secondary monitor

Howdy , oohlolaw… you could try the following:

1-reset smc/pmu*

2- zap pram 4x on startup… restart your iMac, hold down Apple+option+p+r together for 4 chimes/bongs, then let go of all keys and let imac startup as per usual.

3- if you hold down the ‘D’ key on startup that’ll get you apple diagnostics

4- start up in safe mode ( hold down ‘shift’ key on startup until you see ‘safe mode’ on screen) … it’s slower to startup than usual startup so you have to be patient

that’s all i got

John B

* here’s how to do that ( see link) https://www.macworld.com/article/224955/how-to-reset-a-macs-nvram-pram-and-smc.html

It may be as simple an issue as your old display in not compatible. I would also try doing two things.

On the 2013, please create a new account and call it TEST ACCOUNT, log into the account and see if the external display works. If it does that indicates a software issue on your normal account that needs troubleshooting.

The second option is to test another external display and see if that works okay, if it does then you know the problem is with your old display.

The issue on the 2013 to external display also maybe due to the adapters you use. Please use the ones Apple sells as they are high quality and the quality of the cabling and adapters can make a difference.

Does the 2013 iMac's display work normally or do you want the external because the iMac internal display has failed?