Hi All,
I have an old iMac12,2 that is on it’s way out. I’m looking at getting a new iMac but wanted to know if I could use the old one as an additional monitor? If you can, would this be the case even if the logic board died? I currently have a 24" Dell as my second monitor, but that’s shot too (lines everywhere).
Cheers!
For Target Display Mode to work, the internal Logic Board must be functional as it serves as the power source, video source and brightness control for the display.
Not only does it have to be functional, it has to be booted up and logged in. IE you have to use all the power of an Mac at idle to run a second screen.
Bottom line, it’s horribly inefficient way to do things and means you have to have a working iMac.
Something like this would be ideal if you’re keen to mod.
Damn, seems such a waste to have it sitting there doing nothing. It is currently fully functional, I just don’t think it will last much longer. Would be an interesting task to undertake that mod - I might give it a try 
This (along with my need for multiple displays and a hatred of non-matching bezels) is why I never buy iMacs for myself. It’s so frustrating that the Mac mini is so underpowered these days. They used to be great machines (options for quad core, discrete gpus) but now they are clearly entry level only. 
I am using 5x Dell (IPS business range) displays that are 10 years old on my Mac Pro that is 8 years old. In that time, I have clients who have gone through 3 24/27-inch iMacs each. It really is pretty wasteful to have to pitch the screen if you need a faster CPU or GPU. I have upgraded both on my Mac Pro multiple times.
Or alternatively, when another part in the iMac dies, the perfectly functional display is thrown out. In the case of 27-inch iMacs, this is usually when the GPU dies. Had a client recently dispose of 3x 27-inch quad core i7 iMacs as the GPUs had failed in all of them! 
Apple wanted around $1k to replace the video card, making it not economical to repair. It’s crazy how quickly we throw away hardware because of the ‘all in one’ desktop design. I get that the all in one is nice from a ‘neatness’ perspective, but it’s so environmentally irresponsible. Makes more sense on a laptop than a desktop for it to be ‘integrated’. Bring on the new Mac Pro I say!
Whoops. Didn’t meant to turn this thread into another ‘repairability and upgradability’ rant! 
I may have the chance to get my hands on one or two older Mac Pros in the near future when and may look at using one for a main machine as I really want 2 or more decent sized screens which I cannot do with my current MacBook Pro (15" mid 2012).
I’d be interested in feedback on the upgrade options etc. on them once I can get the model details. I suspect that they are about 8 or so years old.
Happy to help. Depending on the model there are various options. A Mac Pro 4,1 is the minimum for legit macOS Sierra support (with a 5,1 firmware flash) so hopefully they are 4,1 or newer.
Unfortunately it looks like they are only 3,1