That temperature control dial is the bane (plight, pest, nuisance) of many Sprinter owners. It's not so bad in the summer and winter, because you're most likely to just leave it set for coldest or hottest all the time. But in the fall and spring, it can be a real PITA.
You start off during the day when it's warm and the setting is on coldest, then the sun goes down and the temperature drops, so you rotate the dial it's hottest setting, yet the air temperature coming out of the van remains as unaffected, open-vented air. You can drive all night and it won't get any warmer.
I've found the only way to get it hot again is to shut the engine off and let it sit there until the engine temperature cools down a bit. 10 or 15 minutes (but I've never done any scientific methods for it), and then start it back up and let it idle until the engine reaches operating temperature again. If you turn the fan on and it blows hot for more than a minute or two, it'l stay hot. Just don't touch that dial again.
Many people have just had their ATC unit replaced with a new one (OEMs are Airtex, Wells and Hella).
eBay $322.33 ($396.99 from Rock Auto),
Airtex or Wells from Europarts $489.87, $444.09 from Berry (now Toliver) Dodge in Corsicana, TX.
One option is to send yours off for a
rebuild for $99.
For an extra $100 (plus another $100 core deposit), they will send you one that's
rebuilt already, then you send in your old one and they'll refund the core deposit.
Just don't buy a used one on the cheap and expect it to work. They will ALL fail for the same reasons. They ALL did within 3 or 4 years, for the same reasons. The rebuilt ones won't fail, though and if it does he'll fix it for free. The bran news ones manufactured after 2007 aren't likely to fail, either. But any original likely has already failed, so a used one won't be any better than the one you've already got. Of course, if you can get a used one for really, really cheap, you can send it off and have it repaired. That's been my plan now for 5 or 6 years.