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Diagnose Onan Generator Problems

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Onan BGM, NHM Series Owner's Manual

Onan Generator Troubleshooting

- contributed by Mickey Mathias.

The following information was provided by Alan Robinson a Cert. Onan Tech.

(The following is a compilation of several e-mails between myself and Alan regarding my genset problem. Some of the information below will only make sense and or apply to my specific case so don't let that confuse you if you try and use this information to assess your problem.)

The problem was engine would start but quit as soon as the start button was released. (Onan Emerald 4kw)

(WEBMASTER COMMENT: this information is specific to the model listed - your particular model may have different specifications and wiring connections. Therefor this article is intended for general information only. Before doing anything, verify your model number and specifications)

Also, check the oil level first - There is an oil pressure sensor that shuts down the genset if the oil pressure drops. When the start button is pressed it bypasses the oil pressure cut off circuit and the engine will run only if the button is pressed in. Check the wire to the oil pressure sensor - the sensor is located under the cover near the oil filter. It may have been knocked off.

It sounds like one of two things - either you are not getting the voltage from the control board to the voltage regulator while cranking (the field flash voltage that establishes some initial output from the genset), or the control board is providing the voltage but there is an open circuit in the voltage regulator, and the field flash voltage isn't being passed along to the brushes.

To tell which is which, locate the voltage regulator module inside the control box (bolted down at bottom left - has a capacitor molded into the top, and a short multi-wire pigtail to a twelve-conductor socket mated to a matching twelve-conductor plug in the wiring harness - J4/P4). Set your voltmeter to dc volts, ground the negative lead, and back-probe pin 7 - you should see approx 12v dc here while cranking.

If you _do_ see 12v here, but don't see 10-11 v at the brushes, the control board is good, but the voltage regulator is bad. * If you _don't_ see 12v at J4/P4 pin 7 while cranking, make the same check at P1 (the connector to the control board) pin5. If you still don't see it here, the control board is bad. * If you see 12v at P1 pin 5 but not at P4 pin 7, the wiring between them, or the connection at the connectors, is bad.

(For your engineering bent, the voltage comes thru a normally open contact on K4 that closes while cranking, and a normally closed contact on K2 that opens once there is output from the genset.) * If you are getting the field flash voltage at P4/pin 7 but nothing is getting to the brushes, one last check is to test continuity from P4/pin 9 to one brush, and from P4/pin 10 to the other brush - should be less than one ohm, and neither should show any continuity to ground.

If all this is true, the voltage regulator is bad, and you'll have to replace it. * If you aren't getting the 12v field flash at P4/pin 7, and it appears to be a control board problem, try using a jumper lead to briefly apply +12v dc toP4/pin 7 while cranking. If the voltage regulator, brushes, and armature are OK, it should take off and start producing output - and continue producing output as long as it is running. *

Gut feeling - based on symptoms and your measured 135 ohms brush to brush - is that the voltage regulator is probably bad. Simplified explanation is the voltage regulator (once the genset is running) gets excitation voltage from the exciter winding (Q1/Q2) and monitors the output voltage (L0/L1). It rectifies the excitation voltage and converts it to a dc voltage that it passes to the brushes to provide a field just strong enough to maintain the desired output voltage.

If there is excess resistance in the field circuit, the regulator has to provide a higher field voltage to get enough current through to provide the necessary field strength. If taken to extremes, the output devices are turned on 100% trying to provide enough field voltage to force enough field current through the resistance to get the output voltage where it belongs - and it doesn't take the device long to fail.

If it's any consolation to you, your new voltage regulator (although under the same part number) is what's called a 'capped' voltage regulator -in this circumstance, it would go to a default minimum voltage out (18v, IIRC) - they changed to this about spec G or H. * It's possible to check the voltage regulator with a meter that has a good diode check function (note that if it fails any of the following, it's bad - if it passes, it *may* be good - or bad - as this is only a static check). *

Open the control box cover and unplug the 12-pin regulator connector. With the meter on 'diode check', test between the following terminal pairs, connecting the positive lead to the first pin and the negative to the second. 5-9, 7-9, 10-9, 11-9, 12-9, 5-10, 5-11, 5-12, 5-3. Voltage regulator is bad if any pair indicates 'short' or 'open' except for pair 5-10, which should indicate open. *

With the regulator still unplugged, you *could* check the windings, control board, etc by jumping 18v dc to pins 9 and 10 in the wiring harness plug (which connect to the brushes) while you have the set running - this should give you a nominal 120v output, or close enough to switch the control board to run mode - then measure ac voltage between pins 2 and 3 (should be output voltage, 120v +/-), and measure the exciter winding output between pins 11 and 12 (should be about 85% of output voltage).

I usually use two 9v transistor batteries and a 5 amp fuse to get the 18v. If it switches to run-mode when you do this, and the output and exciter voltages look reasonable, then everything except the voltage regulator is OK. * Earlier (non-electronic voltage regulator) Onan generators may lose output if the slip rings oxidize, but won't hurt anything. Later electronic-regulator generators have a 'capped' voltage regulator - if the field voltage exceeds what *should* be the maximum for that set, it trips a one-shot and goes to the minimum field voltage (usually 18v) - and all current rev replacement regulators are capped and are protected.

If system is tested by using an 18V supply, is polarity important?

* It would only be important if you were doing it with the voltage regulator still hooked up. Since it won't be, it doesn't really make any difference (when you get a new voltage regulator installed, the field flash will establish - or re-establish - the desired polarity). As far as the generator function itself, the rotor has four poles - 2 will be N and 2 will be S regardless of which polarity you hook the 18v up.

As I didn't have any idea how long it was going to take to get the Onan part in I ordered a new reg from Flight System's.

They are building their own design from scratch. Came with a 1 yr warranty and the reg is repairable. Their design is a drop-in replacement. Biggest revision that can be seen is the large cap is physically smaller and stands upright and directly connected to the pcb. Reg came with a nice 9 page manual that included a section on theory of operation and a block diagram. Spec sheet looks like it should be able to handle the environment. Operation = -55&#deg to 175&#degF. The exciter specs are 250V max with 400V peak and 6A continuous @ 175&#deg and 15A peak.

One of the features mentioned was "no more start-up failures". Don't know if that is when typ failure happens but seem to fit case when my reg failed. Flight systems says they have some control components that Onan no longer mfrs..

If you need to adj the governor what is the correct procedure?

* On the speed/frequency issue: first, take a close look at the genset when it is running with no load - the idle stop screw should NOT be touching, i.e. pressing the throttle lever back against the idle stop should lower the speed to 55 hz, and there should be visible space between the screw and the lever. Adjust this as necessary. *

Next, with the genset stopped, unclip the governor linkage rod from the throttle lever and check that the rod goes in the hole on the throttle lever with the throttle lever just short of being wide open throttle. Adjust the rod length if necessary by loosening the locknut on the other end of the rod (where it threads into the ball and socket joint), turning the rod into - or out of - the joint as needed, then re-tighten the locknut and clip the rod back into the throttle lever. *

Looking at the governor mechanism, you'll see an arm extending outwards, pivoting on a shaft coming up vertically out of the engine, with the ball and socket joint on the end closest to you. About 2/3 of the way down the arm, you'll see a spring extending to the right, with the left end hooked to a link that's threaded on a screw mounted in the arm and the right end hooked to a threaded rod that goes thru a stationary bracket, with a nut on the other side of the bracket. This nut is the speed adjustment nut - by controlling the tension on the governor spring, it controls the speed of the genset. *

The screw mounted in the governor arm is the sensitivity adjustment screw - by moving the point where the governor spring is attached to the arm, it changes the governor sensitivity. With no load, adjust the speed adjustment nut for 62-63 hz. Apply full load - the speed should be at least 58 hz. Spec calls for 2-4hz drop from no load to full load, but in my experience, trying for anything less than 3hz makes the gen unstable at some load levels (whoop whoop whoop as it accelerates/decelerates but never settles on a stable speed).

If your drop from no load to full load is more than 4hz, adjust the sensitivity screw counterclockwise by 1-2 turns, re-adjust the speed nut, then test drop under full load again. Continue until no-load to full-load drop is under 4hz. Do note that this presumes carburetor mixture screws are correctly adjusted first:

With the genset fully warmed up, apply a full 4kw load. Turn the main adjustment inward until frequency drops slightly. Turn main adjustment outward until frequency drops slightly. Locate the point where frequency is highest (approx midway). From this point, turn the adjustment out another /4 turn. Remove the load. Allow genset to stabilize for 30sec-1min, then turn the idle adjustment screw in _slowly_ until frequency drops and engine begins to run rough or starts hunting.

Back out the idle adjustment screw as required for smoothest running/highest frequency without hunting (may take 1/8 - 1/4 turn out from highest frequency to keep set from hunting on sudden load removal). Test with various loads, and transitions between various loads, to ensure stable operation under all conditions.

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