Century 4 AP oscillation Help!

[Tx_Aggie]

Interestingly I just saw the post below on gfc-500 oscillations. 
 

The original Century 4 autopilot in my 1980 M20J has been rock solid since I acquired the plane 3 years ago. About a year ago while in altitude hold mode, the plane dove deeply out of no where. I killed the ap got back to level flight at ~9000’ and reengaged altitude hold. About 5 minutes later it started bobbing up and down chasing 100’ above to 100’ below 9000’, annoyingly. It continued to happen for me almost on every other flight. During annual at Don Maxwell, they could not replicate the issue, but as soon as I picked up the plane and flew it back home - it started to oscillate only it would dive down hundreds of feet instead of just a little bit. Now, sometimes I can’t even engage altitude hold on the autopilot control box. A work around I’ve discovered is just leaving it in ATT mode and keying the UP/DOWN toggle periodically to maintain level flight but this is not ideal obviously. Heading/Nav/ATT mode all continue to work perfectly. It’ll fly approaches like it’s on rails every time, I just can not figure out the altitude hold issue. Since this is a legacy autopilot, I figured someone else must’ve had this issue before too in the last 40 years!

 

I’m open to any suggestions or thoughts. Thanks 

[Bob Weber]

I can help you get this fixed, visit my website to see how I work. webairconsulting.com and email me at bobweber@webairconsulting.com.

There are some voltage checks that can point to the problem.

[jetdriven]

Sounds like the altitude chamber is faulty

[Jake@BevanAviation]

Depending on the dash number for the flight computer it could have the electronic transducer assembly which is known to fail for alt hold functions only.

[Tx_Aggie]

On 9/30/2023 at 12:40 PM, Bob Weber said:

I can help you get this fixed, visit my website to see how I work. webairconsulting.com and email me at bobweber@webairconsulting.com.

There are some voltage checks that can point to the problem.

Hi Bob - I tried to send an email to you yesterday, looking forward to hearing back from you. 
thanks

Matt

[Bob Weber]

On 10/2/2023 at 2:33 PM, Tx_Aggie said:

Hi Bob - I tried to send an email to you yesterday, looking forward to hearing back from you. 
thanks

Matt

Matt, did you send it to bobweber@webairconsulting.com?

You can visit my website webairconsulting.com there is a link at the bottom of the first page.

You can also call me 9am-5pm Eastern mon- fri 616 822 1999.

Incidentally, My Cousin, Lieutenant General Joe Weber, is a name you may recognize as an AGGIE class of '72.

I lived in College Station in the late seventies building prosound systems for discos. GIG EM!

Give a call.

Bob

[Tx_Aggie]

On 10/4/2023 at 9:12 AM, Bob Weber said:

Matt, did you send it to bobweber@webairconsulting.com?

You can visit my website webairconsulting.com there is a link at the bottom of the first page.

You can also call me 9am-5pm Eastern mon- fri 616 822 1999.

Incidentally, My Cousin, Lieutenant General Joe Weber, is a name you may recognize as an AGGIE class of '72.

I lived in College Station in the late seventies building prosound systems for discos. GIG EM!

Give a call.

Bob

Hi Bob,

yes I emailed that address. I tried again just now. I also called the number on your website and left a voicemail. Did you not receive that either?

 

[Bob Weber]

Matt, I did see your emails when I looked back. I will give you a call today, my apologies for the delay.