International Transtar Mode Door Stuck

    This post concerns International HPV trucks, (4300, 4400, Durastar, Transtar, etc.) A fleet customer came in with a Transtar tractor that wouldn’t change HVAC mode. The fan and temp controls worked however, so the first thing I looked at was the mode door motor. Both the temp door and the mode door controllers (motors) are on the left side of the HVAC airbox under the dash.
    The temp motor is on the scroll box containing the blower motor, and is easy to see. The mode motor is higher up and toward the firewall, kind of behind the wire harness for the fusebox. By switching the mode control and watching the motor’s output, I could see the motor was working. I could also see that the motor’s coupler that connects to the door was cracked and not turning the door. This isn’t uncommon, and usually means there is a problem with the doors. It is necessary to pull the scrollbox out and find the problem before putting a new motor on. To remove the scrollbox just remove the fuse cover, unplug the temp door motor, remove a few torx head screws and the scrollbox will slide out.
    Once the inside of the airbox was accessed, it became obvious what the problem was. Somehow the door gaskets on the edges of both mode doors had melted and the sticky residue had stuck the doors shut. Oddly enough the gasket on the temp door (in the scrollbox) was unaffected. The gaskets had the appearance, consistency, and stickiness of tar, and it took some work with a scraper and solvent to get it cleaned off the doors and the housing. I put a new motor on, then moved the mode from one setting to another to make sure the doors stayed free. The doors also didn’t seem to leak badly, so I wasn’t worried about letting the truck back in service without the seals.
    Soon after this repair I had an airbox apart in a 4300, which has the same HVAC system. The gaskets here looked OK, but when I squeezed one a little in my fingers found that it was also sticky and tarry. Obviously the seal material doesn't stand up to the environment it lives in. I have looked online and in Navistar’s tech bulletins, but haven’t seen any reference to the problem. Maybe the truck with the bad seals had overheated or something, but I will be on the lookout for this condition in the future.