Kx Auto focus not functioning

Hello there, I'm a newbie to this forum and I have a problem.
My K-x will not autofocus with the DA16-50 lens. Press the shutter release half-way down and nothing happens. However, all the other Pentax lenses focus fine on the K-x?
If I put the DA16-50 on a K-5 it works fine and there is no problem with autofocus. This problem definately has me scratching my head - hope someone out there can offer a resolution.

Are the other lenses that focus fine on the K-x SDM lenses or screw-drive AF ones?

Just wondering if that could be the key to the problem.

The lenses have an autofocus coupler (screw drive). For an example, I have tried the DA 50-135, DAL 55-300 and DA 12-24 and they work fine.
The DA 16-50 worked at first, and to my recollection has not suffered any abuse.
When I examine the contacts on the K-x I can see no difference compared to the K-5, so suspect not damage there. The same goes when comparing the 16-50 lens to other, the drive coupler and contacts all appear the same.

It looks as if the SDM in the 16-50 is not working. Unfortunately if SDM fails it does not revert to screw drive.

You coud try cleaning the SDM contacts both on the lens and in the camera mount (two brass contacts). Otherwise you will have to send it in for repairs. I hope it is still under warranty.

The lens with the problem is the only one with SDM. The K-x would use SDM if available. It can't 'fall back' to screw drive.

The fact that the lens works on a K-5 would point to the problem being with the K-x.

Clean the contacts on the K-x camera body; try again.

Thank you for your response Archie. I do not understand how the 16-50 would work on the K-5 if it were faulty? I will clean all the contacts, as you say.

The DA 50-135 is SDM and works well on the K-x.
I do not understand why the 16-50 will not?
I have re-cleaned the contacts and suffer with the same results.
Thanks JohnX your ideas are appreciated - have you any further suggestions?

Best of luck with the contact cleaning. The relevant contacts are the pair made of brass just inside the camera mount. They were originally the PZ (power zoom) contacts in the days when Pentax made Power Zoom lenses but are now used to power the SDM motor.

That is very odd. The other possibility is that if the battery in the K-x is low it might not produce sufficient voltage to drive the DA16-50. If the focusing mechanism in the lens was a bit stiff this might explain it. You could try manually focusing from infinity to close up a few times to loosen it then try again with a freshly charged battery.

Interesting point - what sort of batteries are you using in the K-x?

Apologies. Missed the 50-135, and that it works ok.

The suggestion about enough power is a good one to follow, but I note also that the 16-50 '...worked at first...', which suggests something has changed. Different batteries perhaps?

Don't miss what Archie said about cleaning the contacts just inside the camera mount.

Also, look at lens fit. Is the lens lose or tight when mounted compared to the 50-135? Maybe it's just not making proper electrical contact. I've seen solutions to similar problems where the lens is rotated back a little once mounted to ensure contacts are properly aligned.

I had to replace the K-x batteries last week, Helios, no change. The battery indicator indicates that they are good, but I will install a new set just in case.
John Riley - I have been usinf Energizer Lithium batteries, as they normally get me through 2,000 or more shots.
You are right on JohnX, I did not clean the contacts INSIDE the camera mount, but will do so. Will also check the lens fit.
Hey Guys, thanks for all the helpful response. The wife is calling my name, so must run but will let you know the results ASAP.

I believe the focus motor on the K-5 is a bit stronger than previous Pentax cameras. When SDM fails it tends to go slowly over time, maybe that extra power is enough to keep the 50-135mm focusing on the K-5.

I have cleaned and recleaned camera & lens contacts, installed two different sets of new batteries and checked the lense fit to the K-x, which is relatively snug and certainly just as tight as the K-5. While the 50-135 does auto focus on both the K-x & K-5, the 16-50 will auto focus on the K-5 but when the shutter release switch is depressed half-way or fully on the K-x the lens only cycles between infinity and 0.3m and not auto focus and not permit a photo to be taken.
I wish I could determine if the problem lies within the 16-50 lens or in the K-x?
Thanks everyone, for all suggestions. If anyone else should recognize this problem or have a suggestion, I am certainly willing to listen.