Equipment problem.

My 20D / Sigma 500mm combo has recently developed a fault and I was wondering if anyone on here can offer any advice. When the 500 is mounted on the 20D body I cannot change any settings. When a button is pressed (as in the iso button/ af button/info button etc) what normally happens is the LCD shows the setting screen for long enough to change settings. However on mine it won't stay on to allow things to be changed, it just flicks on when pressed and immediately goes off. I have tried the body with several other lenses and even with extension tubes etc and there is no problem. I have tried the 500 with my other body and again no problem so it appears to be specific to the 20D/500 combination. I've tried cleaning the contacts on both the lens and the body but that hasn't made any difference. Any thoughts?

What about the 20D with other lenses? Could just be a problem with your 20D.

Clean the contacts on the lens and the body with a pencil eraser. Be careful not to get any rubber crumbs in the body. Usually works for me.

Robert what is happening is the weight of the 500mm is pulling it ever so slightly away from the contacts of the camera,Check if the screws holding the lens plate on the camera or the lens connection plate on the lens are tight.
If you connect the camera and lens and then look into the sky with the lans check to see if you can change settings or put the lens on the floor standing on the lens hood ,this way there is a full contact.
Good luck

The way you stated the issue, I gather the 20D & 500 used to work and now don't. If that's correct, it rather eliminates any issues with Sigma which was my first suspicion because in years past when I used Sigma, I had numerous issues with them, one of which was communicating with one body but not the other.

Since you said the 20D works with other lenses and the 500 with your other body, it doesn't seem like contact cleaning would be the issue but could still be I guess. Seems to me Avi's answer makes the most sense. Quite intuitive too. Not sure I'd have thought of that but I agree that it sure sounds the most likely. Do post a reply when you have it solved so we know. Good luck.

Thanks all for taking the time to reply with some useful suggestions. John, you are correct in saying I haven't had any problem with the 20D/500 combo in the past and have used it without incident for over 5 years. I will take another look at it when I get home this evening and will post the outcome.
Thanks again
Robert

Problem solved. I checked the fixings on both lens and body but all seemed secure, no loose play / screws. Tried Jim's pencil eraser and presto it worked a treat. I had previously cleaned the contacts with a lens cloth but hadn't tried the eraser before so thanks Jim.