Car Fresh Air Fan Thermal Fuse

John.G

A lot, if not all of the above have a thermal fuse mounted on the resistor pack used to allow different fan speeds. On a 4 speed fan, the current for 3 of the speeds passes through this thermal fuse, 4th (full speed) power is supplied directly to the fan motor. This resistor pack is often inserted in the fan ducting.

Is this fuse rating based both on current flow and the surrounding ambient temperature? and is it used to protect the resistor pack from getting too hot and causing a fire, in the event say of the fan drawing excessive current, or/and the fan itself?. I have seen mention of 115C fuses, does this mean that the fuse will blow if the surrounding air temperature exceeds this temperature for whatever reason without any extra current draw?.

2011

Fuse elements are made of metal. Each metal or alloy of metals have a melting point. Once the temperature of the metal exceeds this threshold it will melt regardless of whether this high temperature is caused by current or ambient conditions.

John.G

Understood but I think this is, basically, a fusible link whose primary function is to melt in the event of the resistor pack overheating due to the fan drawing extra current, worst case (thermally) would be if the fan shorted out, the resistor(s) would then be drawing max current/watts with no air cooling.

As I stated elsewhere the fan is on its third resistor pack even though the fan apparently is drawing normal current and supplying adequate cooling air, the pollen filter was also renewed, so IMO either the resistor1 is continually failing or the fusible link is failing as both of these are in the circuit for fan speeds 1,2&3. The fan is running on speed 2 only, for 99% of the time.

10-10-20

John, look at the physical location of the thermal fuse. If it's downstream of the air-flow over the resistors or installed close-coupled with the resistor, then it's likely a thermal cut-out to protect the vehicle if there is a blockage or the fan shorts', as mentioned.

If the thermal-fuse is a standard type, it only provides thermal cut-out and not an electrical overload cut-out. But there are self-resetting electrical overload fuses which resemble capacitors, so there might be an equivalent thermal and current device... perhaps.

A photo of the resistor board with fuse would help, but also show the air-flow direction.

Just looking at your table there - a 43w resistor must mean that it's a wire-wound loop resembling an air-coil?

John.G

This is a photo of my present resistor pack, this type of blower resistor in very common in all makes and is just slotted into the air ducting immediately after the fan, the air flow through the car vents is quite normal so I don't think the failures are heat related, there is also obviously a gale of wind through the hole in the ducting with the resistor pack removed. The pack is inserted exactly as it looks while removed, it can only be locked in place one way. this failure is showing up in other vag group cars as well, in this Polo, first failure was after 2 years, the second after a further 9 months, the present one is in place ~ 6 months. I have a 17 year old Polo with the exact same resistance readings still on the original resistor pack and we had a Golf for 21 years and a Bora for 19 years both with the original packs. I also include details from the thermal fuse, below.

Thermal Fuse: Microtemp ZJABWA G4A00 Tr 240C

10-10-20

John, reading the spec-sheet for that thermal fuse, even with a simple device like that - there's a fair bit at play.

To answer your first question as to whether these fuses exhibit a response based on current in-circuit - no, there is no in-circuit resistance as the device is mechanical rather than being polymeric.

240C is the "maximum opening temperature" or "functioning opening temperature" and is the upper confidence level as set by the manufacturer on the basis of bench tests. The lower confidence level appears to be the Th value - which is 200C for the same fuse.

That opening temperature is stated to have some variation and these are due to manuf. variation, design, ageing and the application of the fuse.

These fuses suffer from heat ageing and this can cause shrinkage of the thermal pellet or other mechanical faults. The manufacturer states that the fuse needs to be maintained at least 25C below the operating temperature and that consideration needs to be given to the effective trip profile (just like an MCB has a trip class) where they state that the fuse can only be maintained at Th for 168 hours before opening. So in all, in an application where the fuse is running hot - which this application definitely is - you may see nuisance trips.

Lastly, these fuses are significantly de-rated for DC over AC - AC can take up to 21A at 250vAC, but the DC rating is just 5A at 24v DC (for most approval agencies). That's interesting as you worked out that the fan was pulling between 2.4 A to 9.3 A; were these calculations or measurements? In thinking further about the DC application I also realised that your nominal voltage in a vehicle during use is roughly 13.4v DC (and not 12v) and also that the fuse doesn't have a rating for a DC inductive load! DC motors are considered to be resistive AND inductive, and inductive loads are problematic for many components due to the back-EMF (reverse voltage) generated during use. Back EMF is normally snubbed in the circuit by placing a large diode in reverse polarity closest to the source (the motor) as possible. This converts the back EMF into a short which is burned up as heat within that diode. So in thinking about this, I wonder if that diode has failed, would the back-EMF be problematic to the thermal fuse...?

IMHO, your options are:

1. To investigate and test the diode (and series capacitors) and replace if needed.
2. To replace the fuse with a higher spec device (your only option there is the 257C rated device).
3. To shield the fuse from some of the thermals, but still maintain it's safety function.

Option 1 is easy enough to test for if you have access to the blower motor.

Option 2 is problematic as even if you obtain a 257C rated device, you'll need to spot-weld it to the resistor body as the operating temperature of the device is above the temperature of solder!

Option 3 is based on the assumption that the fuse is nuisance tripping from run-away thermals, but it's difficult to know whether these are conductive (through the leads) or radiant heat. You could try shield the device using reflective foil (keeping the foil away from the leads) and see if that offers a longer life.

https://www.fluke.com/en-us/learn/blog/digital-multimeters/how-to-test-diodes

https://www.emerson.ru/documents/commercial-residential/microtemp-thermal-fuses-en-792774.pdf

John.G

Thanks for that detailed response, I might wait to see if the TF is actually blowing and if so go for the higher rating, option 2, I replaced one on someone's car a few years ago and as there were two long wires on the replacement fuse I just clamped them to the resistor's ends with two brass terminals from a terminal strip.

10-10-20

Actually, using brass screw terminals gets rid of a few problems - such as conductive heat. That's sighted as a potential failure point in the data sheet if the differential between the epoxy lead and the case lead is high. Even if I only had the 240C fuse, I'd go with that as a possible solution. You can bend the fuse back down towards the resistor pack anyway.

John.G

Isn't it strange though how only in the last few years that this resistor pack short life is now appearing across a number of VAG models, this set up should be well sorted out now, you might at least have expected 5 years or so specially in a car that has less than 15,000 kms up in over 3 years.