how to replace heater control module in 98 24 V
 

My truck ONLY blows out the defrost vents. It is winter,.. (so I don't know when it began doing this,..I leave it on defrost in winter because that's what matters). I went to switch it on vent or ac so it would blow out the front vents, but nothing changed. I tried all other setting but it just stays on defrost. My first thought was the vacuum line, (I just changed my fuel filter 2 weeks ago) it was loose, so I checked (it had vacuum) and plugged it back in. Hoping that would fix it. But nothing changed. I don't know anything else to check other than to remove the heater control and replace it (or at least check it out). I have NO IDEA how to pull the heater control out, but I suspect I might have to remove the dash, which sucks. I've never had this dash out (that I can remember) Anybody who knows, will you please chime in if you have any direction? I love this truck but like to keep everything functioning.

Maybe something is blocking the blend doors inside the dash?

I've got the same issue, and as soon as it warms up enough for me to work on it, I'll be replacing the blower unit and the resistor pack. While I've got the blower out I'm going to see if I can clean out everthing. I've also got a cheap borescope that I'll probably use to see things a little better.

Your issue is likely a vacuum problem, not the heater control module. The default mode for these trucks is defrost when there is no vacuum to the blend doors. Since you already checked the vacuum line at the fuel filter, try these steps:

Check the vacuum line at the firewall – Follow the vacuum line from the engine bay to the firewall. Look for cracks or leaks.
Check the vacuum reservoir – It is usually under the battery tray. If it is cracked or disconnected, the system won’t work right.
Check the heater control panel – If the vacuum lines are good, you can remove the control panel to check for vacuum at the switch.
Pull off the dash bezel (it pops out).
Remove the screws holding the heater control in place.
Check for vacuum at the back of the switch. If there is none, the issue is before the control.
If vacuum is getting to the control panel but the vents don’t change, the switch may be bad. If there is no vacuum, you have a leak somewhere else. Let me know what you find.

I have it under control and fixed. It's mostly an eyesight problem. Unfortunately, with age, I get stupider and blinder. I had popped off one of the little nubs when I changed the fuel filter, and didn't notice it (even though that's the first place I looked for a leak in the vacuum line).. And I looked right at it quite a few times trying to track down what was going on. Eventually I had the right pair of reading glasses on and a strong flashlight, it was staring me in the face. I sure wasted a lot of energy when a better light and different pair of reading glasses would have done the trick. Ya, I wasted my time popping the bezel off and everything. I ran a vacuum line from the main vacuum to the control module and saw it was all working,..which made me get a stronger light and different pair of reading glasses.