probably something on the circuitboard of the dash itself if you're certain the power supply you tried was good. I've got an '84T one I'll sell if you're interested. 70$ plus shipping (shipping runs about 12-13 for priority mail). email me if interested, spot@zdriver.com

 

I don't know of any way to 'test' either the dash or power supply. I usually just take them apart and examine them. I've only had minimal success in repairing dashes, as it's time consuming and I dont' have the time to spend tracing all the circuits down. The power supplies are less time consuming so I usually have plenty of those on hand that I've already reconditioned.
Please send me your email address so I can send you payment/address info.
I'll need your zip code to determine shipping costs.

my email is <a href=mailto:spot@zdriver.com>spot@zdriver.com</a>

 

oops, sorry bout that, i posted that reply before i checked my mail.

 

to run diagnostic:

turn the key to accessory
pust both trip odometer buttons
while holding buttons, turn key to "on"
release buttons
your dash should start lighting up and testing itself, if not, i would check the connector inside the power supply box. if that is whats bad, its easily fixed. if not, go with what spot says.