Thermal runaway - a warning to all owners

[CovKid]

I should make it clear that I know of no instances of T25 fires due to fan resistors (thinking of Vauxhall Zaphira here) but I'm not a fan of using resistors to control fan speeds at all. Although shoving a resistor in the circuit does work (and this is how the lower dash blower speeds on a T25 are controlled), if you get a thermal runaway with a wirewound resistor, the potential for a fire is great when in close proximity to plastics. By modern standards its a fairly barbaric way to control power anyway and more akin to the mad professors lab. I've seen several T25 blower boxes that show signs of excessive heat around the resistors, and seen it on other vehicles too.

Now, a fuel line fire on an old vehicle you could possibly put down to negligence on the part of the owner. I'm a great believer in preventative measures and like many of us, I inspect them annually and replace them every 2-3 years regardless. However, what makes dash fires in particular more of a worry is the plethora of cheaply made fuses (like those readily available in pound shops for instance) that don't perform correctly, and the tendency of some owners to increase fuse ratings or even bridge connections because one keeps blowing (either blower or rad fan). Blower motors draw more and more current as they age. All the waffle about unseizing them with WD40 doesn't help either. When they're 30 years old, they are at the end of their life. Often by the time someone has decided they need to replace the blower motor, the fuse fitted can be way beyond what I would consider safe. I added a note in the WIKI a few years ago for those with pre-85 vehicles that have fan and wipers on the same fuse and how to split them.

In the case of the zaphira, its possible the wirewound resistors go into a thermal runaway situation and thats difficult to control unless you can pull the plug quickly.

Some time ago, Ghost123UK and I were discussing this and concluded that a dedicated PWM motor speed controller was the way to go when replacing the motor since it doesn't draw as much current as wirewounds and gives greater control. A thermal fuse being a nice touch. I did just that and covered it on youtube: https://www.youtube.com/watch?v=3GzJ8N1Rqp8" onclick="window.open(this.href);return false;

That said, all T25 owners should check CAREFULLY that the correct rated fuse is fitted on items like lights and motors that draw current and NOT fit larger ones to get around blown ones. DO NOT DO IT. Always establish WHY the fuse is blowing - don't increase the rating as you risk the very real potential for a dash fire which is unbelievably scary. Even if you disconnect the power (advisable), the fire may already be well established and too late. Finally, make sure you carry a fire extinguisher - it could save lives as well as your camper.

Dash fires DO occur on T25s though: http://www.thesamba.com/vw/forum/viewto ... &view=next" onclick="window.open(this.href);return false;

[Ralf85]

So in the meantime it sounds best to run the heater fan on maximum when it's needed. Is that right?

[CovKid]

Correct but as I say, I have no evidence that the resistors fail on the T3 (although they do get hot make no mistake) and the greatest threat is from incorrect fuses or bodges.

E D I T: I should add, that its usually the initial startup of an old blower motor that blows the fuse and why people increase the fuse rating but thats just asking for trouble. If you're in that situation, forget WD40 - just replace the blower and ideally change to PWM control which can be cheaper than fitting new resistors anyway. Both blower and PWN controller need to be fitted together. Don't add PWM to an old motor.

[scottbott]

read this a while ago on the samba.com,about using a cheap Jeep resistor and mounting it differently,cannot seem to get a link to it,easy enough to find if you look for 'cheap Jeep resistor'

[lloydy]

i just had new resisters fitted when the fan was changed.

[CovKid]

Its about efficiency too. Resistors are not remotely efficient compared to a PWM controller. Its a bit like using the handbrake to slow you down instead of going down the gears Well worth checking the correct fuse is in place for each circuit though. Most owners seem to end up with a mish mash of fuses that don't protect circuits.

Please don't tell me this is what is fitted in the Zaphira.

Cheap they might be but why would you



At least mosfets have a heatsink. I suppose it could double up as a toaster.

[Synkraut]

Hey CovKid,
thanks a lot for that post! I actually feel directly adressed because after having had problems with the #1 and #2 setting on the blower switch (and a blown fuse) my garage guy fitted a stonger fuse and sayd it'll be ok
I'm planning on installing an new blower motor and a 10" brake booster the next weeks but do you have links to PMW contollers sold on fleabay (or wherever I can order them from Germany) that you consider suitable?

PS: I always have a "grown up" CO fire extinguisher on board but would prefer not having to use it

[CovKid]

Case of hunting through ebay pages (listings change constantly). You want at least 20amp and control on a flylead as it makes it easy to fit.

Yup, bigger fuse not a good idea. Mine came with a 75amp fuse in there!

Er, how about this - under £10 http://www.ebay.co.uk/itm/10-63V-20A-12 ... SwFnFWE5Rq" onclick="window.open(this.href);return false;

You'll also need to disconnect brown earth from blower at earth crowns and fit to negative on controller output instead or it won't work

Again, can't stress this enough do NOT attempt this mod with an old blower motor. Fine with a new one.

[Smosh]

Not really sure what a PMW is tbh, but if you strongly advise using them but equally not putting them on and old blower, what happens when the new blower ages? I think what I'm asking is why not use one on old blower? Cheers,
Josh

[CovKid]

No point putting it on an ailing 30 year old blower. It may not work or could stress the controller - especially if its drawing far too many amps at startup, and most old motors will.

In simple terms PWM is "Pulse Width Modulation" a circuit that turns the supply to the blower motor on and off in rapid succession in the same way that you might disconnect the power to a small model electric motor by tapping a small battery to it - only quicker than you could do it by hand. Its pulsed. Think of it as a warble rather than a single note. The gap between each pulse (or on and off state) is changed by the control knob so the amount of power supplied varies accordingly.

With a wirewound resistor, at lower blower speeds all you're doing is soaking up some or a lot of that power in heat (like a toaster) in an effort to slow the motor down which is why it is so inefficient. With PWM you actually tax your battery or power supply less. Instead of turning the fan motor switch and getting two or three different speeds by routing it through resistors, with PWM you'll get a gradual change from zero up to full speed and by using much less power to do it.

This all forms part of what Ghost and I were trying to achieve, a sort of rudimentary climate control alongside control of the heater, something that VW do now but didn't back then. Frankly, I'm surprised that Vauxhall or any vehicle manufacturer uses resistors now given the above, but I gather it is in conjunction with air conditioning and there may be reasons why they chose to do it that way. Cost could be one but most with an interest in electronics would see resistors as an unimaginative and cheapskate way to do it. Replacement resistors can be bought but if you're pulling out a dash to replace the blower, it makes sense to do the job properly and get complete control too.

I don't strongly advise it, its just a better way of doing it. What I do strongly advise is paying attention to fuse ratings. Resistors (under normal circumstances) work well enough and the correct fuse is in place. Its just a bit Frankenstein in comparison and people will try to solve problems by upping the fuse. Doing so makes the rest of the circuit including wires, a fuse in itself. It all starts to get hot which can lead to fires.

[Synkraut]

Thank you for the link! Ordered one.

[Smosh]

Okay, thanks for explaining that. Certainly sounds worth doing when I get the dash out. As for the rest of the fuses in my van, that sounds like a long afternoon of trying read Haynes diagrams and play hunt the fuse!

[Synkraut]

I've had the dash out a few times already (OK on the other T25, larger brake booster, insulation,...). It's not really that much of a job, nothing compared to swapping the starter on a syncro wbx

[kit]

I have had no heater for the last 10 years but there is no way I intend to strip out the dash to find out out why.

[Mr Bean]

Its about efficiency too. Resistors are not remotely efficient compared to a PWM controller. Its a bit like using the handbrake to slow you down instead of going down the gears Well worth checking the correct fuse is in place for each circuit though. Most owners seem to end up with a mish mash of fuses that don't protect circuits.

Please don't tell me this is what is fitted in the Zaphira.

Cheap they might be but why would you



At least mosfets have a heatsink. I suppose it could double up as a toaster.

Yes I use these little PWM boards for many purposes including flow control on my sink tap water pump in the camper. As already stated:
Never leave permanently powered up and always use appropriate faze values regardles of manufacturers claim re over current protection.
CS