Onan MDJF not charging the start battery

[JoeQ]

I have an Onan MDJF generator that starts and runs fine. It produces AC and does what it is supposed to do as far as mains is concerned. The problem is that there's no DC output to charge the starting battery.

Any onan experts or anyone with a wiring diagram or workshop manual? Any ideas?

 

[cliff]

Joe,

Ask and ye shall receive /forums/images/graemlins/grin.gif

Have a look Cummings Onan Manuals Your model is listed but there are two variants so you need to know which variant.

Got your rigging problems sorted yet?

---------------------

Artificial Intelligence is no match for Natural Stupidity

 

[cawarra]

Hey JoeQ
I certainly am no expert in this field but I understand that some generators do not have a DC charging circuit. They only supply AC which would be used to power the battery charger, which obviously in turn charges the batteries/ genny start battery.HOWEVER, it sounds like it was charging in the past so I am guessing you genny does have DC charging capabilities. Sorry, not really much help am I! Hope you get it sorted though. Have a good weekend....

 

[JoeQ]

[ QUOTE ]
Have a look Cummings Onan Manuals

[/ QUOTE ]
Thanks but I had a look there already but they don't have a manual for download, only operator manual part numbers. I need a workshop manual or wiring diagram.
[ QUOTE ]
Got your rigging problems sorted yet?

[/ QUOTE ]
No, but fortunately I paid Formula Yacht Spars with a credit card so I have a claim against the credit card company. I still need to find a rigging/mast specialist to do a complete refurb.

 

[JoeQ]

It has a 10 amp ammeter that shows a small negative deflection when the genny is running. It would be handy if I could get my hands on a wiring diagram!

 

[Norman_E]

Even if it does have a "12 volt" output, unless that is specifically designed to charge batteries (and is therefore actually 14.4 volts) rather than just run 12 volt equipment, it will be pretty hopeless at charging your starting battery. The other issue is that if the "12 volt" output is limited to say 10 amps, that is not going to charge the battery very quickly. In most cases it is far better to ignore the 12 volt output from generators, so far as battery charging is concerned, and use the mains output via a charger. There are two reasons for this, firstly the generator "12 volt" output most likely acts just as a crude single stage charger even if it is actually 14.4 volts and proper chargers offer multi stage charging, secondly the mains output will run a much more powerful charger.

 

[JoeQ]

This seems logical.

My concern is that there's a stop solenoid that requires current to operate, it drains the battery when the genny is running. I disconnected it and just manually stop the genny when not required. I have a large invertor, if I fit a battery charger for the genny start battery would this impose an unnecessary demand on my house bank?

 

[Norman_E]

Sorry, I thought your concern was charging the main engine battery, rather than the generator starting battery. Your generator should keep its own start battery charged. Whilst you can run a battery charger to charge the generator start battery, doing this via an inverter is inefficient, and begs the question, why not just use jump leads from your house battery if the generator start battery is flat? I think you need to study the Onan manual to find out why the start battery is not being charged. Have you checked that the start battery is actually capable of holding charge? I am a bit puzzled by the stop solenoid drawing constant current. I thought they only draw current when they need to push the stop rod, rather than using current to keep the engine going.

 

[JoeQ]

I don't have an Onan manual nor can I track one down, I've tried. The genny start battery is almost new and I've tested it recently to ensure it's OK. As for the solenoid, it's spring loaded against the "not running" position and requires current to move the rod to allow the genny to start. If this current is removed the solenoid withdraws the rod and the genny stops. If the engine overheats or suffers low oil presure the wire supplying current to the solenoid goes to earth thus stopping the engine. Don't know if I'm describing this properly but I think that's how it works.

 

[Norman_E]

I see, it appears to work the opposite way to many engines, because it is wired in to a "low oil" circuit. Without a manual it is going to be difficult, but within the system there must be a mains driven 12 volt charger, which appears to have failed. Such things are often now electronic, which means that there may not be a big and easily identified transformer. Look very carefully for any fuses in the 12 volt wiring. Is there a thick wire going from the battery positive to the starter, plus a thin wire from the battery positive terminal? If so the thin wire probably comes from the charger. and will show you where to start looking.

 

[JoeQ]

This thing is probably 25 years old. I thought there might be a separate winding to supply charging for the engine start battery but without a wiring diagram I don't want to start poking around. I already had a look for obvious things like blown fuses. I don't recall seeing a smaller wire from the battery and will have a look next time I'm aboard.

Thanks for your help so far.

 

[jeremyshaw]

Maybe I'm being stupid, but is it not just likely to be an alternator problem? Forget the generator stuff, this is a diesel engine with a cranker, so the cranker will get recharged by the alternator. At least that's how my Kohler works TTBOMK&B. So could just be a problem with the regulator on the alternator, or the classic fractured lead. I'd look there first...

 

[bunglebear]

On the Onan MDJE the was two types of battery charging fitted, either automotive type alternator or a set of battery charge windings from the main alternator, These windings are listed as B1 - B2 on the wiring diagram. There is a large resistor fitted in the hot air outlet from the main alternator. If you call cummins onan on 01733 406100 they will be able to supply you with a wiring diagram.