Update on solenoid for Onan generator

As some of you may remember, my generator works, except that the shutdown solenoid is broken. It would run great, but had to be stopped by hand (shutting down the injector pump). Which was not nice since safety shutdowns did not have a chance to work.

The price of the replacement solenoid from Onan was $390, which nearly gave me a heart attack.

I bought a different solenoid from FW Murphy for $44 and received it today. Right away I noted that the return spring was too weak and that there was too much unnecessary pull depth/travel. I decided to exchange some depth for strength and tried to put in the spring from the old solenoid, and to pull the rod a little bit. FWMurphy solenoid had an ability to attach a threaded rod to the back to arrest excess forward travel. The spring from the old solenoid was too small diameter, so I had to widen it, using pliers judiciously.

All solenoids have greater pull strength if the rod is partially pushed in, that's why exchanging depth for strength works.

The spring also had to be clipped a little bit because the solenoid would not pull in all the way due to stopping action of the fully compressed spring.

Fortunately, in the end, it worked out: just the right amount of travel, just the right pushing force etc. I need to make a little fixture to adapt the mounting holes in the new solenoid, to the holes in the engine housing, but it is no big deal and I will do that tomorrow. I am quite hopeful, as it stands. For fastening, I will use Nylocks throughout.

Unlike the onan solenoid which has only two terminals, and internal disconnect for the heavy pull in coil, the FW Murphy solenoid has three terminals, so that there are separate sources of current for the pull in vs. the hold coil. Supposedly, the pull in coil attaches to the starter, and the hold coil attaches to the wires that were used for the solenoid originally.

I do not like this because the pull in coil needs to work for barely a second, whereas cranking will go on for seconds, possibly overheating the solenoid. I will think about what to do with it. It is workable though. I will place an inline marine switch on the wire to the pull in cord, so that, when prolonged (relatively to the burn up time of the solenoid) cranking is required, the solenoid would not overheat and burn.

Or, alternatively, I may add a push button switch to momentarily engage the solenoid while the engine cranks.

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This is probably not a big problem. First, it probably HAS to be this way, as the engine won't develop oil pressure until it starts, so the hold coil won't have power while cranking. Second, the battery voltage will probably dip a bit during cranking, maybe as low as 8 V, depending on the cranking load. That will save the solenoid.

Maybe you could also just get rid of the starting connection entirely, and have a button or lever you hold manually-mechanically when cranking, and then let go of when the engine starts, and the hold coil can then hold it in. Then, the pull-in could could never burn it out.


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Jon Elson

That's my plan exactly, to have a pushbutton switch for the pull in coil. I would depress it when necessary, but it would only get energy during cranking.

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Why not wire the hold coin normally and connect the pull-in winding the gen's starter switch?

Reply to
Jim Levie

Just a thought - that solenoid could have been intermittent before you bought the unit, and the army figured it was easier to scrap the unit than try to diagnose and fix it.

Or, alternatively, you could rig a microswitch at end of travel to cut the pull-in coil power off when the plunger hits home, just like the expensive solenoid had built-in...

Or, alternatively, get a timing relay of some sort as a one-shot timer to cut the pull-in coil power one half second after power is applied...

Get creative - but after you settle on a workable design you have to think through the consequences of all the failure modes you can imagine, to make sure if something goes wrong you don't burn up the solenoid. Or the whole generator, and the house.


Reply to
Bruce L. Bergman


It would have to be a 50 amp rated switch. I could not find one. Making one properly would be a pain.

Murphy sells such a relay, maybe I will ask what the price is.

Good point.

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After more tinkering, installed the FW Murphy solenoid today. It does its job: the engine starts and stops every time and the solenoid holds while the engine is running. Now I need to clean up my act: I used solid 10ga wires (a big no-no due to vibration), I need to close the hole made for old solenoid so that the cooling air circulates properly, I need to install a pushbutton switch, etc. All of these are things that are necessary, but not for immediate generator functioning. I hope to finish it, perhaps sans the pushbutton (would be unlikely to arrive), on this Sunday. Will also clean the engine etc and make final pictures.

Thanks a lot to all.

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