Gear doors not closing on gear extension.
 

We've got a D Model (engine driven hydraulic pump) and the gear doors are not closing on gear extension.

Ground test: battery off, hand pumps gear doors open - battery switch on, hand pumps doors closed, checks ok.

Any ideas on troubleshooting this ?

Thanks!

I have the same model 337 and I have had the same problem. One thing I did was to be fastidious about topping off the hydraulic fluid. Seems that bubbles can be found in the reserve. Cheap fix, but it takes many gear cycles to get out the bubbles. Of course you can use the hand pump instead of swinging the gear. You can also, with the front engine running on the ground, put the gear lever in the down position a few times and that should clear the air out as well.

Also check the connections going into the hydraulic pump. They tend to get loose.

And uplock microswitches
 

It's been a few years, but as I recall,
the system is designed to prevent doors from closing
until all the microswitches confirm the gear is fully locked either up or down.

I vaguely recall an issue like that many years ago:
The micro switches that detect full uplock position were not fully closing,
so it would prevent the gear doors from closing.

Q-Does your uplock light come on? I THINK that requires all uplock switches closed.

Open the doors on ground w hand pump.
"Circuit open (power off), doors open"
"Circuit closed (power on), door close"

Look for the uplock microswitch for each leg.

Press each w finger for affirmative tiny click.
Spray (FAA certified and approved) brakeleen, or equivalent into and around switch.
That will clear (FAA certified and approved) dirt / snot.
Until you feel a nice clean little click when you press.

The airframe uses common industrial (aka 'digikey) microswitch switches.
They are easy to find replacements if needed.

Sounds like it's in the microswitch loop. It's daisy chained so any open connection will not allow the doors to close. Check all the micro switches and inspect each connector very carefully. A loose connection on a terminal or broken wire are sometimes hard to see.

I had this issue as well when I first bought my T337D skymaster. The switches would close on the ground after following the manual procedure for setup but not in the air. The issue is that the airflow over the back of the plane in flight creates negative pressure that forces the gear away from the switches. If these don't all close the gear doors won't close either. A slightly more aggressive adjustment on the switch position fixed the problem. I haven't had it since. One other issue I have consistently is that the hydraulic mule used to swing the gear on the ground would suck hydraulic fluid out on shutdown. So every year I remind the A&P to top up the fluid after annual.

Had this problem with our E model years ago and it was the handle switch on the bottom of the power pack