Gold and silver resistors

[T-Cut]

Indeed. Presumably the Zafira fires have resulted from fried wiring that wasn't suitably protected. I'm guessing, since it's not a story I've followed.

EDIT: A thermal fuse issue in the blower motor system.

TC

[MarkS]

The Zafire issue started with a water leak from the windscreen area, which caused the bearings in the fan motor to start to rust, which caused the increased current drawn by the partially seized motor to dissipate more heat in the series resistor, which radiated more heat to the surrounding plastic, which melted onto the bare resistor, and caught fire. But I could be wrong.

I believe the version which used a PWM speed controller (as opposed to a series resistor arrangement) never caught fire, or was recalled.

Good job, R75s don't suffer from leaks then, do they...

[HarryM1BYT]

Originally Posted by DerekS

Harry ... your earlier suggestion of designing a PWM supply is a good one, and should probably have been the original design . . However unfortunately for us normal spanner monkeys its a step too far. .If it would be possible to design a simple plug&play replacement without a T4 involvement , i would think there would be plenty of customers .
This is one area where the original system could be improved .(similar to the sunroof drains /handbrake etc ) .

I was last involved with a PWM system 30 years ago. It was for a really tiny motor for an endless loop data tape drive and it needed variable speed with speed feedback. I sensed the back EMF, as a way to determine speed and it needed to hold the read/write speed accurately, yet allow full speed for seek. The control system had failed, it was an expensive system at the time, so I delved in to sort it. Whilst I know what can be done using PWM, designing this would be an entirely different ball game, especially due to the currents involved, so I will pass on this one. There are others in the forum, much more qualified than me to design this.

There is no reason why is should appear to the ECU, as any different to a resistor. It just needs the resistor to be replaced with a single output level PWM system, triggered by the existing system. Instead of a resistor dissipating lots of watts, you just have a semi-conductor rapidly switching on and off the 12v so as to provide an average voltage around 50% of the usual 14v. Switching cleanly on and off, produces very little dissipation in the device, so it needs a minimum of cooling.

[HarryM1BYT]

Originally Posted by MarkS

The Zafire issue started with a water leak from the windscreen area, which caused the bearings in the fan motor to start to rust, which caused the increased current drawn by the partially seized motor to dissipate more heat in the series resistor, which radiated more heat to the surrounding plastic, which melted onto the bare resistor, and caught fire. But I could be wrong.

That was my understanding of the issue too. Bear in mind, seizing motor bearings mean the fan will run slower, the slower the fan runs the more current it will try to draw and the less effective will be the resistors cooling - assuming it is it the air duct.

[MSS]

Originally Posted by HarryM1BYT

I was last involved with a PWM system 30 years ago. It was for a really tiny motor for an endless loop data tape drive and it needed variable speed with speed feedback. I sensed the back EMF, as a way to determine speed and it needed to hold the read/write speed accurately, yet allow full speed for seek. The control system had failed, it was an expensive system at the time, so I delved in to sort it. Whilst I know what can be done using PWM, designing this would be an entirely different ball game, especially due to the currents involved, so I will pass on this one. There are others in the forum, much more qualified than me to design this.

There is no reason why is should appear to the ECU, as any different to a resistor. It just needs the resistor to be replaced with a single output level PWM system, triggered by the existing system. Instead of a resistor dissipating lots of watts, you just have a semi-conductor rapidly switching on and off the 12v so as to provide an average voltage around 50% of the usual 14v. Switching cleanly on and off, produces very little dissipation in the device, so it needs a minimum of cooling.

I have been designing and building PWM controllers for 30+ years for my RC model boats. Our motors can draw 100A+ if stalled. These days, designing one to handle 50A is a piece of cake with the availability of ultra low turn-on resistance FETs. When I first started, I had to use darlington bipolar transistors in order to handle anything over 20A and the transistors were mounted on large heatsinks which were water cooled as the boats moved in the water!

PWM would be an elegant solution now but not when our cars were designed. At that time, a high-power resistor was the most reliable and economical solution.

Personally, I think people are worrying too much about problems that do not exist. Even the OEM resistor on my CDT has lasted 4 years. Given a replacement can be purchased for less than £10 and takes 30 minutes to install, we should just consider these to be consumable items with a lifespan of 4-5 years.

[murphyv310]

Originally Posted by mss

I have been designing and building PWM controllers for 30+ years for my RC model boats. Our motors can draw 100A+ if stalled. These days, designing one to handle 50A is a piece of cake with the availability of ultra low turn-on resistance FETs. When I first started, I had to use darlington bipolar transistors in order to handle anything over 20A and the transistors were mounted on large heatsinks which were water cooled as the boats moved in the water!

PWM would be an elegant solution now but not when our cars were designed. At that time, a high-power resistor was the most reliable and economical solution.

Personally, I think people are worrying too much about problems that do not exist. Even the OEM resistor on my CDT has lasted 4 years. Given a replacement can be purchased for less than £10 and takes 30 minutes to install, we should just consider these to be consumable items with a lifespan of 4-5 years.

Absolutely agree.
Like yourself I've built many PWM PSU's for projects over the years. With today's range of fast and low resistance switching MOSFETS they are the obvious answer.
To build one off's though is impracticable. I did think of using the "K" bus at one time and have the fan running on a curve vs temperature, 95c the fan would start with a short M/S ratio making it run at only a few rpm, this would rise in relation to coolant temperature till it was then fully on.
The IRF510 Mosfet was one I considered at the time, a decent snubber circuit and RC filter too.

Going back to the series resistor, perhaps the OE resistor being rated at 50w was not such a bad idea. If the fan was baulked then the current rises, not enough to pop the fuse but enough for the series resistor to blow, just like a fusible resistor, if the system then trips and full speed is selected then the fuse will hopefully blow.
With the 100w resistor there is less likelihood of it blowing but it could get seriously hot and melt something or even cause a fire.
At the end of the day would you replace a "fusible resistor" with a 10 watt ceramic wirewound???

[HarryM1BYT]

Originally Posted by mss

PWM would be an elegant solution now but not when our cars were designed. At that time, a high-power resistor was the most reliable and economical solution.

Well, they used PWM for the climate system interior fan. The climate unit generated the PWM waveform, which is then fed to the MOSFET module which actually powers the interior fan motor.

[T-Cut]

Sounds easy enough.
Right then, who's volunteering? Where do I put my name down for a PWM controller?

TC

[DerekS]

Thanks to all , for an interesting discussion.
Ive got a diesel, with no resistor, and no gas in the aircon . Who needs a fan anyway ?
Why do I bother ????

Cos when youre retired as much as I am , theres not much else to occupy the time !

[T-Cut]

Originally Posted by DerekS

when youre retired as much as I am, there's not much else to occupy the time!

Maybe you could learn how to make 100amp PWMs and start a new business?

TC