Sending unit and gas guage wiring

[62stepside longbox]

Hi everyone, the other day I tried to hook my sending unit up to my gauge. I tested it so I know the sending unit is good. First I grounded it then I hooked the brown wire that comes from the gauge to the to sending unit. Then I turned the key so it would get power and "zap" a small cloud of smoke came from the sending unit and fried it. It no longer works now. So now I got another sending unit and tested it and it works. Any ideas on what went wrong? I don't want to fry this one. Thanks