Heater Air Flow is always low

Going through my list of fixes and have to the HVAC system air always being low.  I have opened up the blower motor and checked for leaves/blockages.  Not much in there but I did clean out a leaf or 3.  Unfortunately, that did not fix the problem.  Not sure what to look at next.  Switch? Resistor? Are there any areas to check voltage that would narrow the problem?

there are basically two different systems that came in these trucks. heat only or heat/ac. they are prone to getting clogged with leaves and rodents are quite common to on neglected vehicles. the heater cores and evaporators can get clogged within the fins and will need to be removed to be fully cleaned out. condition of the blower wheel is another point but usually it will be making noise if it is slipping or broken. both have resister banks mounted in the air flow. heat has two resister coils and one non resistive, heat/ ac will have three resistive and one non resistive. also, there is a thermal fuse mounted in the resister bank which if blown the only fan speed to work will be high. if blown anew resister is needed. past this it gets more involved. there are vacuum operated dampers within the ductwork behind the dash. these "motors" can go bad and not open fully, leaving a less positive change from one setting to the next. another common one is the heat/cool door which changes flow from ac evaporator to the heater core. these are made as a one-piece plastic part which has a thin folding line instead of a proper hinge. guess what breaks. itis riveted into the duct, and this can be a major pain for many. however, before condemning any part and investigating with a big disassembly be sure that it is not just that it blows weaker than you like. none of these blows strong! on heat only systems the damper doors are controlled by a lever and cable. they can get hard to move if the cable gets rusted, but it should still move. on ac systems the dampers are move by vacuum and therefore have a vacuum control switch that also can be leaking and not allowing a positive change between. this switch is mouted to the back of the dash panel.

So should the blower motor spin faster depending on the blower switch position?  Mine is always the same speed.

yes. the high position gives the blower battery voltage but on either of the slower speeds the current gets routed through a resister in order to reduce the voltage and therefore the speed(power) of which the blower spins. if you are willing to make a little wiring jumper. you can unplug the two-wire plug to the blower and run it straight to the battery and prove the motor but that will only prove what the blower is capable of. if it runs stronger this way then you have a breakdown in the wiring. its possible to have a weak blower motor

Pulled the hvac control and nothing was connected to the blower motor switch. Plugged it in and now it works. Although now the off position blows through floor vents

on my 86 had same problem. Found the small white vacume line's and vac. tank had bad spots. Not letting vacume motors to change direction. Changed all line's all work like they should.

Any clue what vacuum line / door is for the hvac off position?

first of all are we dealing with heat /ac system or just heat. dealer air adds another layer to this onion. is it possible that you have a mix match of parts?

Just heat, no ac. Although I guess mixed parts is always a possibility

then you don't have an hvac truck. there should be no vacuum circuit to deal with nor is there a hot/cold diverter door to be broken. the fan switch should go from off at the bottom to high at the top. there it a lever with a cable driven damper to go from the floor to the defrost and a lever to a cable from cool to heat. the blower still has a resister with two coils and one direct to the blower

Sorry, it does have ac, I’m just only concerned with heat. The controls look like this.

IMG_0877.jpeg

User811780 wrote

Sorry, it does have ac, I’m just only concerned with heat. The controls look like this.

Do you mean that the fan flow isn't strong?
I mean that the heat is good but fan doesn't push enough?

*with a vacuum system* Default should be defrost.
Id think you either have a stuck vacuum motor or a vent door that's off it's hinges.

The vacuum troubleshooting section at the back of the EVTM might be helpful.  https://www.garysgaragemahal.com/1981-evtm.html
Check also vacuum systems under: Documentation/Vacuum Systems/ various topics

Plus, there are quite a few threads about repairing vacuum leaks, the coffee can reservoir, and fixing the doors.

If there’s no vacuum to the system, like when the vacuum line under the hood is off or broken it’ll be in defrost. You can see the vacuum line, tee/check valve, reservoir, and line into the cab here:

Keep in mind the 19A566 can is prone to rusting out on the bottom where condensation collects.

Yep, and that kills the vacuum.

I would also check the vacuum line to the transmission modulator. It's on the firewall junction.

The modulators can leak and destroy that vacuum line.

Thanks for the help everyone, after cleaning all the vents and plugging in the blower switch (not sure how it was undone) everything seems to be working normally.

The only problem I have now is the blower motor does not turn off when the lever is in the off position. Which isn’t that big of a deal. Merry Christmas!

You might have a look at component testing (pgs 100&101)

https://www.garysgaragemahal.com/component-testing.html

Thank you Jim. That is very helpful!