Trying to understand the electric window wiring in my new mk2. It has 3 fuses in the system, none of which seem to be 'thermal fuse' type. Seen reference to a 20A thermal fuse- part number 443937105 -in the bracket that holds the relay above the fusebox (No. 12 below)? Looks like this. No.13 in this illustration Presumably (assumption time!) this is to give some protection to the window switches, so they don't take too big a surge as the motors start up? If not, why is it necessary? Thanks!
Normally "Thermal" fuses are wired in series into a transformer winding and if the transformer fails or gets too hot the fuse blows. So in this thinking it might be there to protect the wiring insulation from melting if say the motor is slow and needs alot of current to move the window reg but not enough current to blow the main fuse. An example could be that you have used your windows alot in one day, the cables are getting hot so thermal fuses blows to stop the wiring insulation from melting.
Thanks for the reply Chris. I can picture a little one of those fuses inside a Nokia charger back in the days when they were just a transformer and a couple of diodes. Would make sense if such a fuse were attached to the motor housings or something, if that were the idea. Although relays do get hot, and it appears to be slammed up against the back of the relay holder; so you could be right. On closer inspection, two of the three fuses are in series, feeding the relay permanent 12V battery power, so one of those can probably get deleted. The other seems to be in the 'X' power rail, to trigger the relay, as per diagrams 45/46 in Haynes. The wiring's all a bit/lot on the thin side, especially the ground/0V ones back from the switches, so that may explain the snail-like pace of window motion.
Another thought... What happens when the window gets to the top and bottom of its travel? There's no microswitch type end limits in the motor is there? Does the motor just stall out? If so maybe the thermal fuse is there to handle the stalled motor current without popping, where a normal fuse might pop before you let go of the switch?
That is a possible. However on a car with a D.I.Y electric window kit such as mine all you get is an inline fuse wired up from ignition live. On my clifford concept 600 window roll up it has a current cutoff. So you put a meter in series with the circuit and wind up the window until the motor stops and draws more current. You then set that in the clifford, without that the fuse will pop or worst case the wires burn out/ motor burns out. So a click of my alarm puts my windows up if I forget. lol... Just a flash git really. haha
the thermofuse is just there to cut the motors out if they get jammed or some fool sits there with his finger on the botton after its open/closed. you're reading too much into it lad the wiring is simple as simple can be. thermal fuse on live feed to relay, relay is ignition triggered. the relay powers the switches, which then power the motors. no micro cutout switches anywhere they will just run (or try to) while the button is pressed. sometimes the thermal fuse is replaced with a normal fuse (30a iirc?) edit: it should just have the 1 thermal fuse, no external fuse on the ign live feed (it will be fused by one of the main fuses)
Just trying to understand it mate. If I'm right about what the thermal fuse is for (current limiter, not one-time cut-out) then it may explain why the PO of my car had trouble with fuses popping and switches needing their contacts cleaned regularly. I've ordered a secondhand 443937105 from ebay, so should know more about its characteristics soon. Thanks for your help.
Well the Ebay one didn't happen, but to my great surprise when I was at my local VW dealers on other business, I asked for a price on 443 937 105 and was told that there was one on the shelf. About 12 after discount. Prizing off the metal cover reveals a little bimetallic strip with a coil around it. So the strip carries current up to a certain limit, at which point it warms itself up due to I²R heating of the strip and contact, so bends enough to break the contact that is made by one end of it. The heating coil then takes over to retain the 'brokenness' of the contact until the voltage across the device is removed (finger off switch) and it cools down again. (A smarter man than me figured out this last bit, cheers Steve ). The heating coil is shorted out whenever the main contact is made, so virtually no current will flow through it until there's an overload; its resistance is 30-odd Ohms. So it's fully and automatically resettable, and should prevent sustained currents flowing that might damage switch contacts, wiring, and other fuses (if fitted). Neat.
