HVAC Heater Blower Motor Resistor

[pbeach]

Morning gentlemen.

Hope your weekend’s going famously.

Installed an HVAC Heater Blower Motor Resistor within the last two years (think it was Dorman). Seems it’s shit the bed. Not surprising after reading various reviews of this and others.

Anyone had luck with a vendor/brand? Holstein? Dorman? Crown? …?

Thx.

~ Peter

 

[Wyatt Morris]

I installed a Dorman Resistor a year ago and it's holding up.

I've seen multiple threads mentioning that the blower motor needs to be replaced alongside the resistor. From what I understand, the old blower motor ends up drawing too much current and blows the resistor.

 

[Jerry Bransford]

The usual root cause of a blown resistor pack is the fan motor windings shorting together causing it to draw excessive current through the resistor pack and wiring. Sometimes the fan speed switch is damaged too. The motor windings are made with bare copper wire insulated with shellac. When the shellac starts breaking down the windings start shorting together.

So I'd replace both the fan motor and the resistor pack together. Closely examine the wiring for burned spots and/or melted insulation too.

Replacing just the resistor pack usually results in it blowing again from the bad fan motor.

 

[TrueTexas]

The advice from @Jerry Bransford is good. Issues with the motor can certainly cause excessive resistance and heat which will destroy the weakest link in the chain.

Note that the factory wire sizes used in the HVAC system were adequate at best, when the Jeep was new. Over time, they can degrade through oxidation etc... and then they become inadequate, especially if what Jerry mentions happens when the blower motor wears.

My issue was wiring. After the second new OEM blower motor, I was still reading excessive amps being pulled by the fan. I ended up doing a voltage drop test and found the last leg of the 12v+ was dropping significantly. Replaced that wire and the fan amps dropped into the normal range. Note this is a very abridged version of what I did to build a rock solid HVAC system.

Before investing in a fan, it may be good to bench test it with good wiring to see how many amps it wants to pull when directly wired to the battery. If it is in the 10-15 amp range, the fan is good and your issue lies in the wiring.

Search the forum, lots of info on the HVAC issues.

 

[pbeach]

The usual root cause of a blown resistor pack is the fan motor windings shorting together causing it to draw excessive current through the resistor pack and wiring. Sometimes the fan speed switch is damaged too. The motor windings are made with bare copper wire insulated with shellac. When the shellac starts breaking down the windings start shorting together.

So I'd replace both the fan motor and the resistor pack together. Closely examine the wiring for burned spots and/or melted insulation too.

Replacing just the resistor pack usually results in it blowing again from the bad fan motor.

Jerry for the win. Thx, man.

 

[pbeach]

The advice from @Jerry Bransford is good. Issues with the motor can certainly cause excessive resistance and heat which will destroy the weakest link in the chain.

Note that the factory wire sizes used in the HVAC system were adequate at best, when the Jeep was new. Over time, they can degrade through oxidation etc... and then they become inadequate, especially if what Jerry mentions happens when the blower motor wears.

My issue was wiring. After the second new OEM blower motor, I was still reading excessive amps being pulled by the fan. I ended up doing a voltage drop test and found the last leg of the 12v+ was dropping significantly. Replaced that wire and the fan amps dropped into the normal range. Note this is a very abridged version of what I did to build a rock solid HVAC system.

Before investing in a fan, it may be good to bench test it with good wiring to see how many amps it wants to pull when directly wired to the battery. If it is in the 10-15 amp range, the fan is good and your issue lies in the wiring.

Search the forum, lots of info on the HVAC issues.

Appreciate the great info. Thx, TT.

 

[Jerry Bransford]

Issues with the motor can certainly cause excessive resistance and heat which will destroy the weakest link in the chain.

The shorted motor windings described above don't create excessive resistance, more resistance would actually reduce the current it draws and cause reduced heat in the surrounding circuit. Shorted windings are shorts which causes reduced or zero resistance which is the root cause of it drawing excessive current through the resistor pack and other parts of the fan circuit overheating them.

 

[TrueTexas]

The shorted motor windings described above don't create excessive resistance, more resistance would actually reduce the current it draws and cause reduced heat in the surrounding circuit. Shorted windings are shorts which causes reduced or zero resistance which is the root cause of it drawing excessive current through the resistor pack and other parts of the fan circuit overheating them.

Fair point. I stand corrected.

The main point I was trying to make was that although a bad motor is often the case, there may be other reason the resistor fails or the wires and harnesses overheat and fail / melt.

In my case it was 100% the last leg of the 12v+ to the blower motor that was badly corroded. Can't remember exactly how much the voltage drop was at that leg but it caused excessive amps to be pulled and the wiring and relays were getting super hot. After multiple OEM blower motors did the same thing, replacing all the wiring and taking the high current off the components (by using a bunch of relays) solved it for me. Overkill? I'm sure. But I have a rock solid system now.

 

[Jerry Bransford]

Voltage drop across resistance, a bad/corroded connection, or corroded wire, doesn't increase amperage, it decreases it. Especially on the wiring providing the motor's 12 volts. Shorts and decreased resistance where it shouldn't be is what increases current to get things hot. A good basic electronics course wouldn't be a bad idea.

 

[Dr. Internet]

Voltage drop across resistance, a bad/corroded connection, or corroded wire, doesn't increase amperage, it decreases it. Especially on the wiring providing the motor's 12 volts. Shorts and decreased resistance where it shouldn't be is what increases current to get things hot. A good basic electronics course wouldn't be a bad idea.

When all else fails, remember: E = I x R