# A problem with heat damaging Blower Motor Resistor Block & Connectors

#### Lumenosity

Joined Mar 1, 2017
469
Hello again.

I am wrestling with a common automotive problem. On many vehicles, the speed of the Blower Motor that circulates air inside the cabin of vehicles uses a Resistor Block consisting of various length and thickness windings of steel coil as resistors to vary the blower speed. (See photos 2 & 3 below)

The common problem is that there is so much heat generated in the process that the Resistor Block itself, along with connectors often fail in time due to the excessive heat. Connectors can be themselves be expensive costing over $100.00 for some vehicles. Generally, the resistor blocks themselves are not as expensive as the connectors. One typical way in this particular instance to mitigate the problem is avoid using lower blower speeds as more current is resisted to achieve the voltage drop in order to get lower blower fan speeds (is that correct?) Using the higher speeds requires less current resistance (correct?) There was a time these could be easily found in self service pick your own part automotive salvage yards. However, my vehicle is about 20 years old now and it's becoming rare to find a donor vehicle. The usual fix for the Resistor Block is a simple buy and replace (readily available, cheap & easy) But when the Connector itself has been damaged (as often occurs, See photo 1 immediately below), the fix is more costly and more involved as it involves cutting the pigtail and splicing in new wires and the new connector. Here is a picture of the damaged connector in my vehicle. The damage is mostly on the Left side which probably corresponds to the connector with the highest current resistance. I was able to exactly duplicate this connector using CAD design and successfully printed a duplicate on a 3D printer, but the problem is that even with ABS plastic, the copy produced on the 3D printer will not withstand the heat which I estimate to reach 325 F where the wire from the harness connects with the terminal on the Resistor Block.at times. The Glass Transition Temp of ABS used in 3D printing is only 220F. Of course I can buy a new connector and it will last a few years. But I'm wondering if there's a way to mitigate the heat at the connector. Here's two photos of the resistor block and it's connecting tabs where the plastic connector plugs in My temporary solution is to eliminate the plastic connector altogether and instead, carefully wrap the end of the wire and each female connector with Kapton Tape and them simply push each wire onto the spade connector on the Resistor Block shown in the photo immediately above. The risk with that is the HOT wire coming off and contacting a ground in the engine compartment. So what I'm looking for is a way to mitigate the heat at the connectors. Any ideas would be appreciated. Thanks for your thoughts #### Attachments • 82.8 KB Views: 1 #### Reloadron Joined Jan 15, 2015 5,499 Yeah, they fail on a pretty regular basis. Most I have replaced simply burn open and that speed is lost. Most just run straight power to the blower for High and add resistances for the slower speeds. When connectors like those pictured, using 1/4" spade lugs fail it is normally a result of a poor connection. This is where it is nice to have female connectors which mate tight with the male pins. Heat from a faulty connection will literally crawl away from the problem and the problem starts to snowball. The more the resistance at the terminal gets the more heat until something cooks itself open. The best way to mitigate the heat is to have good quality connectors which mate tight. Ideally the entire resistor block network could be redesigned I guess using better type connectors and resistors if someone really wanted to. With female spade lug connectors I have had some luck just pinching them with pliers so they fit the male pins tighter. Really depends on how far you want to take it. Ron #### shortbus Joined Sep 30, 2009 7,756 Modern cars don't use them anymore, do they? Aren't they now PWM controlled, from the BCM(body control module)? #### Reloadron Joined Jan 15, 2015 5,499 Modern cars don't use them anymore, do they? Aren't they now PWM controlled, from the BCM(body control module)? I really don't know. My truck is a 2007 and it uses them and I replaced one on a friends 2011 car last year. I think the car was a Nissan. Now that you mention it though it would be pretty easy to make a small PWM card with a single MOSFET which should work fine. Ron #### Dodgydave Joined Jun 22, 2012 9,200 Hello again. I am wrestling with a common automotive problem. On many vehicles, the speed of the Blower Motor that circulates air inside the cabin of vehicles uses a Resistor Block consisting of various length and thickness windings of steel coil as resistors to vary the blower speed. (See photos 2 & 3 below) The common problem is that there is so much heat generated in the process that the Resistor Block itself, along with connectors often fail in time due to the excessive heat. Connectors can be themselves be expensive costing over$100.00 for some vehicles. Generally, the resistor blocks themselves are not as expensive as the connectors.

One typical way in this particular instance to mitigate the problem is avoid using lower blower speeds as more current is resisted to achieve the voltage drop in order to get lower blower fan speeds (is that correct?) Using the higher speeds requires less current resistance (correct?)

There was a time these could be easily found in self service pick your own part automotive salvage yards. However, my vehicle is about 20 years old now and it's becoming rare to find a donor vehicle.

The usual fix for the Resistor Block is a simple buy and replace (readily available, cheap & easy)
But when the Connector itself has been damaged (as often occurs, See photo 1 immediately below), the fix is more costly and more involved as it involves cutting the pigtail and splicing in new wires and the new connector. Here is a picture of the damaged connector in my vehicle.
The damage is mostly on the Left side which probably corresponds to the connector with the highest current resistance.

I was able to exactly duplicate this connector using CAD design and successfully printed a duplicate on a 3D printer, but the problem is that even with ABS plastic, the copy produced on the 3D printer will not withstand the heat which I estimate to reach 325 F where the wire from the harness connects with the terminal on the Resistor Block.at times. The Glass Transition Temp of ABS used in 3D printing is only 220F.

Of course I can buy a new connector and it will last a few years. But I'm wondering if there's a way to mitigate the heat at the connector.
Here's two photos of the resistor block and it's connecting tabs where the plastic connector plugs in

My temporary solution is to eliminate the plastic connector altogether and instead, carefully wrap the end of the wire and each female connector with Kapton Tape and them simply push each wire onto the spade connector on the Resistor Block shown in the photo immediately above. The risk with that is the HOT wire coming off and contacting a ground in the engine compartment.

So what I'm looking for is a way to mitigate the heat at the connectors. Any ideas would be appreciated.

I would ditch the resistors , and use a Pwm module for speed control, plenty on Ebay search for 555 pwm controller.

https://m.ebay.co.uk/itm/DC-Motor-Speed-Controller-Reversible-PWM-Reversing-Control-Switch-12V-40V-10A/163093766103?hash=item25f9255fd7:m:mWMCyrNL0oy8LyX-DSszMLw&var=462241249004

Joined Jan 15, 2015
5,499
Before you go with a PWM solution you may want to take a lookie at the blower motor fuse to see what it is rated at. That will give you some idea of how much current a blower motor draws. If a blower motor is fused at 25 amps you want a PWM circuit capable of driving that much load to leave yourself some headroom.

Ron

#### shortbus

Joined Sep 30, 2009
7,756
I really don't know. My truck is a 2007 and it uses them and I replaced one on a friends 2011 car last year. I think the car was a Nissan. Now that you mention it though it would be pretty easy to make a small PWM card with a single MOSFET which should work fine.

Ron
Knock on wood, in all of my years of driving and the many cars, mostly used, I've never replaced one.

Joined Jan 15, 2015
5,499
Knock on wood, in all of my years of driving and the many cars, mostly used, I've never replaced one.
The ones I have replaced just required being a contortionist and I don't bend as well as I once did. Generally under dash on passenger side.

Ron.

#### gerty

Joined Aug 30, 2007
1,287
The ones I've seen were placed in the heat/ac box so the air from the fan motor it controls blows across the resistor to provide some cooling..

#### cork_ie

Joined Oct 8, 2011
428
If you replaced the "Pollen filter" in the air intake more often, the problem shouldn't happen.
Most cases of resistor overheating are either:
1) Insufficient airflow through heater to keep resistor cool.
2) Stiff bearings / bushing in blower fan motor causing it to draw excess current and hence dissipating more power in resistor block.
an occasional clean out of the motor assembly and one or two drops of oil on motor bearings works wonders.

#### Lumenosity

Joined Mar 1, 2017
469
If you replaced the "Pollen filter" in the air intake more often, the problem shouldn't happen.
Most cases of resistor overheating are either:
1) Insufficient airflow through heater to keep resistor cool.
2) Stiff bearings / bushing in blower fan motor causing it to draw excess current and hence dissipating more power in resistor block.
an occasional clean out of the motor assembly and one or two drops of oil on motor bearings works wonders.
There is no pollen filter in this vehicle. Totally unobstructed air flow. But I do get where you're coming from.

#### Lumenosity

Joined Mar 1, 2017
469
Knock on wood, in all of my years of driving and the many cars, mostly used, I've never replaced one.
In all those years, didn't you miss heat and AC ?

Seriously, it's such a common thing, you were beyond "lucky"

Most of them will actually work on HIGH setting even without the resistor block by design, so as to not leave a family stranded with no heat or AC. Maybe you always thought there was only one speed.....HIGH ?

Joined Jul 18, 2013
20,874
Modern cars don't use them anymore, do they? Aren't they now PWM controlled, from the BCM(body control module)?
My (GM) car is not that 'Modern' and that uses a PWM on just about all the motors on it.
Max.

#### shortbus

Joined Sep 30, 2009
7,756
My (GM) car is not that 'Modern' and that uses a PWM on just about all the motors on it.
Max.
My 04 Grand Prix was like that, no resistors.

#### shortbus

Joined Sep 30, 2009
7,756
In all those years, didn't you miss heat and AC ?
I only had a few cars that even had AC. But all of them had heat, and still never replaced a resistor block. I did mechanic work on the side for others and for my father in laws used car lot, and can only recall replacing one during that whole time, maybe over 10 years for the car lot and over 20 for other people.

So maybe it makes you out to be unlucky?

#### Lumenosity

Joined Mar 1, 2017
469
I only had a few cars that even had AC. But all of them had heat, and still never replaced a resistor block. I did mechanic work on the side for others and for my father in laws used car lot, and can only recall replacing one during that whole time, maybe over 10 years for the car lot and over 20 for other people.

So maybe it makes you out to be unlucky?
Well, TBH, I've only had to replace two in my entire life with all my vehicles, but BOTH with the same vehicle. But it's still a common problem.
Dodge probably should have used a PWM setup with these vehicles.

As far as the being "unlucky"......lol....You have no clue just how wrong you are.

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