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Onan 8KW MDKD Generator Fuel Solenoid...voltage...?

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    Onan 8KW MDKD Generator Fuel Solenoid...voltage...?

    Last night I went to start the generator and it wouldn't start...distinctive sound of no fuel. Checked 12V to fuel pump and fuel solenoid and only see 8.8V. If I push the solenoid it finally activates and holds until I "STOP".

    So I'm wondering if the fuel solenoid is supposed to work at 8.8V or if the problem is not getting the full 12V to the solenoid.

    Couple of other diagnostics...battery is fully charged at 13.3V. Battery goes down to 11.7 while cranking. Removed arm to fuel rack and solenoid shaft is free going in and springing back (manually). If I push the solenoid in while cranking it starts immediately and runs "forever" so no other alarms. All connections are clean and tight. After a "user assisted start" the voltage at the fuel solenoid is at full charging voltage of 14V+.

    Should the voltage going to the fuel solenoid be reduced to 8.8V by design or do you think I have a more fundamental problem of something reducing the voltage...?

    Thanks in advance...Nick
    1988 3888
    Twin Cummins 6BT's 210hp
    Onan 8KW

    #2
    Nick,
    I can’t imagine why they would use less then 12 vdc at the fuel solenoid. Have you measured voltage at the start switch to see if it reads 8 volts there or 12?It would be very helpful if you had a schematic for the generator.
    Gibraltar, Mi.
    1986- 3870- Hino 175's
    1988 26' Shamrock/ Diesel
    14' Zodiac Bay Runner

    Comment


      #3
      Originally posted by Sunbird View Post
      Nick,
      I can’t imagine why they would use less then 12 vdc at the fuel solenoid. Have you measured voltage at the start switch to see if it reads 8 volts there or 12?It would be very helpful if you had a schematic for the generator.

      I read battery volts at the switch (Start/Stop)...I have the schematic and don't see anything in the circuit that should cut down the voltage. Odd thing is that after the generator starts (with my help at the fuel solenoid), I see full charging volts at the fuel solenoid and fuel pump (14+V)...

      I also don't see anything in the schematic that would indicate start vs run voltage. I was beginning to think the battery was running down during cranking but when I put a good meter on it, it only drops a little over 1 volt (down to 11.3V while cranking).

      You have me thinking though...there are a couple of more places I haven't yet checked voltage...and will take a better look at Start/Stop switch (both sides). The service manual schematic shows a direct path from battery to switch to fuel solenoid to fuel pump. But it also says it's a "typical schematic". You're making me think there must be more in the circuit. Thanks for that...
      1988 3888
      Twin Cummins 6BT's 210hp
      Onan 8KW

      Comment


        #4
        The Onan run solenoid needs 12V. Onan's electronics on those older engines are insanely and unnecessarily complicated. I've had a couple of MDJs - one on a converted bus and another on our Defever. In both cases I completely disabled the Onan junk and rewired them with a couple of Ford relays. Its been a while now but as I recall one relay powered the run solenoid with the relay coil energized through a NO oil pressure switch. The 2nd relay grounded the run solenoid through a NC high temp switch. So when you started cranking and oil pressure built the oil pressure switch would close and the run solenoid would be energized. At that point the solenoid would have a ground because the NC temp switch was cold and therefore closed. If the gennie ever overheated the NC temp switch would have opened, dropping the ground to the run solenoid and stopping the genset. Similarly if I had ever lost oil pressure the run solenoid would have lost power. The run solenoids for my MDJ were made of unobtanium 10 years ago. I expect yours is similar. The run solenoids draw a ton of amps so any corrosion in that circuit could be your voltage drop. They also have both a pull in and a hold coil. The theory is that the pull in coil draws higher amperage and then the hold coil takes less watts to hold the solenoid closed.
        R.J.(Bob) Evans
        Buchanan, SK
        Cierra 2755
        Previously 43 Defever, Response LX
        Various runabouts, canoes & kayaks

        Comment


          #5
          Originally posted by bobofthenorth View Post
          The Onan run solenoid needs 12V. Onan's electronics on those older engines are insanely and unnecessarily complicated. I've had a couple of MDJs - one on a converted bus and another on our Defever. In both cases I completely disabled the Onan junk and rewired them with a couple of Ford relays. Its been a while now but as I recall one relay powered the run solenoid with the relay coil energized through a NO oil pressure switch. The 2nd relay grounded the run solenoid through a NC high temp switch. So when you started cranking and oil pressure built the oil pressure switch would close and the run solenoid would be energized. At that point the solenoid would have a ground because the NC temp switch was cold and therefore closed. If the gennie ever overheated the NC temp switch would have opened, dropping the ground to the run solenoid and stopping the genset. Similarly if I had ever lost oil pressure the run solenoid would have lost power. The run solenoids for my MDJ were made of unobtanium 10 years ago. I expect yours is similar. The run solenoids draw a ton of amps so any corrosion in that circuit could be your voltage drop. They also have both a pull in and a hold coil. The theory is that the pull in coil draws higher amperage and then the hold coil takes less watts to hold the solenoid closed.
          I think I've check most if not all of the points you've mentioned. All of what you described however does not happen until the generator output AC voltage comes up to 90VAC. Then a latching relay takes over and continues to provide power (negative) to the fuel solenoid and fuel pump. That latch doesn't engage if any of the NC/NO sensors don't operate properly (oil, temp, etc). Typically, the generator would start and then shut down. My problem happens prior to that during the start cycle.
          You've answered one of my questions and that is the need for the full 12V (battery) at the fuel solenoid during start. As of now I've only checked the hot side...time to check the negative side...thanks...

          Thanks for the additional info...I have more points to check but it was so hot and humid today I was starting to slide all over the bilge from my own sweat. Gonna hit more testing tomorrow morning in the cooler temps...
          1988 3888
          Twin Cummins 6BT's 210hp
          Onan 8KW

          Comment


            #6
            I think you know this but just so its 100% clear. I disabled ALL the Onan crap. My gennies ran on 2 Ford relays, a NO oil pressure switch and a NC high temp switch. I couldn't conceive of a shutdown situation that wasn't covered by that very simple system. That whole Onan business of lifting grounds to control circuits was just silly to my mind. And ridiculously complicated to troubleshoot.
            R.J.(Bob) Evans
            Buchanan, SK
            Cierra 2755
            Previously 43 Defever, Response LX
            Various runabouts, canoes & kayaks

            Comment


              #7
              Nick- you could also disconnect both wires from the solenoid and measure voltage holding the start switch. If you get 12vdc it could be that the solenoid coil is bad or leaking to ground. Just a suggestion-
              Gibraltar, Mi.
              1986- 3870- Hino 175's
              1988 26' Shamrock/ Diesel
              14' Zodiac Bay Runner

              Comment


                #8
                Originally posted by bobofthenorth View Post
                I think you know this but just so its 100% clear. I disabled ALL the Onan crap. My gennies ran on 2 Ford relays, a NO oil pressure switch and a NC high temp switch. I couldn't conceive of a shutdown situation that wasn't covered by that very simple system. That whole Onan business of lifting grounds to control circuits was just silly to my mind. And ridiculously complicated to troubleshoot.

                You're right, Bob...I was focused on diagnostics and missed your recommendation. I see your point and certainly easy enough for me to explore. After all, my Cummins don't have much more than what you've described.

                I appreciate the recommendation...thanks...
                1988 3888
                Twin Cummins 6BT's 210hp
                Onan 8KW

                Comment


                  #9
                  Originally posted by Sunbird View Post
                  Nick- you could also disconnect both wires from the solenoid and measure voltage holding the start switch. If you get 12vdc it could be that the solenoid coil is bad or leaking to ground. Just a suggestion-

                  Good point...I did check with the leads off the solenoid but after spending time last night with the schematic and the service manual I'm headed back to the switch. Good point on the switch...the generator worked just a couple of days before I started having problems. I may just have an intermittent problem at the switch. I've got a couple of momentary switches and if it's the Start/Stop switch, they're going in.

                  And I've got a couple of weeks before my trip to Block Island so I think I'm going for Bob's recommendation and bypass all that "Onan doo-doo-cacka" ...
                  1988 3888
                  Twin Cummins 6BT's 210hp
                  Onan 8KW

                  Comment


                    #10
                    Well...some more treoubleshooting. I took the Start/Stop switch out of the circuit but it did no good. I can see Battery Voltage at the input to the switch but I don't think it's actually coming from the Battery. The schematic shows that the supply voltage comes from the battery but I don't believe it. I get full voltage until I crank to start and then it changes to 8.8V which does not energize the fuel solenoid and fuel pump. I disconnected the Stop side of the switch and no change. I checked voltage at the starter (full battery voltage supplied through the Perko). I bypassed the Perko and no change.

                    Something in the circuit is changing the source of the B+ that the generator uses for Start and Stop...

                    And again, when I help the fuel solenoid and the engine starts, the voltage to the fuel solenoid and Start/Stop switch changes from 8.8V to the full charging voltage of 14.4V.

                    So, whatever it is that supplies B+ is changing the source from the battery to something internal in the control box. And guess what...I can't get to the relays in the control box because the lid is too close to the underside of the deck to be able to remove it.

                    Tonight I'm going to re-draw the schematic to something more simple and then "deduce" my way through it. It's 96deg out there so I'm quitting for the day... I bet it's gonna turn out to be something simple...
                    1988 3888
                    Twin Cummins 6BT's 210hp
                    Onan 8KW

                    Comment


                      #11
                      Originally posted by bobofthenorth View Post
                      I think you know this but just so its 100% clear. I disabled ALL the Onan crap. My gennies ran on 2 Ford relays, a NO oil pressure switch and a NC high temp switch. I couldn't conceive of a shutdown situation that wasn't covered by that very simple system. That whole Onan business of lifting grounds to control circuits was just silly to my mind. And ridiculously complicated to troubleshoot.
                      Getting ready to make your mod on mine...couple of questions...

                      1. by "run solenoid" do you mean the fuel solenoid...?
                      2. did you use Onan's cranking circuit or did you do your own...?
                      3. can I assume that you shut it down by putting a switch in series with your relays and the fuel solenoid...?
                      4. did you parallel the fuel solenoid and fuel pump power through your Ford relay powered through the Oil Press switch...? Basically powering the solenoid and pump with the power connected to the oil press switch...? Just wondering about the current draw through the Oil Press switch...

                      Thanks for your recommendation...it's a good one and I'm in the process of doing up a quick schematic based on your description above...wanted to make sure I understood it...
                      1988 3888
                      Twin Cummins 6BT's 210hp
                      Onan 8KW

                      Comment


                        #12
                        Originally posted by Nickp View Post
                        1. by "run solenoid" do you mean the fuel solenoid...?
                        Yes - the big bugger that holds the fuel pump open.

                        2. did you use Onan's cranking circuit or did you do your own...?
                        I don't recall changing anything there.

                        3. can I assume that you shut it down by putting a switch in series with your relays and the fuel solenoid...?
                        I believe I used switched power to the relays. My goal was that if anything - literally anything - failed then the engine shut itself down. So the default was "power off --- shutdown".

                        4. did you parallel the fuel solenoid and fuel pump power through your Ford relay powered through the Oil Press switch...? Basically powering the solenoid and pump with the power connected to the oil press switch...? Just wondering about the current draw through the Oil Press switch...
                        One of my MDJs was entirely mechanical so the fuel lift pump didn't enter into the situation. On the other I did as you suggested but there is no change in power through the oil pressure sender - its just controls the Ford relay. The relays I use are rated for 30 amps so they aren't an issue.

                        Couple of points to watch for:
                        - the oil pressure switch isn't going to replace your existing oil pressure monitor - gauge, light, whatever. It needs to close at a relatively low pressure - 8 or 10 PSI - so that your starter can generate enough oil pressure to close the circuit. There are different sensitivities so make sure you get a low pressure switch.
                        - I had to buy a card of the high temp switches - 10 or a dozen as I recall. I think I know where they are - if you want a couple PM me and I'll mail them but that may not happen quickly enough for your schedule.
                        R.J.(Bob) Evans
                        Buchanan, SK
                        Cierra 2755
                        Previously 43 Defever, Response LX
                        Various runabouts, canoes & kayaks

                        Comment


                          #13
                          Originally posted by bobofthenorth View Post
                          Yes - the big bugger that holds the fuel pump open.


                          I don't recall changing anything there.


                          I believe I used switched power to the relays. My goal was that if anything - literally anything - failed then the engine shut itself down. So the default was "power off --- shutdown".


                          One of my MDJs was entirely mechanical so the fuel lift pump didn't enter into the situation. On the other I did as you suggested but there is no change in power through the oil pressure sender - its just controls the Ford relay. The relays I use are rated for 30 amps so they aren't an issue.

                          Thanks, Bob...then I think I'm on track for doing the same as you. I'll be hitting the Autozone store in the next few days...hopefully get it done before my trip to Block Island. Much appreciate you sharing the info...thanks...
                          1988 3888
                          Twin Cummins 6BT's 210hp
                          Onan 8KW

                          Comment

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