Low air flow from vent registers

bad_decision

Member
Aug 14, 2020
9
2
13
37
Houston, TX
Trying to troubleshoot an issue I've had on my 91 convertible since I purchase it a few months ago. There is very little air output from any selected air vents - I know these cars are aged but there is no way it was this weak from the factory in 91. I have replaced the blower resistor... when I got the car the fan would ONLY blow when set to high. When set to low the fan would not move at all. New resistor fixed that problem.

At all speeds you can hear the fan doing work, and at higher settings it sounds like it should be putting out a lot of air... but even then you don't feel much coming from the dash vents. Vent selection works - blows only to dash vents when they are selected, only to floor when that is selected, only to top of dash when that is selected. The 'max' setting works as well - you can see that panel inside the car open up when you select max ac.

Tonight I pulled down the blower assembly. It appears to be the original housing and blower motor.... blower motor has a September 90 datestamp. But it is obvious somebody messed around with the wiring as you can see the orange motor wire was hacked up. I also found the black motor wire was tapped into a blue/green wire AND a separate black wire that was not connected to anything... just sitting under the carpet. Does anyone know what that blue/green wire is?? I looked at the diagrams posted on veryuseful.com and couldn't find that one.

2nd - I was looking for obstructions and am not seeing anything. Not seeing anything out of order other than the wiring and the foam looks beat up. The evaporator core looks clean. All fan blades intact, and I can watch the fan spin fast and freely. Really don't want to throw parts at this one... any ideas on what's going on?

wires.jpg



blower.jpg
 
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ebone84

Member
Jun 8, 2004
81
17
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I believe that silver gadget actuator should have a vacuum tube on it also.... In the last picture to the right
 

bad_decision

Member
Aug 14, 2020
9
2
13
37
Houston, TX
I believe that silver gadget actuator should have a vacuum tube on it also.... In the last picture to the right
Absolutely right. That moves the door that switches between fresh and recirculated air. I should have clarified - that hose was connected before I removed the blower housing. That picture is after I dropped the blower housing and have it sitting on the passenger floor board. I was trying to show the condition of the fan... looks good to me... and the foam in the blower exit vent, which looks beat up and I'm not sure if it's supposed to go all the way around or not.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
36,124
12,040
224
Massachusetts
Here’s the wiring and vacuum diagram for a 91

The blue/green wire you are looking for is wire 249 from the heater controls to the motor.

0CEDBAC8-0FA5-4A71-B5B2-33986348ABCA.jpeg
7879380F-D43B-4E7E-990C-254B68047DD8.jpeg
 
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Reactions: 1 user

bad_decision

Member
Aug 14, 2020
9
2
13
37
Houston, TX
Here’s the wiring and vacuum diagram for a 91

The blue/green wire you are looking for is wire 249 from the heater controls to the motor.

0CEDBAC8-0FA5-4A71-B5B2-33986348ABCA.jpeg
7879380F-D43B-4E7E-990C-254B68047DD8.jpeg

Awesome! That is super helpful. So that mystery is solved. Now to figure out why everything seems to be in good shape, but still have low vent output. Something must be clogged up or not sealing properly.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
36,124
12,040
224
Massachusetts
Set it to floor/defrost. This is the default position of there is no vac to power the various flappers. The flow should be fairly strong. If not, check the seal between the hvac box and the outlet on the dash hvac plumbing. Make sure it’s seated right. You can probably get your hand in behind the dash and feel for airflow that should not be there
 

7991LXnSHO

10 Year Member
Sep 1, 2010
4,959
1,479
184
Kearney, NE
Are the heater core and evap core both unblocked and clean? Is there a mouse nest elsewhere in the system blocking air?
I found a heater core blocked by house insulation fiberglass early one winter. There were no mouse droppings, but I had no other explanation beyond sabotage.
 

bad_decision

Member
Aug 14, 2020
9
2
13
37
Houston, TX
Looks like the main problem was the blower box not being properly sealed against the rest of the hvac box! This was the first time I messed with the housing, and based off the tips here I realized those bottom two screws that secure the blower box to the housing were missing. Mine was just held in place by the 7/16 screw behind the glove box! Since I had the blower box out, I cleaned around the evap core as much as I could. There was some dust and pine straw in there but nothing crazy. Not sure how the heater core looks yet.

Since I like to track progress, I got a cheap wind speed measuring tool to measure vent flow before and after. This is what it looked like before and after... all numbers are in MPH and the four vents are listed driver to passenger side. Fan speed 4 and set to max a/c at idle:

Blower box loose - not cleaned - no lights or radio
4.5, 5.1, 4.7, 4.5
Average 4.7 MPH

Blower box loose - not cleaned - headlights on, radio loud
4.2, 4.9, 4.5, 4.2
Average 4.45 MPH

Blower box cleaned and secured, no lights or radio
5.1, 6.3, 5.6, 5.4
Average 5.6 MPH

Blower box cleaned and secured, light on, radio loud
5.1, 5.8, 5.1, 4.9
Average 5.225 MPH

So about an 18% airflow increase overall, I'll take it for a free fix. This is with a stock alternator but the 3g upgrade is on my short list. I'll be curious to see if the flow numbers increase with either condition (no load, radio+lights).

Thanks for the tips guys!