Gold and silver resistors

[steve-45]

I like the idea of mounting it elsewhere, especially away from the elements. If it was mounted under the slam panel - with some silicone heatsink paste to aid conductivity, would the lack of air flow be a problem?

[MarkS]

Originally Posted by T-Cut

According to the theory as to why they 'explode', the twins in parallel approach is no less likely to blow than the standard system. They'll both get above 100C internally. The 40amp fuse is a wise move IMO. Let us know how you get along with the fitting. It'll be worth taking plenty of photos for a HowTo, so please do come back on this. They need to be 1ohm each of course.

TC

Just as a little aside, you recommend fitting a 40amp fuse. I notice the R75 fusebox only have 50, 60, 80 and 100amp fuses fitted. Where would/did you get the 40amp fuse from ?

[SD1too]

Originally Posted by MarkS

Just as a little aside, you recommend fitting a 40amp fuse.

I think some explanation and caution would be wise here.

KV6 and diesel engines are fitted with a 450 watt motor which, when running at full speed (according to the simple electrical equation) draws about 33 amps.

T-Cut, who said he had fitted a 40 amp fuse, has the 1.8 engine with, most likely, a 350 watt motor which draws about 30 amps at full speed.

In both cases, should the resistor become open circuit, there will be a considerable surge when the motor starts at full speed. In these circumstances, there isn't much margin for error in the case of the 450 watt motor; the fuse is at risk of blowing when a fault is not present and resulting in overheating of the KV6 engine.

Fuses provide only short circuit protection and they work extremely quickly, probably a matter of milliseconds. They are not designed to detect a relatively slow increase in current flow resulting from an under-rated resistor becoming too hot and partially disintegrating. When this happens, the resistor itself behaves like a thermal switch.

To put this in perspective, excluding the initial non-resistor design which is not susceptible, we have lived with failing resistors for fifteen years now. I do not remember a single case of a catastrophic fire being caused, or a burned out loom for that matter, but no doubt someone will now come forward with just such a story!

As you might have seen from a recent rather passionate debate, I am in favour of improving upon manufacturer's recommendations when justified. In this particular case though, I'd stick with the 80 amp fuse, just for "peace of mind" as they say.

Simon

[HarryM1BYT]

Originally Posted by MarkS

Just as a little aside, you recommend fitting a 40amp fuse. I notice the R75 fusebox only have 50, 60, 80 and 100amp fuses fitted. Where would/did you get the 40amp fuse from ?

Any motor factors should stock them, or Ebay...

http://www.ebay.co.uk/itm/MIDI-STRIP...9Z8OIsmYstDCmA

[murphyv310]

Hi.
There are a few other factors though. Firstly I'd be pulling the fan fuse and check the current with an ammeter where the fuse was. Now it would be a good idea to measure both speeds. At high speed the loading will be higher and it's possible the current drawn could be higher than the rating of the motor would make you believe. The disadvantage of de rating the fuse is it could overheat rather than blow, this could ruin contacts making the problem worse leading to a fire.

Let's not forget the 50w original resistor would act more like a fusible resistor than the 100w type.

[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 ) .

[T-Cut]

Originally Posted by SD1too

In both cases, should the resistor become open circuit, there will be a considerable surge when the motor starts at full speed.

Presumably if the ECU trips it on. The initial surge current was discussed when I raised the idea a year or so back. I thought it was said the startup load would be similar for all speeds. Whatever, the 40amp fuse is my starting point and is more or less arbitrary. It's worked for me so far, but it's only had normal aircon duty. A call for the high speed might blow it I suppose. It's something I might test. I keep the 80amp link in the car anyway. It's basically an idea for discussion and experiment. But frying the fan harness isn't acceptable in any circumstance and clearly should be prevented.

TC

[MarkS]

Originally Posted by T-Cut

Presumably if the ECU trips it on. The initial surge current was discussed when I raised the idea a year or so back. I thought it was said the startup load would be similar for all speeds. Whatever, the 40amp fuse is my starting point and is more or less arbitrary. It's worked for me so far, but it's only had normal aircon duty. A call for the high speed might blow it I suppose. It's something I might test. I keep the 80amp link in the car anyway. It's basically an idea for discussion and experiment. But frying the fan harness isn't acceptable in any circumstance and clearly should be prevented.

TC

Suggestion:

How about for a 450W motor:

1) Use a pair of 100W 1 ohm resistors, connected in parallel, mounted back to back in the normal position
1) Reducing the 80A fuse to say 50A. Keep the 80A fuse in the car as a spare.
2) Putting a lower rated fuse in series with the resistor, say 25A.

Then if the 25A fuse blows, the car still has full speed to fall back on, preventing overheating.
The resistor is better protected from dramatic failure by the 25A fuse.
The 2pcs 100W will operate at half the temperature of a single 100W resistor, which if below 100 degrees C, will stop steam generation, and subsequent explosion.

BTW: Is the fan motor a series wound motor ? I seem to remember the current increases dramatically if the motor is jammed. And fans are awfully vunerable in the event of a front end shunt (which aren't exactly unheard of); or jamming on an undertray (as Mike Trident found out!).

[T-Cut]

Originally Posted by MarkS

Suggestion:
How about - - - -

I honestly don't see that the current single resister arrangement as a problem worth worrying about. There are umpteen reports of the resistor popping for whatever reason, but there are umpteen thousand resistors in normal trouble-free operation and have been for over a decade. The overheating/explosion issue is out of proportion with the reality.

Of much greater concern in my opinion, is a few reports of overheated or fried wiring with smoke and indications of fire. There should be no possibility of a wiring fire from any system in these cars, including the high ampage fan system. The resistor is as much the victim in this as the wiring. I reckon that's where we need to focus the discussion. My 40 amp fuse is simply a starter for one.

TC

[MarkS]

Check the scrapyards for unused fire extinguishers in Vauxhall Zafires ? ;-)