Hey guys - I picked up a 1993 Dakota Cub Cab V6 back in April, and it's been a project truck/daily driver since then. I've fixed some minor issues, mainly replacing the water pump, master cylinder and the idle air control valve. Outside of the minor issues (and general ugliness, see the pictures), she's been pretty reliable - I can trust that when I put the key in, the truck will start and go where I point it.

Last week, I noticed while driving to work that the cool air from the A/C would blow from the front vents for a few seconds, and then it would divert to the defroster vents pointing at the windshield....then it would switch back to the front vents. It did this for a few days, and after I really started paying attention, I noticed that when the truck is idling, coasting, or with a very mild throttle input, it comes out of the vents like normal. If I press the pedal more, like when starting from a stop, or going up an incline, the air diverts to the defroster. I have manual hvac controls, and when this is happening, it doesn't matter what button is pressed for the output.

Any ideas? Possibly a vacuum issue, or is the control switch going bad? I appreciate any input!

 

There is a check valve in the vacuum lines. Replace it.

 

Update: I did buy a generic replacement vacuum check valve, but the ends were not the same diameter as the old one, so it didn't fit snug into the vacuum hose. However, while finagling and pulling on things, the other end of that line broke loose where it enters the firewall. The line that actually goes into the dash is plastic, and maybe 1/8 or 3/16 in diamater; the rubber vacuum line from the check valve was larger, and there was a clear hose in between them - an odd arrangement, if you ask me. The clear hose had gotten brittle and cracked, I expect I was losing vacuum there even before it broke.

So, I've now slid the rubber vacuum hose over the plastic line that goes into the firewall, and wrapped it up with electrical tape...and it seems to have solved the problem, for now. It's an ugly interim fix, but at least I can make the two hour drive home tomorrow with the air conditioner actually blowing into the cab. I'll come up with a longer term solution this coming week, when I'm at home.

Thanks for the lead!

 

thanks i have the same problem, ill check the check valve and see if it fixes it.