Home » Public Forums » GMCnet » Re: [GMCnet] Onan Dino board problem and fix - my story
| Re: [GMCnet] Onan Dino board problem and fix - my story [message #306073] |
Sun, 28 August 2016 17:43  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Nice analysis and writeup, Jim. While I no longer fool with Onans, I agree
with your evaluation of the problem. I suspect it is equally applicable to
the Onan boards. In fact, I think Johnny Bridges is adding the larger
dropping resistor already.
Y'All are going to keep on until replacing the Onan no longer makes sense!
(Wonder if my son will let me have back the home emergency 4kW he's had
for about 8 years?)
Ken H.
On Sun, Aug 28, 2016 at 6:19 PM, Jim Miller wrote:
> I’ve had two Dinosaur boards fail in the exact same fashion over the past
> couple of years; each board failed after a couple tens of hours of runtime
> and they failed in the exact same fashion. A search of GMCnet led to Ken
> Burton’s posting regarding some modifications that Dinosaur had made - and
> then the trail led to George Zhookoff who had similar problems and went
> through a couple iterations with Dinosaur’s engineering team to get it
> fixed. My fix and how I arrived at it is described below.
>
> Failure description: Onan would crank and try to start but would
> immediately fall over before reaching governed speed. Running a jumper from
> battery + to coil + allowed the engine to start and run perfectly.
>
> Unit specifics: Onan is connected to the house battery for starting and
> has the charger voltage regulator disconnected. Flywheel alternator is used
> only for providing a signal to the control board to confirm a successful
> start. Flywheel alternator voltage at governed speed = 30 VAC measured.
>
> Control board circuit description: flywheel alternator voltage of 30VAC is
> half-wave rectified and fed through a series dropping resistor (~50 ohms)
> to relay K2, a 12VDC relay with coil resistance 270 ohms. Once the engine
> reaches around 700 RPM the voltage has built up to the point where K2’s
> coil pulls in. This has the effect of de-energizing the starter and
> latching +12V to the fuel pump and ignition system so that the engine will
> continue to run.
>
> My analysis of the failure: over-voltage at the coil of relay K2 caused
> excessive heating of the coil windings. Continued heating results in a
> cascading failure where the insulation on the coil windings starts to melt
> and the windings start shorting together- which reduces the resistance of
> the coil and then produces even more heating. Eventually the resistance
> through the coil is so low - and the current in the circuit is so high -
> that the series dropping resistor fails. On my two failed boards the coil
> resistance was down to around 15 Ohms - a long way away from the 270 that
> it should have been - and the series dropping resistor was smoked. Visual
> inspection of the relay coils confirmed the melted insulation.
>
> I theorize that the flywheel alternator voltage is higher than it would
> have been on the original Onan design which had the battery charger in
> place - and thus the control board is now seeing a higher voltage than it
> was designed for. Approximately 24V delivered across the 270 ohm coil of
> the original 12V relay would produce a power dissipation in the coil of
> just over 2 Watts. The datasheet for the relay describes an operating power
> of around 0.5W. As the coil windings start to short together the resistance
> will decrease and the power dissipated as heat will increase.
>
> I wasn’t quite happy with the repairs and modifications that I had seen so
> I set out to try and come up with a mod of my own that would/should
> hopefully be bulletproof. After experimenting with different relays and
> dropping resistors I arrived at a 200 Ohm dropping resistor and a drop-in
> replacement 24V relay, P/N JW2SN-DC24V ($4 each at Mouser). With this
> arrangement the voltage at the relay coil is about 23.5V and I should think
> the relay will obviously live much longer than the 12V piece that had been
> in that spot on the original Dino board. Note that a 150 Ohm resistor will
> work in place of the 200 Ohm but it seemed to me that this caused the
> starter to drop out a little too soon. There is a critical relationship
> between the dropping resistor’s value and the pull-in voltage of the relay.
>
> I put a few pictures at: http://www.jcmco.com/gallery/album30
>
> Naturally I cannot guarantee that this fix will be helpful for anyone
> else’s Onan but I thought I’d write it up for the GMCnet nonetheless.
>
> —Jim
>
> Jim Miller
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH
>
>
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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