I have a 1993 C3500 with a 6.5 turbo diesel. i bought last week. I did alot of maintenance to it getting it running.
New fuel filter glow plugs change oil and transmission fluid (both filters)
i took it out pulling for the first time and man it was weak i pull a 7500 pound 5th wheel and its so weak i start on the bottom of a hill 55 and at the top im doing 30.

I started losing overdrive and all i had to do was stop restart the truck and its back.

Ok now the transmission fluid was burnt but the pan and magnet were pretty clean.

I start looking and i find the cold advance solenoid unplugged. I search and find conflicting information. Some people say it causes all of my problems. today i jumped out the sensor and my truck ran great but its in cold idle. i pulled my 5th wheel camper back with much more power. If i replace this sensor will it fix my problems or do i need to look further?
Also my 1st to second shift is really hard and 3rd to 4th (when i have it) is way early.