Sticky Solenoids

Pictured below is a pickup solenoid from a LaserJet 4100's Tray 2. It's in its normal, de-energized state.

And here it is stuck in its energized state, in the absence of any applied energy.

On the solenoid's frame, there's a tiny rectangle of foam rubber adhered with pressure-sensitive adhesive; it's there to quiet the solenoid's operation. The foam rubber deteriorates with age, and the adhesive bleeds through. The result is a sticky solenoid that will cause the clutch it's controlling to double-cycle. That can result in some quite mystifying symptoms. Here's a view of the ruined foam rubber.

HP doesn't break out this solenoid as a service replacement part, but it's not at all difficult to get a sticky solenoid working properly.

First, you have to remove all traces of the adhesive with lacquer thinner. (The adhesive is tenacious stuff, but it does come off. Be sure to get rid of all traces of it.) Wrap four thicknesses of electrical tape around the flapper and reassemble the solenoid.

The solenoid will work fine, but it won't be as quiet as it was. It will make an audible, though not objectionable, muffled 'click' every time it's energized.

This style of solenoid is widely used to control clutches, and they can all get sticky with age. The symptom this one produced in a 4100 was rather odd.

On a multi-page print run, every third page would be a blank sheet. No error would be reported, but the printer would stop printing, and just keep running doing nothing. Opening and closing the lid would cause printing to resume for two pages, then the same blank sheet incident would occur.

Looking in the back of Tray 2, I could see that the pickup rollers were double-cycling. 'Explained a lot.

The double-cycling didn't affect the first page's timing, of course, and it didn't affect the second page either. But it always caused the third sheet to enter the printer too early. Whenever a sheet is fed too early, a misprint (blank sheet) ensues. The curious thing is that there was no error displayed. Misprints are supposed to give '41' errors. It looks to me like the 4100 has a bug in its firmware, such that it can't interpret the particular condition I was seeing. It just gets bewildered, and goes into a 'do nothing useful' loop.

Any solenoid that sticks long enough to cause double-cycling of a clutch will cause trouble of some sort. Any time you have access to a solenoid of this type, take a minute to press its flapper closed and see that it doesn't stick.

When you encounter a sticky solenoid in a machine, and the machine has other, similar solenoids in it, examine them all. The odds are good that they'll be sticky as well. The printer this solenoid was in had a Tray 1 solenoid that was even worse for stickiness. I think there might be some environmental factor that contributes to the foam rubber's deterioration.

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