If your chassis uses vacuum powered controls for your Heat/Ventilation/Air Conditioning (HVAC) system, then a Diesel probably uses an Electric Vacuum Pump to provide the vacuum for those controls. This is the most popular automotive vacuum reserve tank/reserve reservoir on the planet.
There are two models: Dorman 47076 has a built-in check valve. Purpose of check valve is to retain maximum vacuum in the reservoir if the source vacuum (which I think is a pump in your case) reduces or stops. This happens in gas engine vehicles when the throttle plate opens. Dorman 47077 has no check valve and at least with gas engines it needs an external check valve. If you have such a reservoir and it's cracked, one of the two Dorman models should fix your problem. 47076 has two connectors and 47077 has only one.
Your pump should run only occasionally. If it runs continuously or cycles often, something's leaking.
On gas engine vehicles, the problem is often BEFORE the actual HVAC control panel in the dashboard.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB