Why is my Kenworth blower motor not blowing hard? This is the question that we get asked quite frequently. Here are the 6 things that can be causing your blower motor to produce lackluster results. These are also the steps that we walk customers through to help diagnose their blower motor issue.

1. Clogged Evaporator & Heater Core

Water and dust don’t mix when it comes to your HVAC unit. Unless you have blocked off fresh air or have a really good air filter, your HVAC box will suck in dust. Your evaporator creates condensation. The dust and condensation can create a wall of dried mud in your unit, not allowing air to pass through the coils like it is supposed to. 

It is good to check and clean the evaporator and heater core at least once a year to get the most air flow out of your system.


2. Air flap stuck in the wrong position

We had a customer call stating that the blower motor produced an amazing amount of air when the lid on their heater box was off. But when the lid was on, barely any air came through the vents. 

I followed up with this question: “Did you block off the fresh air intake?” 

When the customer said yes, I asked them to send pictures of the air flap. The air flap was stuck in the wrong position. It should be able to flap open towards the fan. When the air in the cab is recirculated, this flap should have the ability to blow open. 

The purpose of the air flap is to prevent air from blowing back through the fresh air intake and into the cab of the truck. This flap comes in handy when your sitting at a truck stop in the middle of winter. No one wants that ice cold air blowing through their heater vents. However, this flap is supposed to be able to open when the air is recirculated. The picture below shows a flap that was stuck on the wrong side of the box. This flap should swing freely.


Flap is stuck behind the firewall

3. Cracked Vent Tubes/Ducting/ Broken Blend Door

There are vent tubes as well as plastic 1 ½ inch corrugated hoses and plastic ducting that will sometimes get cracked, allowing air to not flow correctly through the vents – as well as the blend door that will sometimes not close all the way. When the door doesn’t close correctly, the air will blow in every direction but not be concentrated where you want it. All of these things can produce poor airflow.


Broken blend door

4. Blower Motor Wired Backwards

We have seen blower motors get wired backwards. We see this a lot on popular Kenworth models like the W900, T800, etc. For these style Kenworths, the wiring harness on the truck side has two wires running to the blower motor. The solid black wire is ground, while the blue wire with the black stripe is positive. When the wires get switched around, it causes the fan to blow backwards, which results in half of the output. Here’s a picture of our blower motor wired up correctly.


Blower motor wired correctly

5. Your HVAC Box Has Holes In It 

We see this a lot in the salt belt states, where rust corrodes boxes to the point that air is escaping from the box. Instead of getting concentrated airflow in the cab, the air is diverted outside of the HVAC box, as well as fighting with the outside temperature. In the case of a rusted Kenworth heater box, it’s best to replace it.


A rusted Kenworth heater box

6. Blower Motor Is Failing

Unfortunately, blower motors get old and stop performing like they used to. After you rule out all the above scenarios, it’s time to look into replacing your blower motor. 

We would recommend choosing one of our Kenworth Blower Motor Upgrade Kits instead of replacing the motor with an OEM motor. We have found that our blower motors get at least a 15% increase in CFM over a brand-new, out-of-the-box OEM.


Kenworth blower motor upgrade kit

Hopefully, I have successfully answered your question as to why your Kenworth Blower Motor is not blowing hard. As usual, if you have any other questions, reach out to us via phone or text at 616-831-1503. We would love to talk to you.

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