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Onan mdja not running.


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  #1  
Old 10-21-2007, 05:33:11 PM
Brent S Brent S is offline
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Default Onan mdja not running.

Hello everyone,
I have been lurking in the background for a while trying to solve a small problem with my 1970 vintage genset, in my 36 foot Grand Banks Trawler.
I have recently replaced the head gasket and raw water pump on my 3 kw mdja unit. However, I cant start the engine without removing the decompression solenoid.
I have no voltage to decompression solenoid.
I do have voltage on the ammeter.
I have checked the oil pressure switch and it seems ok.
I have taken the cover of the reverse current relay, and if I manually push down on the switch, the engine seems to run and stop perfectly.

Any advice would be appreciated.

thanks Brent
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Old 10-21-2007, 08:43:22 PM
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Talking Re: Onan mdja not running.

Brent, Do you have a wiring diagram for your MDJA? If not I can send one to
you if you give me an email address that can accept attachments. Look
at the HIgh Temp Switch, and the Shutdown Relay, as well as all the other
Control Wiring. I own two MDJA's, so I am very familiar with them.
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Old 10-21-2007, 10:48:08 PM
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Default Re: Onan mdja not running.

Bruce, thank you.
Yes I have a full operation and parts manual.
Looking at your previous posts, I think I might have two independant problems.

Firstly, the Reverse current relay, appears to supply current to the battery once the generator is producing 12 volts. I suspect it might be faulty, and assuming I cant buy one I am starting to think about diodes. Do you think I could replace the faulty relay with a diode? I figure that the K3 relay doesnt affect the voltage to k4 the decompression relay.Do you agree?

Second problem, is the lack of voltage to the decompression solenoid, looking at the wiring diagram, it looks like once the generator is revolving it creates voltage that passes thru the start solenoid, then the variable resistor R2 then R3 then decompression relay and finally retracts the decompression solenoid. So If I dont have any voltage at terminal 2 of the k4 relay then the problem probably rests with the supply of 12 volts from the generator.

What is your experience?

Ive been at this for a few days now, and starting to obsess.

thanks Brent
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Old 10-22-2007, 05:13:23 PM
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Talking Re: Onan mdja not running.

Brent, the way the Safety Shutdown System works is:
When you hit the Start Switch, it supplies Battery to the DC Series/Shunt
Windings on the Genset/Starter, which causes the engine to turn over, and
build Oil Pressure. When the Oil Pressure Swich closes, it then causes
Said Battery Voltage to cause K4 Shutdown Relay to pull in and close.
This then causes Battery Voltage thru the Fuse/Ammeter and the K4 Points
to flow to the High Temp Switch, which is Normally Closed, and energize
the Decompression Solenoid, to Ground. Once the Decompression Solenoid
energizes the engine will then try to Fire and run, If it does run, then the
DC Series/Shunt Windings will start to produce Volatge hugher than the
Battery Voltage, and send current backwards thru R2 to cause the Reverse
Current Relay to close, and keep the DeCompression Solenoid energized
and the engine running, as well as charging the Battery back up. R2 is
adustable to set the amount of Charge Current back thru the
Ammweter/Fuse to less than the Fused Current. One Thing I do to ALL
my xDJx Gensets, it to change out the Fuse/Fuse Holder for a 10 Amp
Circuit Breaker. You never have a Fuse handy when the one in the
genset BLOWS.
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Old 10-23-2007, 04:43:29 PM
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Smile Re: Onan mdja not running.

Bruce,
thanks for the insight on how the electrical system works.
Your description enabled me to understand the operation and yesterday I got it to work.
There seemed to be a few problems, some associated with the oil/ water mixture when the raw water pump failed, eg the oil pressure switch, was not working as well as it should.
The flicker switch was permanently closed, dont know how that happened. I have adjusted it to the maximum opening which is .025, but I am concerned how I will maintain this gap as the points wear.The movement from the tappet that pushes on the points with each revolution of the cam shaft is very small , is that normal? So I am wondering if I need to some how replace the points, any idea what else can be used?

Finally the biggest problem seemed to be the R2 resistor, it had burnt out, but I managed to flip it end for end , alter the variable tap and now its producing a nice 2 amps to charge the starting battery. I will be looking for a permanent replacement over the next week or so.

But essentially your advice allowed me to fix the old girl and I am really thankful for your generousity.

Regards Brent Stallard
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Old 10-23-2007, 05:10:52 PM
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Talking Re: Onan mdja not running.

Brent, Good on you, Mate Glad I could help. The Flicker Switch just shorts out
the Field Winding Resistor and adds a bit of resistance to the Field Winding,
just as the Engine fires on the Compression Stroke. It helps keep the Voltage
on the AC Side regulated, a bit more even. What most folks do is make there
own when they are needed, and if the plunger wears down a bit they just
weld them back up and repolish them. Only the single Cyl (A version) use
the Flicker Switch, so they are specific to that unit. There isn't a lot of current
passing those points, and there should be a capacitor across the points to
eliminate any arcing. You can Buy a repacment Resistor from Ohmite, for
R2, as they were the OEM that Onan used. As I recall they were about $10US
ea. in single quantities.
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Old 10-23-2007, 05:17:48 PM
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Default Re: Onan mdja not running.

Brent, also if you ever decide to Scrap the Old Girl, I would be extremely
interested in the DeCompression Solenoid and the Raw Water Pump. Never
hurts to advertise a bit.........
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Old 10-24-2007, 04:37:37 PM
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Default Re: Onan mdja not running.

Bruce , I have just rebuilt the raw water pump, it was totally screwed.
I went to a local bearing supply company and they pulled a suitable shaft and bearing assembly straight of the shelf, then it was on to an engineering company who shortened the shaft and did some other minor work, assembled it for me and charged me hardly anything. I researched and found a place in NY, who could source the pump but it was really expensive compared with the route I took.
thanks Mate!!
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