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Heater fan runs with switch 'off'
Topic Started: Feb 9 2017, 09:23 PM (615 Views)
dorset tony
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An annoying little problem on my 35.10 camper, the heater fan comes on even when switched off.
It doesn't always happen and usually after a while it goes off.When it's not going by itself, the switch works as it should - on/off and and all the speeds work.
I thought it might be a faulty switch but when I removed it to test it I discovered that the fan will still run with no switch!
No obvious signs of dampness but could a wet CIU/fusebox cause this?
Any ideas?

Thanks, Tony.
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ivecojoe
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they are known to have all sorts of electrical shorts when a leak develops and soaks the fuse box, I would suggest untill you have time to pull fuse box out and check, disconnect earth from battery when not in use....save a lot of possible problems. Obviously a short somewheres...Joe
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graysworld
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Mine was the heater resistor located in thhe right side (as you look at the engine) of the heater box under the bonnet. They are stupid money and as yet I have not got around to trying to source a different resistor. I managed to get a second hand one.

Graeme
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dorset tony
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Thanks for the replies!
Just had the fusebox out and it's dry and no signs of damage or shorting.
While sitting in the van having a cuppa after replacing the fusebox, the ignition was on but the fan swtiched off. After a few minutes I heard a muffled click
and then the fan came on full blast.
I moved the air control lever to the middle position to stop the fan but when I put it back to the fresh air position the fan stayed off so I feel the problem may be to do with the heater controls so I'm trying to get the centre section of the dash out.
I've managed to release the top but can't work out what's holding the bottom edge in place, so any help with
that would be welcome.

Thanks, Tony.
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ivecojoe
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if you heard a click that sounds like relay, something shorting powering it up? check that resistor as suggested they are known to fry.......
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graysworld
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My money is still on the resistor :) I tried everything else.



Graeme
Edited by graysworld, Feb 11 2017, 03:53 PM.
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dorset tony
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Got to the bottom of this eventually............
It turns out that there is a bi-metal thermostat on the resistor which, when activated, bypasses the resistor
and connects the fan at full speed irrespective of the switch position.
When a slow fan speed is selected either using the switch or the recirculate position on the air control, the
resistor heats up and the thermostat contacts slowly come together and the fan runs until the stat cools
and breaks the connection.
The faster fan speeds don't cause this to happen due to less heat from the resistor and higher airflow over the stat.
The stat can also be activated after switching the fan off due to residual heat from the resistor.

I think the 'click' that I heard just before the fan came on was hesitation of the motor starting due to dirty contacts.

I presume this stat is there to protect the resistor?
Maybe the resistors that burn out don't have a functioning thermostat?

I think my stat is probably closing at too low a temperature but I think I can live with it now that I know
what's happening to safeguard the expensive resistor.
One other thing that is not immediately obvious is that the motor is always live (with the ignition on) and the circuit
is made by the fan switch connecting to earth through the resistor windings.

Thanks to Joe and Graeme for their suggestions,
Tony
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ivecojoe
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Tony glad you got a result, and indeed appreciate the thankyou...it so rarely happens on here, really good to post your findings to help others.joe
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