How do I know if the TIU is worth saving?

If no sticker, they would be Rev G, the original.  MTH did correct a deficiency in these.  You could open them up and check all connections, or you could send them to a smart tech like GGG for a quick check.  Unless something simple, I wouldn't spend much repairing them.  

If you're powering them through AUX, try powering thru FIXED 1, and vice versa

Parts, it is all about the parts!   

True, but other than the case & screws, what parts would you re-use?

For one thing, the binding posts are useful items to have.  I have a couple of old dead TIU's, I've already robbed a couple of the binding posts.  You'd be surprised how often people smack the posts and break them off.

Hang on.... When fixed 1 is powered  (or an aux power supply is used)  does the red led on top of the TIU light up? Does it blink? and if it blinks how many times?

Gregg, unless broken, the red light always lights when the TIU is powered, regardless of whether thru AUX or FIXED 1.  It blinks when power is so applied.  It blinks a number of times equal  to the TIU's address.

 Yes Robert, That's my point... unless broken. Do these tiu light up and if they do how many blinks? perhaps the remote is talking to the wrong tiu?

That's only the first step, next is do they put out a dcs signal?

I still have my original G

Excuse me.  I thought you were asking a general question, as opposed to a query to desertrailer.

I actually have 2 G's.  I plan to use one, plus an oriignal issue remote, in setting up a layout for my grandson.

desertrailer, Gregg's questions are good ones.  Forum contributors may solve the problem, but we have to get info one step at a time.

Yes when aux power is applied the red light comes on...it blinks once. It is the only TIU powered. When I attempt to address it via the remote to see what upgrade it has...it does not find it. I have tried to assigned it TIU #4...(I only have one TIU on the layout and it is #1) Same thing happens when I try to upgrade it via the computer..(USB plug to Serial Port)...program can not find it?

It seems the remote( tethered or not ) & computer can't find it and it is a one blinker. tiu #1. You do have have tiu #1 added to the remote? I think you're beat.   You mention you had 2 faulty ones  , same thing with the other one?

Hard to believe that both would go bad at the same time.  Did both fail at the same time, or did both work when they were put away?