Onan generator DC issue

Estragon · July 2017

Maybe not the right place to ask, but thought some off-gridders might have better knowledge than I.

Fired up my rarely used but 30+ year old diesel to charge a few weeks back, and as the inverters added charging load, the old girl sounded like she was struggling. I didn't quite make it to the AC out disconnect before she gave up, but she did seem to be shaking some trying to hang in there. She's a 4kw CKD.

Since then, pretty much dead. I got a bit of a crank once or twice, but basically nothing, so my working assumption is one of two things has happened. First, the shaking messed something up, maybe a wire shook loose, chafed, or whatever. The second is the fuel solenoid, which to me has seemed stiff for some time. The fuel solenoid does seem to actuate when I jump B+ to the pull terminal though, so I'm working on the first possibility for now.

So far, I can see B+ voltage where it should be at the start switch and glow plug switch. Both show continuity when on. Both show 0v on the switched side when connected on the switched side though, and low resistance to ground. My thinking is I should be seeing B+ on the switched side and high resistance to ground with either switch on, so a short to ground somewhere?

mcgivor · July 2017

The only requirements for a diesel to run is fuel and air, the air intake is generally unrestricted, open to atmosphere, fuel metering is used to control speed, so no fuel means no run. Fuel cut solenoids are a common problem, if you don't hear a click, or draws no current, that would definitely cause a no run condition, if not burned out often removal, clean, reinstall solves the problem. If the coil has failed the plunger and spring can be removed, this however is the means to shut down, there may be a manual shut down lever, if not choking the air intake will have the same result.
That would be the first thing to check IMO, years ago I paid a guy $500 for a VW Jetta worth $6000 because his mechanic told him the engine need to be replaced, after handing him the cash, I removed the spring and plunger from the fuel cut solenoid, started up and drove away, can still see that stunned look, with cash in hand.

Estragon · July 2017

The fuel solenoid was my first thought. It seems sticky, and I wondered if that was why everything seemed shorted to ground. I unplugged all the leads and connected the pull lead to B+ and it actuated though.

There is no click on either the start or fuel solenoids when normally connected. I can short them to actuate, but not crank normally. Can the fuel solenoid fail in a way that grounds the start circuit so it doesn't energize?

Thanks for your help

mcgivor · July 2017

Have you checked the fuses, do you get battery + on the disconnected solenoid wire in the on position, is the start battery voltage at the starter motor good while cranking, terminals and connectors good? resistance check the solenoid should be fairly high, I guess it could fail short but the circuit should be protected by a fuse. A few ideas and thoughts.

Ralph Day · July 2017

My old diesel set would shut down for no apparent reason. I never used it in "auto" mode, and needed to go out to it's shed to switch from grid to gen anyway, so I just disabled the solenoid. To shut down I have to go out and actuate the fuel cut off by hand. Not a probem.

If the fuel is old maybe you've got a fuel starvation problem. My old girl had not been used for years when I purchased it. The fuel in the filters and system was the consistency of honey. Hand pumped that out, new filters and ran like new...until I had fuel starvation because my summer fuel wasn't ready for -20C temperatures!

Ralph

Dave Angelini · July 2017

Some of the marine Perkins gens and propulsion had a fine screen in the air intake to keep "big things" out. They never got plugged.
The manuals over the years were changed to say, they were hard to get plugged.

mike95490 · July 2017

sputtering to a stop is not a solenoid issue. Look in the fuel system for something plugged at a fitting or filter, even the joint at the solenoid can catch debris and plug the line.

Estragon · July 2017

mcgivor said:

Have you checked the fuses, do you get battery + on the disconnected solenoid wire in the on position, is the start battery voltage at the starter motor good while cranking, terminals and connectors good? resistance check the solenoid should be fairly high, I guess it could fail short but the circuit should be protected by a fuse. A few ideas and thoughts.

The only fuse is on the starter motor solenoid +, and it looks good. Two circuit breakers (control on/off, and fault) both seem okay (continuity from line to lead). The battery is pretty new AGM, and resting voltage is good. It doesn't even try to crank, so I don't know about sag. I'll check resistance through the solenoid when I get back. I'm in the city printing off some schematics and wiring diagrams etc. It was getting old trying to read them on my phone while hunched over the genset.

Aguarancher · July 2017

" It doesn't even try to crank"

you should be able to jump the starter solenoid and get the engine to turn over regardless of any wiring issues except battery cables. have you taken off the fuel filter and checked for water?

westbranch · July 2017

when you try to start it do you hear a Click/Klack or rattleing sort of sound when you turn the key on and off? if so start with either the starter battery and the solenoid or the connections, the sound comes from the solenoid and indicates not enough Amps & Volts getting to the starter... how are the starter cables and connections?
Agree with Mike about the slow stopping due to lack of fuel delivery.

Estragon · July 2017

It initially stopped, I think, because it got loaded by inverters to charge batteries before it got warmed up enough. There may also have been a fuel delivery issue involved. It hadn't run for a while, and some air in the line, or maybe gunk growing, is a possibility. In any case, my theory at the moment is the shaking as it stopped is involved in the no-start problem.

Since then, it does absolutely nothing when trying to start. No clicking. Nada.

I'll be back there in the next couple of days armed with hard copies of schematics, wiring diagrams, and relevant parts of a service manual. One possible culprit I'm suspecting now is a latch switch that is supposed to close when oil pressure gets to ~5psi to disconnect the starter once it gets going. Maybe the shaking left the switch closed?

westbranch · July 2017

I have an old 1930 gas unit and I went to the Onan site for help.... oh ya there is a member here 'Onan Parts' try a PM to him for leads.
he hangs out more on the |Mid\nite forum though, maybe try there too...

Estragon · July 2017

@westbranch - good to know, thanks

mike95490 · July 2017

Violent shaking could cause the Low Oil sensor to trip or break. In some engines, that will kill it.
But if a diesel is at 70F and started, and ran for 90 seconds while inverter/charger qualified it, it should have been warm enough to carry load. Might have smoked for a bit. All reasons I went with 1930's diesel. I'm the oil sensor, starter handle cranker and day tank filler. Only a bridge diode in the alternator to fail from EMP.

wooffi · July 2017

I had auto start issues with my Kobota diesel, tried all kinds of tricks, it has two fuel filters, changed both. Finally traced it to a 12v fuel pump, somehow restricting to flow. Took the pump out, fuel is gravity fed anyway, she's been starting like clockwork since.

Estragon · July 2017

There's a lot to be said for simplicity. My Onan is early 80s vintage, and I think it has three different start-disconnect systems! One with the oil pressure triggered latch, one based on AC voltage, and a third based on DC.

It's in a partly buried shed made of 6x6 timbers, which moderates temperatures, so would have been more like 50f. There could well have been a problem as well as premature loading as I think it should have been able to recover after the inverter cut it loose as AC fell out of spec. I should check to see if the inverter ACin spec is settable - maybe it's set too wide.

Estragon · July 2017

Still working on this. I've ruled lots of things out, but still haven't found the problem.

One possible culprit is a resistor on the control board. I can't get a resistance reading on any scale on my multimeter, so I'm guessing it may have failed open. Should I be able to test it by touching MM probes to each lead on the resistor?

I couldn't make out band markings in the available light, but it's pretty small (~3/8-1/2" long).

BB. · July 2017

Make sure there is no voltage across the resistor (use meter to measure zero volts AC and DC across resistor, all power is killed to the control board).

It is difficult to know if you are measuring the resistor resistance or total resistance (resistor + any circuitry in the area) without looking at a schematic. I assume you are measuring "infinite" resistance rather than zero Ohms?

Generally, if a resistor fails, you should see indications. "Cooked" (brown/black resistor and/or circuit board). And/Or mechanical failure (cracked case, broken leads into case, cracked solder joint(s), corrosion, failed traces).

-Bill

mcgivor · July 2017

Attempting to measure the resistance whilst still in circuit is an exercise in futility, the resistor must be removed to prevent reading resistance through other components in the circuitry, Bill is correct resistor failure usually leaves some evidence.

Estragon · July 2017

Yes, it was infinite resistance (shows as "1" on my MM. The resistor itself is quite dark - could be dark brown or black. The band colours are nearly indistinguishable. The leads are about 1/2" on each side, and they and surrounding board look okay visually. I'll check for voltages, but don't expect to see any. On the schematic, it's between the line side of a dc breaker on one side, and through an open latch switch to ground and a normally open switch to start relay on the other. The latch switch is supposed to close when oil pressure reaches ~5psi. The normally open switch is supposed to close as part of start sequence, and return to open when alternator output gets to ~90vac.

The resistor is soldered to the board, and I'm not sure I could remove it without messing something up. I suppose if I'm going to end up replacing the board (assuming they're still available) anyway, it might be worth having a go at it though.

mcgivor · July 2017

If a replacement board is an option, it would probably be the simplest solution, generally when a single component fails, there is a cascade effect, meaning other components related to the resistor, could have failed, usually the case, or the reverse, when another component fails, it could have caused the resistor to fail, best of luck.

BB. · July 2017

To unsolder, use copper braid and a soldering iron to suck up the solder. Shake a little as it cools. Use a small pliers to lift out the component.

Solder suckers (spring loaded) are cheap too.

- Bill

Estragon · July 2017

After much wailing and gnashing of teeth, I've found that what I think is a circuit to send +12v to the starter solenoid is reading almost no resistance to ground. Would I be correct in thinking there should be some resistance across the coil, and the lack of same suggests the solenoid is shot?

mcgivor · July 2017

The solenoid would have a very low resistance, jumping the circuitry with a wire from battery + directly to the terminal, would confirm if it works, if there is a spark but no action, the plunger could be stuck, open and clean, or use the special service tool, strike it with a hammer, not recommended as a fix though.

Estragon · July 2017

Jumping b+ to the terminal did nothing. No click, just nothing. Tried banging with a socket handle, but no joy. Could it have welded itself closed? Maybe I'll try taking it apart tomorrow.

mcgivor · July 2017

No spark would indicate an open circuit, failed coil, hense no current, you could put a clamp on ammeter to confirm this, I've found using an automotive test light for diagnostic testing in such applications invaluable, a DVM is often too sensitive, FWIW.

BB. · July 2017

Also check for grounding (return signal ground) for the starter relay. Many automotive stuff simply uses the engine block/sheet metal.

But, there is usually one or more bonding straps (engine block to sheet metal, etc.). Many times you have rubber motor mounts and if the bonding strap fails, the engine is "floating" and none of the negative ground stuff for the motor will work right.

-Bill

Estragon · July 2017

@mcgivor -there was a spark jumping the circuit, just no crank or click.

@BB. - the resistance to ground I measured is to local ground in the generator. The "none of the negative ground stuff will work right" seems to fit. I can trigger relays by connecting external voltage across coils, but not through generator wiring. All the coils seem to be around 85ohm from local ground, except the starter, which is less than 1. The b- is connected to the block. Maybe a flakey connection there?

mcgivor · July 2017

Sounds like a stuck solenoid, cleaning all negative connections and coating with anti size compound wouldn't hurt either.

BB. · July 2017

Not sure yet.

A large starting relay coil coil easily be 2 Ohms or so resistance. Your 1 Ohm sounds ok.

All circuits need two wires / round trip connection. An open ground can certainly be an issue.

Using a 12 volt bulb (trail light or similar) can work better than a $300 DMM. the bulb draws a significant current. The meter draws only a few millionths of an amp. (As suggested above by McGivor).

-Bill

Estragon · July 2017

I'm not sure I really understand the use of the bulb. I'm getting there, but not there yet. Would I put it across the +b to the solenoid?

Onan generator DC issue

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