Rear Windscreen De-Fogger Button

Cricketer007

Standard Member
Joined
Feb 9, 2021
Messages
32
Reaction score
40
Points
22
Location
Surrey, United Kingdom
Driving
Hi all. Anyone else experience a clicking sound when pressing the rear windscreen defogger button?
No continuous clicking, just once when you press and then again when you turn it off. Seems like the sound is coming from the glove compartment.

Normal?
 
I can't say I've noticed it but it's probably a relay in the fuse box behind the glove box
 
Er, that’s the relay operating. The rear window element draws a lot of current - hence being relay controlled
 
Yes, I’ve heard it. It sounds like a relay to me, but I thought they were no longer used.
 
Yes, I’ve heard it. It sounds like a relay to me, but I thought they were no longer used.
Why would you think that? Not being funny - just genuinely interested. There are other ways of controlling large currents, but tend to be much more expensive. The humble relay is a very cost effective little device which is reliable and has proven itself over many years. You can also hear them clicking away when the car starts it’s charging activity ?
 
In the digital world it seems an old solution. Do the wipers also have a relay for flashing and sound? Perhaps it’s just a cost thing.
 
In the digital world it seems an old solution. Do the wipers also have a relay for flashing and sound? Perhaps it’s just a cost thing.
Depends totally on the amount of current being switched. The heated rear window takes a LOT of current, hence using a relay. It might be the MG have chosen another option for the wiper motor, or, it could be that the relay is under the bonnet - in which case you won’t hear it.

Solid state relays are available, but can be many times the cost of a relay - hence not cost effective to use in all cases. Relays might be “old tech”, but are cheap, small, easy to fault-find and fix and also provide complete electrical isolation between the two parts of the circuit.

Almost certainly financial & if it aint broken then don’t fix it ?
 
Yes - not sure about this ‘if it aint broke don’t fix it‘ mantra though - we’d never make any progress if we didn’t introduce new things and push out the boundaries using new tech. The old ICE version of our car wasn’t ’broke’
 
Hi all. Anyone else experience a clicking sound when pressing the rear windscreen defogger button?
No continuous clicking, just once when you press and then again when you turn it off. Seems like the sound is coming from the glove compartment.

Normal?
Normal.
 
Perhaps the MG5 doesn’t use a relay?
Relays need to be used in order to manage the switching of large currents, using low current control circuitry.
Otherwise the control circuity needs to be at high power too, which is not for many reasons what you want.

- I am not an electronics expert, so take this as I'm only 99% certain about all this!
 
So not something I need to be concerned about?
I would say that Owners of EV cars notice all sorts of noises in their cars that they did not notice in their ice cars. The EV is so quiet all these noises are a bit more noticeable. These noises are there in the ICE cars as well you just don’t hear it above the engine noise?
 
Relays need to be used in order to manage the switching of large currents, using low current control circuitry.
Otherwise the control circuity needs to be at high power too, which is not for many reasons what you want.

- I am not an electronics expert, so take this as I'm only 99% certain about all this!
Mosfets can be used to switch larger currents but they produce more heat.
 
Mosfets can be used to switch larger currents but they produce more heat.
True, but by the time you take the cost of the power MOSFET into account and then add the issue of a heatsink which will probably be needed, plus its location and cost - then the relay tends to win again. For work we actually use MOSFETS for just such a purpose in an automotive environment- but the reasons are quite specific and relays are used as well.
 
Yes - not sure about this ‘if it aint broke don’t fix it‘ mantra though - we’d never make any progress if we didn’t introduce new things and push out the boundaries using new tech. The old ICE version of our car wasn’t ’broke’
It depends on whether there is a better solution at an appropriate price, ice cars are being made obsolete, technically there’s nothing wrong with them! It is an environmental issue. Although with fossil fuel power stations it is currently moving an issue not curing it!
 
Support us by becoming a Premium Member

Latest MG EVs video

MG ZS EV Retrospective & First Look at the MGS5 EV | Live Q&A with Owners & MGEVs Panel
Subscribe to our YouTube channel
Back
Top Bottom