I've posted about this before:
https://www.thirdgen.org/forums/inte...ster-time.html
and it seemed like the answer was "get an new defroster timer/relay." OK, like I posted there, I have 2, one of which was out of my '87 formula parts car that I know was working correctly when I last drove it. It does the same thing, stays on for a couple of minutes the first time, and then more like 30seconds after that.

Ok, maybe I had 2 fail the same way. I bought another, supposedly good one... tried it tonight. SAME EXACT THING.

So what's the deal? There has to be something else that affects timing of the circuit besides the relay timer box. I've found information about the 4th gen ones turning off if voltage drops too low... well I wondered about that myself early on and tested that- no deal, voltage doesn't drop before it turns off, for that matter I just put a new alternator in it last week, and it still acts exactly the same.

Anyone have any clue what is going on? Anyone have any idea how this thing works? Any ideas what else to check???

 

There's definitely something else going on.

I had the bright idea of grabbing one of the timer/relays and showing it to a couple of the electronics guys at work. They actually checked the capacitor values... hooked up the thing to a scope and confirmed that it's working right.

Based on the wiring diagrams it's doubtful that it's a problem on the power/grid side of the circuit... I guess I'll start by trying to run it without the switch connected and see if it still stops early

 

I have not looked at the scematic, but I would see what your resistance is across the defroster element. If the resistance is too high, you could be building up excess heat and it may have a bimetal switch (like a self resetting circut breaker) safety feature to prevent it from cracking the glass. Bounce the ohm reading from the one in your parts car to the one in your driver and see if there is a difference. I have also seen the bolt that holds the lift struts to the body rust due to a leaking hatch; that may cause a grounding issue for the circut.