I have a 2000 Lariat 4x4 with a strange HVAC problem. For a while after the heater or A/C is first turned on, everything works fine, but after about 15 minutes, the air flow starts to decline. After about 30 minutes, the air is down to a light breath, even with the blower on high. The blower itself is working fine - all speed positions work, and you can hear it trying to push air. The temperature control is also working - what little air there flows is the at the desired temperature. Sometimes flipping the selector around to the various positions (floor/defrost/panel/AC, etc.) seems to ge the air moving again, but usually it is only corrected by shutting the HVAC completely off for a while. It always seems to work fine after the truck has been off for a a few hours.

The best description is this: it seems like a door starts open but then gradulally closes over time. I don't think it is the blend door because I can still adjust the temp from cold to hot.

Does anyone have any idea what might be causing this problem?

Dave

 

Your condition sounds like an evaporator core freezing up. However, that shouldn't happen if your selector switch has been in FLOOR, VENT, or OFF since the compressor shouldn't be on in any of those positions. Any other position, then yes, freezeup is a possible explanation.

A sure sign of freezup is: Park the truck when it acts up. Is there a large puddle of water under it shortly (up to 30 minutes) thereafter?

Evaporator core freezeup is usually caused by a stuck cycling switch. Sometimes, an improper charge can do it.

Steve

 

Thanks for your quick feedback, Steve. I'll follow up on your suggestions. I do recall seening puddles of water under the truck after leaving it sit, but I assumed that was normal for a vehicle with A/C due to frost buildup. Is this correct? And if it were evaporator freezeup, would the puddle be in a different location than the "normal" drips from the A/C?

 

A small puddle is normal.

A really large puddle means that the evaporator core had a pretty good sized chunk of frost in there.

Same drain and spot as normal if it came out of the evaporator only. Often, a frosted system will have some of the hoses and the accumulaor also covered in ice, these will melt and drain wherever they can.

Steve

 

Thanks again, Steve. I think you've nailed the problem - there definately is a relatively large puddle of water that melts out after running the compressor - and, once the compressor is engaged, it doesn't seem to kick off. This explains why the system works fine as long as it runs on vent or floor, but that doesn't help much when the windshield is fogged or frosted! I also had the refrigerant topped up in the spring, so maybe they added too much...

I don't have a service manual yet - I guess I should get on line and order one! Can you tell me where the cycling switch is located (it's a 2000 truck with a 5.4)?

Again, many thanks for your help.

Dave

 

It screws into the top of the aluminum accumulator can mounted on the firewall, just below and towards the truck center from the plastic cover over the mega-fuses and the starter relay.

You will probably have to remove that cover and move some harnesses around to get to it.

There is a Schraeder valve under the switch so it can be replaced without loosing refrigerant charge. The switch should be available at just about any auto parts store for well under $20.

Steve

 

Thanks again, Steve. You've been tremendously helpful.

Dave