Electronic 4WD failure

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Hi Folks, new here so hope this is in the correct place Smile

I've had my Fourtrak EL since new back in 1989, and it was in daily use until 3 years ago when it went into dry dock.

I've had a problem now for around 8 years whereby the electronic 4wd has failed. I've replaced the solenoids twice but its still failed. In the end I took the unit apart, and tested with a vacuum pump. I found that the I could only just engage the 4WD with a very low pressure vacuum, even after taking the unit apart and giving it a good clean. I did hovever find it relatively easy to "pull" the gear in by hand.

My questions are as follows.

1. Is this a common fault?

2. Am I going about fixing this wrongly?

3. Does this fault mean that I need to change the transfer box?

4. If I've got to got to that length, can I retro fit a manual 'box from an earlier model?

Many thanks,

Matt