Does anyone have a diagram of how the heating ductwork on an 1984 pickup runs?

The heated air doesn't feel as warm as it should. I am almost positive that air is bypassing the heater core somehow, but I can't follow exactly how the air is moving. There is a sliding door to let fresh air into the cab on the passenger side. The temp lever on the dash flips a small door that you can see through the sliding door. With the lever flipped to "warm", it shuts off the cold outside air. I can stick my hand through the sliding door, reach up and touch the heater core. With the fan off, it is too hot to keep my fingers on it. With the fan on, the core feels cold and there is lots of cold air rushing from there, past the sliding door, up to the other side of the heater core, and on to the floor and defrost openings. With the fan removed you can see the heater core just inside the opening. I used compressed air to blow through the core from the fan side, and it blew leaves down and out the little sliding door. It seems to me that the fan should be pulling air the other direction.

Yep! You do, in section 24 of your factory service manual. (Seriously, the FSM is at the very top of the list of tools you need to work on your truck. It's a "now" purchase, not a "later" one.)

Poor heat output could be a result of restricted airflow through the heater core, cold air bypassing the heater core, or restricted waterflow through the heater core. It can be very difficult to discern one from the other. If cold air is indeed bypassing the heater core, it could be because of faulty seals in the heater box, a misadjusted temperature control cable, or a faulty or broken door in the heater box.

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Too many people who were born on third base actually believe they've hit a triple.

Lack of funds has been keeping it on the later list. I agree I need to get it as soon as I can. You and others on the forum have been very helpful in the absence of having a manual to look at.

Concerning the heater performance, I had someone else drive the pickup while I felt the air movement inside the sliding vent door. There is cold air (very cold; it's 6* here) blowing through the system with the fan off when you are moving, and with the fan on when you are sitting still. I think if I can find where the cold air is getting in, I can help the situation.

If pickups are like vans, they draw the fresh air from the fendrwells.

My first suspicion would be that a vacuum modulator in the heater system has failed. Of course, if you don't have a factory A/C system then your heter is all operated by cables and you don't have any vacuum modulators.

Have you tried disassembling the heater core housing, confirming the free and correct movement of all the heater doors, and backflushing the heater core? The most common reasons for low/no heat output is a faulty or wrong temperature thermostat (too low) or a clogged heater core or head.

Try taking your heater core to a local radiator shop and see if they can boil it out for you. Replace the thermostat with a 195 while you are in there.

From Autozone's website, here is how to remove and replace the heater core in a truck without A/C:

1981-88 Models


WITH STANDARD HEATER AND NO AIR CONDITIONING


Disconnect the battery ground cable.
Drain the cooling system.
Disconnect the heater hoses at the core tubes. Plug the tubes.
Make sure you have an ample supply of rags for any accidental coolant spills. Keep them within reach while removing and installing the heater core.

Remove the right cowl side trim panel, if equipped.
Remove the glove box by removing the 4 screws at the base and swinging out from the bottom to avoid catch and stops.
Working through the glove box opening, remove the support brace.
Remove the 8 screws from the right side of the instrument panel lower reinforcement. Disconnect the ground strap.
Disconnect the control cables.
Disconnect the blower motor wires.
Disconnect the wiring at the resistor block.
Remove the heater unit retaining screw from the cowl side sheet metal.
Remove the 6 mounting nuts from the engine side of the firewall.
Take out the heater unit, then remove its cover and lift out the core.

To install:


Insert the core and install the cover, then position the heater unit.
Install the 6 mounting nuts on the engine side of the firewall.
Install the heater unit retaining screw at the cowl side sheet metal.
Connect the wiring at the resistor block.
Connect the blower motor wires.
Connect the control cables.
Install the 8 screws at the right side of the instrument panel lower reinforcement. Connect the ground strap.
Working through the glove box opening, install the support brace.
Install the glove box.
Install the right cowl side trim panel, if equipped.
Connect the heater hoses at the core tubes.
Connect the battery ground cable.
Fill the cooling system.

WITH BI-LEVEL HEATER AND NO AIR CONDITIONING


Disconnect the battery ground cable.
Drain the cooling system.
Disconnect the heater hoses at the core tubes. Plug the tubes.
Make sure you have an ample supply of rags for any accidental coolant spills. Keep them within reach while removing and installing the heater core.

Remove the drain tube.
Position the gearshift and transfer case levers away from the instrument panel.
Remove the glove box.
Working through the glove box opening, remove the support brace.
Remove the ashtray.
Remove the 8 screws from the right side of the instrument panel lower reinforcement. Disconnect the ground strap.
Remove the center distribution duct.
Disconnect the control cables.
Disconnect the vacuum lines.
Disconnect the blower motor wires.
Disconnect the wiring at the resistor block.
Pull the carpet back and make sure that the grommet is free from the firewall.
Remove the heater unit retaining screw from the cowl side sheet metal.
Remove the 7 mounting nuts from the engine side of the firewall.
Remove the heater unit. You may have to slightly bend the plastic instrument panel trim for clearance.
Remove the 10 screws and 3 door cranks, then separate the cover from the housing.
Carefully slide out the core.

To install:


Install the core, then assemble its housing.
Install the 7 mounting nuts on the engine side of the firewall.
Install the heater unit retaining screw at the cowl side sheet metal.
Install and align the carpet.
Connect the vacuum lines on engine side, and make sure the grommet is seated in dash panel.
Connect the wiring at the resistor block.
Connect the blower motor wires.
Connect the control cables.
Install the 8 screws at the right side of the instrument panel lower reinforcement. Connect the ground strap.
Working through the glove box opening, install the support brace.
Install the center distribution duct.
Install the glove box.
Install the ashtray.
Connect the heater hoses at the core tubes.
Connect the battery ground cable.
Fill the cooling system.

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Casually looking for a Clifford hyperpak intake for cheap.

This pickup is a very basic version, no a/c or ps.
I put in a 195* thermostat a couple weeks ago when I replaced some hoses.
I don't think the core is clogged. I am going to do some more investigating and check the flow through the core in a couple days.

Well, if the thermostat is a new 195, then my money is on a clogged core or even a clogged head. Take the core out and either back flush it or have it boiled out at a radiator shop. Boiling out is better.

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Casually looking for a Clifford hyperpak intake for cheap.

I worked on my heater system today. I began by using a mirror, a fan, and an air compressor blowgun with a 2' hose on the exhaust side to figure out the direction the airflow was supposed to go. I found that even though there was some warm air coming out the floor and defrost ducts, there was also warm air going out to the grill below the windshield where fresh air is supposed to come in. Also, instead of air being pulled through the core into the fan, It was somehow blowing it through the core the opposite way. It doesn't really make sense to me how it was doing this.

Even though I had used compressed air to blow out the core, I thought that maybe it might still be causing low air flow. The idea came to me to use my one of my sawdust blowers from my woodshop to blow through the core. Using a 2 hp, 1500 cfm blower, with a hose that fit perfectly into the fan intake opening, I blew air through the core for a couple minutes, then I put the hose on the intake side of the blower and pulled air through the core for a couple minutes. I then put the heater fan back in and now it pulls air through the core like it should. It feels noticably warmer, but it is 50* today. the real test will be how warm it gets in the cab when the cold weather returns.

One other thing I noticed when I had the blower out, is that it looks like the blades on the fan blower wheel are angled wrong, which would be another reason for low air flow.