RPM (non)sense loss

[hefele68]

Boy I am neck deep in problems with my home standby generator. Thought I'd give this forum a shot...I am working with a Generac 17kw model 0058850, serial 6277915. I originally had an oil pressure problem, found the head gasket on one cylinder was blown or about to blow, replaced both gaskets still not oil pressure... troubleshot to an oil pump. Brought the unit in my shop, pulled the rotor/stator and accessed the crankcase. There lied all the pieces of the gear. Replaced pump. Thought I could fire the motor w/o the r/s but found out that wasn't happening. Replaced that then I had an rpm sense loss. Verified the starter's ok, battery is brand new, all wires tight. Gas pressure good (I work for a propane company hooking these up regularly.... that was the easy part). I replaced both coils and cleaned the flywheel magnet with some plumber's mesh, The choke is closing off under the air filter cover. I have not checked my compression. My suspicion at this point is the fuel servo/controller. The armature moves once only, sometimes. on other units (that work) I see they jerk one way, then settle back to a "run" position. I'm sure there's a way to test this set-up. A link to: or the testing numbers/technique would be great if anyone knows? I tried tinkering with the armature and I can get the unit readout to do an "over crank" fault vs. an rpm loss...either way it fires but for only like 2 seconds at most. Go easy on my, I'm learning as I go. Any help or attempt at help would be appreciated. Thanks!

[Chris]

Regarding the RPM loss, does the unit actually crank, or is it not cranking at all when you are getting that alarm? You mentioned checking the starter and battery and things, so that's why I ask.

The servo (stepper motor) will only move to the correct position once until the engine starts, and then it will move to where it needs to go on these types of units. The Evolution units act differently, but also use a totally different setup.

[hefele68]

Yes, it cranks and ignites. In checking the "#18" wire to ground from the coils, my voltmeter needle (yes, I'm using an old UEI analog) doesn't flinch. Shuts of right away, always.

[Chris]

I'm not even sure what you're measuring with the meter, so I can't say whether that's a good test or not, but the coils would be the first place I'd be looking...

When you did the head, were you near the coils when removing parts and pieces? f you damaged the 18 wire on either coil, or it was yanked off, that would be your issue.

[Sammtillman]

I think your talking about the mixer assembly. There are two Venturi on the mixer assembly. One Venturi is open for idling. Can inspect the linkage. Don’t open up much when idling . Can’t see the throttle plates unless you pulled off the intake. I had a plate that broke off once. My dealer said the unit had been place for years close to a pool. I always pull out my manometer and check the pressure drop across the demand regulator. If you are having a mixer issue it’s not creating enoug vacuum to pull the regulator diaphragms in. The pressure across the regulator is less than an inch w.c. When it is open. A restricted air filter is a good example that will cause the regulator not to open. You can also open up the throttle plates manually by hand but don’t open much your only wanting to idle. If you look at the two ventures you will see one that is always partially open . It does have an air choke which will. Open some on idle. There’s a lot of different mixers basically all work the same. If you do choose to do the manual operation disconnect the stepper motor first. You must be a pretty good mechanic to replace the pump. We changed the crankcase seal the other day and took us 4 hours.

[78buckshot]

He is working on a 5800 series, it's the older throttle body with the round air filter. Stepper should open the throttle to the full speed position for starting, as the engine starts and reaches a little speed, the stepper should partly close the throttle.

[hefele68]

OK buck shots post is where I'm at. I ordered and replaced the stepper motor, same (Yes, I am playing swap-tronics now) problem. On startup, the motor pushes the armature left, the engine fires then stops. The armature never pulls back any and the armature stays in the far-left position. Most times now it'll try to crank a few times before eventually quitting on RPM Sense loss, sometimes it'll go direct to the sense loss.

[Chris]

My vote again is still coils or damage to the coil ground wire (18 wire).