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Erratic rpm, Miller Trailblazer 250 G

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  • Erratic rpm, Miller Trailblazer 250 G

    Hello, I’ve got a Trailblazer 250G, two cylinder engine, is an Onan. At idle, switch set on auto, engine runs erratically, rpm goes up and down. I set the switch to fast rpm the engine rpm evens out and runs at the increased weld output rpm. Could this be carberation or something to do with the electronics, throttle solenoid ? Also could someone steer me to a tech manual for this unit. I’ve tried the Miller forum no response.

    When I first got the unit it would crank over but had no fuel to the carb. I did change the fuel pump and OH the carb. Cleaned the tank checked all the fuel lines. Just finished reinstalling the carburetor and has the same erratic rpm problem. So I have eliminated the fuel as being a problem. Engine runs really good with the switch at high rpm lock position, (so I would not think it to be an ignition problem) but erratic at low rpm with the switch at the automatic position. Visibly checked the electronic boards for burned components or signs of shorts, none were seen. I get 14 volts to the throttle solenoid with the switch at the auto position, I can disconnect one of the throttle solenoid wies and the engine goes into high rpm mode, and there’s no voltage to the throttle solenoid when the switch is at the high rpm lock position. If the throttle solenoid is not energized at high rpm what forces are involved to increase engine rpm for welding rpm, I don’t get it, something has to physically open the throttle, is it the govenor? This would make sense if the throttle solenoid is energized only at low or idle rpm. If that’s the case the problem is either in the throttle solenoid itself, govener, or the carb linkage arm. Also if the electronics is only feeding voltage to the throttle solenoid at idle rpm I will need to check to see if the voltage is intermittent or dropping off somehow. Anyone had this experience?

    Did more troubleshooting. I disconnect the throttle solenoid (only two wires) and the engine rpm increases to weld output and stays there until I reconnect the wires. I also disconnected the wires again and applied 12 volt DC to the throttle solenoid, it tried to pull in to idle the engine but could not hold the throttle there, could it be that the throttle solenoid could be weak, I checked ohm reading on it, shows to have 13 ohms. Checked the intake manifold for cracks, sprayed starting fluid around it, no changes. I am thinking now the problem is the throttle solenoid or the electronics, I’ll test that today. Thanks in advance, Jeff.
    Last edited by jeff-; 01-30-2019, 06:28 PM.
    sigpicJeff's CNC Plasma Cutting

  • #2
    My only experience is that these engines have a lousy fuel pump. My dad bypassed his and used an aftermarket inline automotive one and solved most of his issues on a Bobcat 225.

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    • #3
      The fuel pump I had just replaced and is working perfectly. So it's not fuel related. I called tech support got the info I needed for the electronics and wiring. The tech sent me the complete tech manual. Tech support also gave me instruction on adjustment for the governor linkage, the link arm to the carb and throttle solenoid. Through my troubleshooting I found that the throttle solenoid only gets voltage when the auto/fast rpm switch is in the auto position. At this point the throttle solenoid is suppose to pull the throttle in the idle position and hold it there until u strike an arch. There's a relay that gets voltage/energized from the electronics with this same switch in auto. When the relay is energized it's set of points closes and furnishes voltage from the battery to the throttle solenoid. I found the problem at this point between the linkage, carb idle adjustment and the throttle solenoid adjustment. So glad it's wasn't in the electronics or the relay, as I disconnected the throttle solenoid and connected the two wires straight to the battery, it tried to pull in to the idle position but could not hold it there. So I adjusted everything according to the tech manual, that fixed the erratic idling problem. Was just adjustments with the carb and the throttle solenoid.
      sigpicJeff's CNC Plasma Cutting

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      • #4
        It was brought to my attention to check weld output Hz frequency. Thank u so much for that tip, although I did not mess with the high rpm just the idle, I would imagine it should be checked. The tech manual did not mention that, at least the section I read. I'll look to see if it states the frequency somewhere. Thanks.
        sigpicJeff's CNC Plasma Cutting

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        • #5
          I don't think that is weld output, probably auxiliary power, should be 62.5 no load, drops to 60 under a load.

          Just want to mention, I have had four of these flathead Onan powered Trailblazers over the years, some comments.

          First, that particular engine is very sensitive to dirt, water, any other contaminants in the fuel. Sounds like you already covered that base. However, if this is a machine you use intermittently, do make it a habit of keeping the tank full, and do use fuel stabilizer, to prevent water and condensation buildup. This is especially important in the later models, that have the fill cap on top, that 2" tube going down to the tank is perfect for creating condensation. This is one of the reasons why my current "service welder" mounted on my truck is propane fueled.

          Second, you are correct, it takes power to energize the idle solenoid. The 12v circuit is created by the alternator, which is built into the flywheel. That part has never been a problem, it is the rectifier (like an electronic voltage regulator) that goes bad. If you are not getting at least 13 + volts to the battery with the engine running, that idle solenoid will not work reliably. I have been dealing with both Wisconsin engines and Onan engines for years, first thing I check is the rectifier.

          I still have one gasoline powered Trailblazer, but with the OHV Subaru/Robin engine, it has its own problems.





          *** Disclaimer ***

          As I have no wish to toy with anybody's life, I suggest you take this and all other posts with a certain amount of skepticism. Carefully evaluate, and if necessary, research on your own any suggestions or advice you might pick up here, especially those from my posts, as I obviously haven't the skill and experience exhibited by some of the more illustrious and more successful members of this forum. I'm not responsible for anything I say, as I drank toxic water when young.

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          • #6


            I sure want to thank u for this reply, I went through everything u stated, alternator was doing its job, I had 14 volts at the battery when the engine was running. I did find out what had happened after getting the unit repaired. The customer had overhauled the carb and messed with the throttle idle screw so it was low 1800 rpm at idle. Also messed with the throttle solenoid, so both were out of adjustment. What clued me in to the correct assessment was the fact that having 14 volts and connecting the solenoid directly to the battery it would still not after pulling in, hold the throttle at the idle position. Then figured it had to be out of adjustment. After reading the manual, adjusted both and the idle rpm to 2200, everything worked as it should after that. Let the customer test the weld output, it worked like a champ. Customer was really happy. I have repaired three other machines not the same model one was a Trailblazer 350D had a diesel engine on it. Had a problem with not welding at all, would not increase in rpm at all. The guy tried to repair it himself, after spending $1700.00 on new pc boards and a throttle solenoid and still not working, he brought it to me. I checked the old boards and the old throttle solenoid, there was nothing wrong with them. There are three boards in this unit and the board he did not replace was the problem, had one blown diode, I replaced the diode, put the machine all back together with all the old parts and the machine ran beautifully. one diode cause the whole problem.
            sigpicJeff's CNC Plasma Cutting

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            • #7
              Here is the board I repaired.
              sigpicJeff's CNC Plasma Cutting

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              • #8
                Most of these small engine drives tend to set idle speeds on the high side, mainly to reduce the reaction time when an arc is struck. If it's low, it might affect the idle solenoid, I don't know, I never tried lowering the idle speed from factory specs. Logically, it would seem it would reduce voltage slightly, to the point where the solenoid operates erratically, which it was when you got it.

                I would mention that anytime you take the carb off on these engines, the intake manifold has to come off, you might as well adjust the valves at the same time. Little valve cover plates are under the manifold, it is easy to do, and the valves do go out of adjustment. A little hack -- if the manifold gaskets look to be in good shape, and you don't have a new set on hand -- reinstall them each on the opposite side. This works the majority of the time.

                What Mac said about the fuel pump is true. Electric pump is more reliable and consistent than the vacuum pump. I believe at the higher end of output range, running the pump off vacuum would rob power from the engine. I replaced the pump on my current 301G (OHV engine, but same fuel pump) with electric. When I use air-arc, dial is turned to max, I can run the carbon rods far better than before, even when it was brand new. However, under normal use, the vacuum pump should be fine, as long as there is no vacuum leaks in the pump or the hose


                Last edited by calweld; 02-06-2019, 06:31 AM.
                *** Disclaimer ***

                As I have no wish to toy with anybody's life, I suggest you take this and all other posts with a certain amount of skepticism. Carefully evaluate, and if necessary, research on your own any suggestions or advice you might pick up here, especially those from my posts, as I obviously haven't the skill and experience exhibited by some of the more illustrious and more successful members of this forum. I'm not responsible for anything I say, as I drank toxic water when young.

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