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Onan Manuals
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 ofthe information below will only make sense and or apply to my specific case so don't let that confuse you if youtry 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 Emerald4kw)
(WEBMASTER COMMENT: this information is specific to the model listed - your particular model mayhave different specifications and wiring connections. Therefor this article is intended for general informationonly. 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 bypassesthe 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 thenegative 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 voltageregulator is bad. * If you _don't_ see 12v at J4/P4 pin 7 while cranking, make the same check at P1 (theconnector to the control board) pin5. If you still don't see it here, the control board is bad. * If you see12v 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 thefield flash voltage at P4/pin 7 but nothing is getting to the brushes, one last check is to test continuity fromP4/pin 9 to one brush, and from P4/pin 10 to the other brush - should be less than one ohm, and neither should showany continuity to ground.
If all this is true, the voltage regulator is bad, and you'll have to replace it. * If you aren't gettingthe 12v field flash at P4/pin 7, and it appears to be a control board problem, try using a jumper lead to brieflyapply +12v dc toP4/pin 7 while cranking. If the voltage regulator, brushes, and armature are OK, it should take offand 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 regulatoris probably bad. Simplified explanation is the voltage regulator (once the genset is running) getsexcitation voltage from the exciter winding (Q1/Q2) and monitors the output voltage (L0/L1). It rectifies theexcitation voltage and converts it to a dc voltage that it passes to the brushes to provide a field just strongenough to maintain the desired output voltage.
If there is excess resistance in the field circuit, the regulator has to provide a higher field voltage toget enough current through to provide the necessary field strength. If taken to extremes, the output devices areturned on 100% trying to provide enough field voltage to force enough field current through the resistance to getthe 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 agood 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 thesecond. 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 dcto pins 9 and 10 in the wiring harness plug (which connect to the brushes) while you have the set running - thisshould give you a nominal 120v output, or close enough to switch the control board to run mode - then measure acvoltage between pins 2 and 3 (should be output voltage, 120v +/-), and measure the exciter winding outputbetween 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 whenyou do this, and the output and exciter voltages look reasonable, then everything except the voltage regulator isOK. * Earlier (non-electronic voltage regulator) Onan generators may lose output if the slip rings oxidize, butwon't hurt anything. Later electronic-regulator generators have a 'capped' voltage regulator - if the field voltageexceeds 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 willestablish - or re-establish - the desired polarity). As far as the generator function itself, the rotor hasfour 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 FlightSystem's.
They are building their own design from scratch. Came with a 1 yr warranty and the reg is repairable. Theirdesign is a drop-in replacement. Biggest revision that can be seen is the large cap is physically smaller andstands upright and directly connected to the pcb. Reg came with a nice 9 page manual that included a section ontheory 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 failurehappens but seem to fit case when my reg failed. Flight systems says they have some control components that Onan nolonger 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 - theidle stop screw should NOT be touching, i.e. pressing the throttle lever back against the idle stop should lowerthe 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 therod goes in the hole on the throttle lever with the throttle lever just short of being wide open throttle. Adjustthe rod length if necessary by loosening the locknut on the other end of the rod (where it threads into the balland socket joint), turning the rod into - or out of - the joint as needed, then re-tighten the locknut and clipthe rod back into the throttle lever. *
Looking at the governor mechanism, you'll see an arm extending outwards, pivoting on a shaft coming upvertically out of the engine, with the ball and socket joint on the end closest to you. About 2/3 of the way downthe arm, you'll see a spring extending to the right, with the left end hooked to a link that's threaded on a screwmounted in the arm and the right end hooked to a threaded rod that goes thru a stationary bracket, with a nut onthe other side of the bracket. This nut is the speed adjustment nut - by controlling the tension on the governorspring, it controls the speed of the genset. *
The screw mounted in the governor arm is the sensitivity adjustment screw - by moving the point where thegovernor spring is attached to the arm, it changes the governor sensitivity. With no load, adjust the speedadjustment nut for 62-63 hz. Apply full load - the speed should be at least 58 hz. Spec calls for 2-4hz drop fromno load to full load, but in my experience, trying for anything less than 3hz makes the gen unstable at some loadlevels (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 by1-2 turns, re-adjust the speed nut, then test drop under full load again. Continue until no-load to full-load dropis 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 frequencydrops slightly. Turn main adjustment outward until frequency drops slightly. Locate the point where frequency ishighest (approx midway). From this point, turn the adjustment out another /4 turn. Remove the load. Allowgenset to stabilize for 30sec-1min, then turn the idle adjustment screw in _slowly_ until frequency drops andengine begins to run rough or starts hunting.
Back out the idle adjustment screw as required for smoothest running/highest frequency without hunting (maytake 1/8 - 1/4 turn out from highest frequency to keep set from hunting on sudden load removal). Test withvarious loads, and transitions between various loads, to ensure stable operation under all conditions.
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