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Old 12-06-2010, 09:01 AM   #1
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4KW Onan Microquiet - fuel pump location, starting question

Hi -I have been doing some work around the generator and noticed that a PO has installed an aftermarket fuel pump outside of the generator cabinet. Where is the fuel pump usually located? I do not see a convenient place for it inside the cabinet.
The reason for the work was that the generator would not start. I think I have traced the problem to between the start switch and the fuel pump. Any suggestions for me b4 I start taking stuff apart?

Thanks
Les
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Old 12-06-2010, 11:13 AM   #2
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Onan MQ

Les - the Onan MQ has the original fuel pump located under the starter solenoid and the control board/module located at the lower left side when you have the service front cover off - the solenoid and the module have to be removed to get the original pump out - if you look under the generator you will see the fuel line going into the base at that location- if the PO did not remove the original pump and just added a new pump (taking for granted that he has installed the correct fuel pump for that application) then it should work but make sure he has connected the new pump to the original pump power wire - also the new pump should be mounted at around the same level as the old one or close to it - most of the RV manufactures have the fuel line from the tank to the generator positioned so that fuel will be in the line close to the pump so that when you prime the pump ( prime position on the Onan start button ) the pump does not have to refill the line every time ( some replacement pumps are self priming and some are not ) - if the PO positioned the pump higher then the fuel may drain back towards the tank - I have seen this on two other applications and repositioning the new replacement pump fixed the problem - also make sure there is no restriciton in the original pump - you can do this by disconnecting the fuel line to the inlet of the original pump and put it into a can or glass jar and try and prime the system and see what comes out - if you have fuel with the line disconnected then there may be a restriciton inside the original pump and you will have to bypass it and go directly to the carb - if you add any fuel line make sure it is line made for fuel systems and keep in mind that it gets very hot inside the generator case when operating- and make sure you have at least a 1/4 to 1/3 gas in the tank since the fuel pickup for the generator is placed above the bottom of the tank so that the generator will run out of fuel first and you will not drain the main fuel tank - let us know what you find.
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Old 12-06-2010, 02:14 PM   #3
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Thank you. I will look in that location for the original pump. The other pump is at the same level. Just outside of the cabinet. The fuel line goes from the "new" pump to the carb.
The second part to the question is that it seems that the pump is not working when connected to the generator. This is a low pressuer/low volumne pump. I assume it is supposed to run when the generator is running.
Les
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Old 12-06-2010, 08:42 PM   #4
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http://www.poweredgenerators.com/ona...als/a-1399.pdf

Don't know if this helps.
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Old 12-06-2010, 10:54 PM   #5
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Thanks for the input. The info does not show the location of the stock fuel pump. The email before yours told me where the original pump is located.

Les
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Old 12-07-2010, 05:11 AM   #6
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Les - the control board takes care of all the functions - on the MQ 4000 when you push the prime part of the start buton its like a momentary contact switch - as long as you hold the prime end down it signals the control board and then the fuel pump receives voltage until you release the button to prime the carb - then when you start the engine and when the control board receives signals that the engine is running then it keeps the voltage to the pump on constantly - check for this with a multimeter at the power wire for the pump - also check to see if the replacement pump has to be grounded to the case (or chassis ) as well and make sure that ground is good - is the engine cranking o.k ? - did the PO install a fuel filter as well ? - the original filter installs into the original fuel pump ( one end is threaded) and is the same kind that Ford used for any years on their carb engines - you can use an in-line filter with your setup.
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Old 12-07-2010, 10:04 AM   #7
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Originally Posted by RV Mech Tech View Post
Les - the control board takes care of all the functions - on the MQ 4000 when you push the prime part of the start buton its like a momentary contact switch - as long as you hold the prime end down it signals the control board and then the fuel pump receives voltage until you release the button to prime the carb - then when you start the engine and when the control board receives signals that the engine is running then it keeps the voltage to the pump on constantly - check for this with a multimeter at the power wire for the pump - also check to see if the replacement pump has to be grounded to the case (or chassis ) as well and make sure that ground is good - is the engine cranking o.k ? - did the PO install a fuel filter as well ? - the original filter installs into the original fuel pump ( one end is threaded) and is the same kind that Ford used for any years on their carb engines - you can use an in-line filter with your setup.
The engine is cranking okay. There is a fuel filter installed ahead of the pump. Another question is what is the spec. for the original pump - pressure and gph? We recently acquired this coach and the generator ran when we purchased it. So something has gone wrong in the last couple of months. Hopefully when I am done with this, it will be as simple as a disconnected or broken wire.
Les
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Old 12-07-2010, 06:53 PM   #8
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Quote:
Originally Posted by RV Mech Tech View Post
Les - the control board takes care of all the functions - on the MQ 4000 when you push the prime part of the start buton its like a momentary contact switch - as long as you hold the prime end down it signals the control board and then the fuel pump receives voltage until you release the button to prime the carb - then when you start the engine and when the control board receives signals that the engine is running then it keeps the voltage to the pump on constantly - check for this with a multimeter at the power wire for the pump - also check to see if the replacement pump has to be grounded to the case (or chassis ) as well and make sure that ground is good - is the engine cranking o.k ? - did the PO install a fuel filter as well ? - the original filter installs into the original fuel pump ( one end is threaded) and is the same kind that Ford used for any years on their carb engines - you can use an in-line filter with your setup.
Now that I know where to look I can see the original pump and filter. I guess someone got lazy and installed the aftermarket pump.
I would like to check the original pump and filter. What do I need to do to remove the control board? It looks like two screws on the exterior of the cabinet and uncouple the connector. And maybe disconnect the battery cable from the solenoid (and properly insulate the cable end) and loosen the solenoid from the mount.
Thanks
Les
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Old 12-08-2010, 08:13 PM   #9
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Onan fuel pump

Les - I do not have the exact GPH spec but the fuel pressure for this generator is between 2.5 to 4 psi - anything more and it can cause flooding in the carb- also these pumps are 'self priming' types instead of 'gravity feed' style meaning that is has the capacity to pull fuel from the tank - not all pumps have this feature - the original fuel pump power wire is brown and you can test at that point for voltage when priming the pump - if you have at least 12.5 volts at that wire and the pump still does not work then the replacement pump has stopped working as well (making sure the ground is o.k if required) - the control board/module pump has a bracket that the two front screws secure as you described and the starter solenoid usually has its own fasteners - one other thing - if the fuel is not treated (for storage ) with fuel stabilizer fuel deposits can damage a pump such as varnish and gum deposits same as the carb that can plug up and this is another possibility- try tapping the pump body and see if it starts working - one of the reasons for using a non Onan pump is the cost -aftermarket pumps are less expensive (usually) - so check for fuel at the inlet of the pump, check for full voltage at the pump wire as well and make sure you have enough fuelin the tank - also if a non self priming pump was installed it may have taken longer to prime it and may have damaged it - this is only speculation - verify everything and let us know what you find - also if you want to get a factory manual ( it has all the updated information including wiring diagrams ) take your Spec and Model number of the front decal and you can get one through Onan - very valuable to have.
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Old 12-10-2010, 08:38 PM   #10
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Thank you. Sorry for the late reply, I was out of town for a couple of days. At this point I am going to put in a new original fuel pump and pull in new fuel line from the tank (just in case it it cracked). I always get concerned when people start changing equipment on the newer coaches. After making these replacements, if I still have a problem, I will look at the other possibilities. It will take a couple of weeks to complete these tasks. I have to fit them in with other things we have planned.
Merry Christmas and Happy New Year.
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Old 01-02-2011, 04:22 PM   #11
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Quote:
Originally Posted by RV Mech Tech View Post
Les - I do not have the exact GPH spec but the fuel pressure for this generator is between 2.5 to 4 psi - anything more and it can cause flooding in the carb- also these pumps are 'self priming' types instead of 'gravity feed' style meaning that is has the capacity to pull fuel from the tank - not all pumps have this feature - the original fuel pump power wire is brown and you can test at that point for voltage when priming the pump - if you have at least 12.5 volts at that wire and the pump still does not work then the replacement pump has stopped working as well (making sure the ground is o.k if required) - the control board/module pump has a bracket that the two front screws secure as you described and the starter solenoid usually has its own fasteners - one other thing - if the fuel is not treated (for storage ) with fuel stabilizer fuel deposits can damage a pump such as varnish and gum deposits same as the carb that can plug up and this is another possibility- try tapping the pump body and see if it starts working - one of the reasons for using a non Onan pump is the cost -aftermarket pumps are less expensive (usually) - so check for fuel at the inlet of the pump, check for full voltage at the pump wire as well and make sure you have enough fuelin the tank - also if a non self priming pump was installed it may have taken longer to prime it and may have damaged it - this is only speculation - verify everything and let us know what you find - also if you want to get a factory manual ( it has all the updated information including wiring diagrams ) take your Spec and Model number of the front decal and you can get one through Onan - very valuable to have.
Hi - time to close this issue. Due to waiting for parts, pre-Holdiay trips, Christmas trips, etc, I finally got this work completed last week - new OEM fuel pump & filter and new fuel lines from the tank. And the generator fired right up. My guess is that the replacement non-OEM fuel pump was at fault, it makes no difference the cause of the fault, as long as the unit functions.
Thank you all for your input.
Les
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