A/C blows through Defrost vents. on 2000 Ford F-250 Super Duty

A/C blows through Defrost vents. Fuse chart shows # 28 which serves the back-up lamps too, which are not working. Vacuum Pump (Source on Diesels) has no power to it. All pointing to fuse #28. Fuse good. Power on both sides. Any suggestions?

by in North Richland Hills, TX on June 01, 2011
Get Your Car Fixed at a RepairPal Certified Shop.
We certify shops based on expertise, quality work, and verified customer reviews
2 answers 1 comment
ANSWER by , June 01, 2011
ISSUE: Some 1999-2006 F-Super Duty and 2000-2005 Excursion vehicles built before 6/27/2005 and 1999-2006 F650/750 vehicles, all vehicles equipped with the 7.3L or 6.0L diesel engine, may exhibit a vacuum pump that is inoperative or running continuously. If the pump is inoperative, the climate control system will default to defrost mode and the 4x4 system may also be affected. ACTION: Install a new vacuum pump. Before replacing the pump, check for and repair any pinched or leaking vacuum line(s). PART NUMBER PART NAME 6C3Z-2A451-BA Vacuum Pump
COMMENT by , June 01, 2011
Brian, Thanks for your reply. I took a volt meter and with the motor running I had no voltage at the incoming wires to the vacuum pump. It's late here. Tomorrow I will use a jumper pair and apply direct power to the vacuum pump and see where it goes from there. My backup lamps not working and being assigned to the same fuse #28 makes me suspect an open circuit from th efuse box - out. I will let you know what I find and really appreciate your quick reply. Doug Potter
ANSWER by , June 02, 2011
Doug, it is generally a vacuum leak at some place. More common at the pump and lines. I've seen a few on the pass. side where sometimes people have accidentally kicked the lines behind the glove box etc. Thanks, Dan