onan help

Tim Bartlett · 02-13-2016, 06:26 PM

Need help on onan 7500 diesel generator it is a 1996 model in a freightliner chassis motor home. The unit cranks fine but after it runs 2/3 seconds it appears the starter trys to engage again. Have replaced the starter, solenoid, both rocker switches and relay. Any help is appreciated

Rich Biebel · 02-16-2016, 09:16 PM

I have not run across one that ever did that.

I will look into it for you tomorrow.

I am an Onan repair tech and have worked on many of those....

A 7500 diesel should be a variable speed inverter/generator......Yes?

Unfortunately the inverter section is a sealed unit and the control is part of that inverter section....NOT serviceable....

I will look at a machine's electrical print and trace out the starter circuitry but it is not a good scenario since you have changed most components already.....

If you can give me the Model Info off the I.D. label it would help.....

Looking in the Service Manual for a typical Cummins-Onan 7500 diesel inverter-generator this morning did not show me anything that is likely to be a simple fix for this machine.

The starter is controlled directly by the Controller PC board located inside the Inverter section and as I said not serviceable...

The starter should be de-energized when the engine reaches 800 rpm and the controller is always monitoring engine speed from output of the generator end......

Unless a wire is damaged, shorted, or possibly output is incorrect.......your Inverter-Controller is likely bad.

Not an uncommon issue on those units and since it is 20 years old.....anything is possible......

I would start the unit with the covers removed and pull the starter solenoid B+ wire off the starter and put a voltmeter on it to see if it is actually re-energizing after the unit is running.

I would also monitor output power and see if there is anything changing there at the same time.

The Inverter-generators can do some very difficult to diagnose things...

I had one that would "full field" the power end during cranking. The starter could barely turn the engine over due to the high field strength.....You think the starter or battery/cables are bad but it was the Inverter-control unit....

Tim Bartlett · 02-17-2016, 10:51 PM

Rich the model number is 7.5 dkdfj and the serial number is L953459253. If I start the unit and pull the b+ wire off should the unit continue to run? It appears the same wire goes to the fuel pump also. You can crank the unit and when the starter trys to reengage if you pull the b+ wire off the noise goes away but the unit stops running. Where is the inverter/controller located. Thanks for your help

Rich Biebel · 02-18-2016, 08:14 AM

Ok Tim, ignore everything I previously posted......

A DKD is a completely different generator from what I was thinking you were talking about.

You have an industrial type open unit. I rarely see them anymore....

I cannot find anything in the service manual or operators manual that clearly says what is used for crank disconnect as it is called.

Something inputs to the control board mounted in the square switch box area to signal the engine is running.

If I was working on it and had a visual on it I could better tell you what it has or doesn't have.

Some Onan's used a simple switch that worked off the governor.....as speed came up the switch opened....I don't think it has one but would have to look for it.....Speed is fixed on those machines and does not have a governor.....Uses injection pump stop to set speed. It might still have that switch and it might be under the rear cover area.....generator end...

Might use AC output.....Might use alternator output....possibly oil pressure......

I have not worked on one of those in years......

Tim Bartlett · 02-18-2016, 09:44 PM

Rich thanks again for the help. I have the original installation manual which includes a wiring schematic. I am far from an expert when it comes to reading the schematic but there is a item in the diagram labeled relay starter protection, do you think something wrong there could be the problem. That item has two circles labeled AC attached to it.

Rich Biebel · 02-19-2016, 08:43 AM

Tim it is possible that the output from the alternator is used as a signal to the controls to indicate the engine is running. That is used on a lot of old generators. I doubt it though as most machines that used that had an alternator with what was called a Tach output terminal.....

A battery charging alternator produces ac and is rectified and regulated to charge the battery. As I recall those DKD's used a small alternator just like the one sold as a Shogun for racecars....Had a separate rectifier/regulator.

You should be able to run the unit and pull the starter solenoid wire off the solenoid without it shutting off in my mind. Starter solenoid just gets 12volts + to crank and de-energizes once it runs. If you can run it without the starter kicking back in you can test for proper battery charging output.....

I can look at circuit diagrams when I have a chance and if I see something that helps I'll get back to you...

The info shown online for those machines is old.....and covers a lot of different models......

Rich Biebel · 02-19-2016, 10:07 AM

Ok Tim I found a print for a marine version of one of those and it sounds like yours is similar.

AC voltage is applied to a relay K-15 and it is used as a crank disconnect along with possibly a second circuit tied to the battery charge circuit.

At 90 volts AC the K-15 relay energizes for crank disconnect.

The old Onan's were very tough to fix as they used a lot of relays and their wiring along with other components. Weather, deterioration all can cause issues like you have.

I spent many hours on old Onan's including marine units and machines that have no starter.....they motorize the gen end to crank.....more components to deal with.....

I can't copy and paste the print I am looking at but look at the top where it shows 2 circles with AC written inside them.....they go to K-15 relay and that would be the coil portion of the relay.....

Somewhere else on the print is the contacts for K-15 and the wiring they are connected to. Probably a normally closed set of contacts(points).......Power comes up and coil energizes and contacts open....

Check the voltage going to K-15 and the wiring.....maybe the relay is just failing......

On older machines all the relays and wiring were separate items.
Later units may have all the components on a circuit board and not something you can replace one item on...

Without looking at your control section I can't tell what you have there so it's very difficult for me to steer you in the right direction...

Onan wiring is all printed with numbers and letters. Should somewhat match the print you have and that is how you can I.D. the components since they may not be labeled.......This is not easy to do but other than just replacing things the only way to fix it....

Tim Bartlett · 02-21-2016, 10:54 AM

Rich just wanted to give you an update. There are 3 control type boxes associated with the generator one appears to be just a on off switch box the second is mounted on top and toward the rear of the motor and is the a/c control box with the rectifier,suppressor,etc. The third is the main control box and is mounted to the side of the unit. In reviewing the wiring schematic the reference to the K15 relay-starter protection is in conjunction with A11 which is the PCB assy- engine monitor, a circuit board. There are 3 relay type items attached to this board,which one is hopefully the K15. I had the part number for board in the manual I have. Found the 300-2604 board at flight systems and have it ordered. Hopefully this is the problem. If not I guess we eliminated everything on that board. Will let you know thanks for your help.

Rich Biebel · 02-21-2016, 06:30 PM

That is the right thing to change since those relays are on the board. A board from Flight or ASAP should work fine. The K-15 relay is probably going bad or a solder joint, connection. Is failing. I have replaced those boards on DKD's. I had a customer who had one mounted in the back of a Ford diesel Van. She had show dogs. Dobermans. I also recall one Ina Motorhome. It was stored in a big barn on a horse farm. That was the only MH I ever saw with a DKD. A diesel pusher. Generator was up front on a power slide. Hope the board fixes it for you.

Tim Bartlett · 02-28-2016, 02:28 PM

Rich, just wanted to let you know that I received the control board and got it installed. Cranked the unit and all appears to be ok. Unit ran fine I shut it off and used the external start switch and it work fine from there also. Appears that part and your help fixed it. Want to thank you again for your help. If you could give me a call at 601-937-0891.

02-13-2016, 06:26 PM	#1
Tim Bartlett Junior Member Join Date: Apr 2011 Posts: 13 Likes: 1 Liked 0 Times in 0 Posts	onan help Need help on onan 7500 diesel generator it is a 1996 model in a freightliner chassis motor home. The unit cranks fine but after it runs 2/3 seconds it appears the starter trys to engage again. Have replaced the starter, solenoid, both rocker switches and relay. Any help is appreciated Last edited by Tim Bartlett; 02-13-2016 at 07:26 PM.

02-16-2016, 09:16 PM	#2
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help I have not run across one that ever did that. I will look into it for you tomorrow. I am an Onan repair tech and have worked on many of those.... A 7500 diesel should be a variable speed inverter/generator......Yes? Unfortunately the inverter section is a sealed unit and the control is part of that inverter section....NOT serviceable.... I will look at a machine's electrical print and trace out the starter circuitry but it is not a good scenario since you have changed most components already..... If you can give me the Model Info off the I.D. label it would help..... Looking in the Service Manual for a typical Cummins-Onan 7500 diesel inverter-generator this morning did not show me anything that is likely to be a simple fix for this machine. The starter is controlled directly by the Controller PC board located inside the Inverter section and as I said not serviceable... The starter should be de-energized when the engine reaches 800 rpm and the controller is always monitoring engine speed from output of the generator end...... Unless a wire is damaged, shorted, or possibly output is incorrect.......your Inverter-Controller is likely bad. Not an uncommon issue on those units and since it is 20 years old.....anything is possible...... I would start the unit with the covers removed and pull the starter solenoid B+ wire off the starter and put a voltmeter on it to see if it is actually re-energizing after the unit is running. I would also monitor output power and see if there is anything changing there at the same time. The Inverter-generators can do some very difficult to diagnose things... I had one that would "full field" the power end during cranking. The starter could barely turn the engine over due to the high field strength.....You think the starter or battery/cables are bad but it was the Inverter-control unit.... __________________ Rich Biebel S/C 1479 Last edited by Rich Biebel; 02-17-2016 at 09:48 AM.

02-17-2016, 10:51 PM	#3
Tim Bartlett Junior Member Join Date: Apr 2011 Posts: 13 Likes: 1 Liked 0 Times in 0 Posts	Re: onan help Rich the model number is 7.5 dkdfj and the serial number is L953459253. If I start the unit and pull the b+ wire off should the unit continue to run? It appears the same wire goes to the fuel pump also. You can crank the unit and when the starter trys to reengage if you pull the b+ wire off the noise goes away but the unit stops running. Where is the inverter/controller located. Thanks for your help

02-18-2016, 08:14 AM	#4
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help Ok Tim, ignore everything I previously posted...... A DKD is a completely different generator from what I was thinking you were talking about. You have an industrial type open unit. I rarely see them anymore.... I cannot find anything in the service manual or operators manual that clearly says what is used for crank disconnect as it is called. Something inputs to the control board mounted in the square switch box area to signal the engine is running. If I was working on it and had a visual on it I could better tell you what it has or doesn't have. Some Onan's used a simple switch that worked off the governor.....as speed came up the switch opened....I don't think it has one but would have to look for it.....Speed is fixed on those machines and does not have a governor.....Uses injection pump stop to set speed. It might still have that switch and it might be under the rear cover area.....generator end... Might use AC output.....Might use alternator output....possibly oil pressure...... I have not worked on one of those in years...... __________________ Rich Biebel S/C 1479 Last edited by Rich Biebel; 02-18-2016 at 08:18 AM.

02-18-2016, 09:44 PM	#5
Tim Bartlett Junior Member Join Date: Apr 2011 Posts: 13 Likes: 1 Liked 0 Times in 0 Posts	Re: onan help Rich thanks again for the help. I have the original installation manual which includes a wiring schematic. I am far from an expert when it comes to reading the schematic but there is a item in the diagram labeled relay starter protection, do you think something wrong there could be the problem. That item has two circles labeled AC attached to it.

02-19-2016, 08:43 AM	#6
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help Tim it is possible that the output from the alternator is used as a signal to the controls to indicate the engine is running. That is used on a lot of old generators. I doubt it though as most machines that used that had an alternator with what was called a Tach output terminal..... A battery charging alternator produces ac and is rectified and regulated to charge the battery. As I recall those DKD's used a small alternator just like the one sold as a Shogun for racecars....Had a separate rectifier/regulator. You should be able to run the unit and pull the starter solenoid wire off the solenoid without it shutting off in my mind. Starter solenoid just gets 12volts + to crank and de-energizes once it runs. If you can run it without the starter kicking back in you can test for proper battery charging output..... I can look at circuit diagrams when I have a chance and if I see something that helps I'll get back to you... The info shown online for those machines is old.....and covers a lot of different models...... __________________ Rich Biebel S/C 1479

02-19-2016, 10:07 AM	#7
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help Ok Tim I found a print for a marine version of one of those and it sounds like yours is similar. AC voltage is applied to a relay K-15 and it is used as a crank disconnect along with possibly a second circuit tied to the battery charge circuit. At 90 volts AC the K-15 relay energizes for crank disconnect. The old Onan's were very tough to fix as they used a lot of relays and their wiring along with other components. Weather, deterioration all can cause issues like you have. I spent many hours on old Onan's including marine units and machines that have no starter.....they motorize the gen end to crank.....more components to deal with..... I can't copy and paste the print I am looking at but look at the top where it shows 2 circles with AC written inside them.....they go to K-15 relay and that would be the coil portion of the relay..... Somewhere else on the print is the contacts for K-15 and the wiring they are connected to. Probably a normally closed set of contacts(points).......Power comes up and coil energizes and contacts open.... Check the voltage going to K-15 and the wiring.....maybe the relay is just failing...... On older machines all the relays and wiring were separate items. Later units may have all the components on a circuit board and not something you can replace one item on... Without looking at your control section I can't tell what you have there so it's very difficult for me to steer you in the right direction... Onan wiring is all printed with numbers and letters. Should somewhat match the print you have and that is how you can I.D. the components since they may not be labeled.......This is not easy to do but other than just replacing things the only way to fix it.... __________________ Rich Biebel S/C 1479 Last edited by Rich Biebel; 02-19-2016 at 10:17 AM.

02-21-2016, 10:54 AM	#8
Tim Bartlett Junior Member Join Date: Apr 2011 Posts: 13 Likes: 1 Liked 0 Times in 0 Posts	Re: onan help Rich just wanted to give you an update. There are 3 control type boxes associated with the generator one appears to be just a on off switch box the second is mounted on top and toward the rear of the motor and is the a/c control box with the rectifier,suppressor,etc. The third is the main control box and is mounted to the side of the unit. In reviewing the wiring schematic the reference to the K15 relay-starter protection is in conjunction with A11 which is the PCB assy- engine monitor, a circuit board. There are 3 relay type items attached to this board,which one is hopefully the K15. I had the part number for board in the manual I have. Found the 300-2604 board at flight systems and have it ordered. Hopefully this is the problem. If not I guess we eliminated everything on that board. Will let you know thanks for your help.

02-21-2016, 06:30 PM	#9
Rich Biebel VIP Member Join Date: Jun 2007 Location: Northern New Jersey suburbs Posts: 2,275 Likes: 25 Liked 510 Times in 200 Posts	Re: onan help That is the right thing to change since those relays are on the board. A board from Flight or ASAP should work fine. The K-15 relay is probably going bad or a solder joint, connection. Is failing. I have replaced those boards on DKD's. I had a customer who had one mounted in the back of a Ford diesel Van. She had show dogs. Dobermans. I also recall one Ina Motorhome. It was stored in a big barn on a horse farm. That was the only MH I ever saw with a DKD. A diesel pusher. Generator was up front on a power slide. Hope the board fixes it for you. __________________ Rich Biebel S/C 1479

02-28-2016, 02:28 PM	#10
Tim Bartlett Junior Member Join Date: Apr 2011 Posts: 13 Likes: 1 Liked 0 Times in 0 Posts	Re: onan help Rich, just wanted to let you know that I received the control board and got it installed. Cranked the unit and all appears to be ok. Unit ran fine I shut it off and used the external start switch and it work fine from there also. Appears that part and your help fixed it. Want to thank you again for your help. If you could give me a call at 601-937-0891.