stator is OK and the regulator is bad. Hope this is helpful.

Good Luck and Happy New Year,