Heated windscreen help

[curly head]

hello, new to the scene and restoring my first ever bus - its seeming to takes ages

apologies for dumb questions but i am dumb :roll:

i was wondering if anyone could help me in deciding whats the best way to wire in a heated screen as i am currently running a dynamo. whats the difference between that and an alternator? i heave heard that i need to ditch the dynamo for a 85A alternator and wire through 2no 40a relays.

thought i would ask if anyone has experienced this as it seemed a bit overkill. i was originally just going to use a single 40a relay.

cheers for any help

cant wait till i get it on the road

 

[VeeDubMan]

I wouldn't have thought the dynamo vs alternator was an issue when it comes to the windscreen.

Have you got the windscreen yet? I would've thought it would come with everything (I'm sure the JK ones do).

When I wired in my electrically heated front window on my bug, I used a single relay. Some years ago now though, so don't remember exact details.

 

[curly head]

thanks for your help VeeDubMan.

i already have the screen, bought it from a distant mate of a mate. spoke to JK and they said they cant help me as their screen doesnt come with any instructions!!! wierd

all i can find are 75A alternators. does anyone know if there is such a thing as 85A or where i could get one from?

 

[birchyman]

im not sure what a heated screen will pull in amps but it carnt be a lot my guess going off what other things pull would be 4-5 amps tops
you should wire it through a relay but the main battery feed to your fuse box will be fine
get a 4 pin relay pin 30 to battery pin 85 to earth pin 87 is the output in this case to the screen
then 86 is your switch find a ign live put it through your switch bobs your uncle
people seem to think the bigger the relay the more power, not the case a 40amp will let 40 amp through
but your appliance would need to be pulling 40 amp
relays dont deliver power like a pump they are just a switch
good luck and be carefull with the wires on the screen they have a bad habit of snapping when you fit the rubber

 

[mjknight71]

...just quickly looked at fuse ratings on a modern car...and heated rear screen uses a 25A.....

....the relay is only switching high current (thicker cable) using a low current cct (thin cable).......

Re: the generator it may struggle with lights/heated screen/stereo/wipers/blower fan all on at the same time :lol: ...you'll just get a generator light and start to draw off the battery...especially at low revs :roll:

 

[curly head]

thanks for your help guys.

i think i will upgrade dynamo to a 75A alternator and chuck in a 40a relay. i assume it doesnt hurt to oversize the relay?

 

[dubdubz]

ohms law is surely your friend?

I suppose it depends if the heated screen has varying resistance when on?

but this link may help? just get a resistance reading on the 2 ends of the screen element and work on 13.8v ....or is this duff?

edit yes link would help!!

http://www.the12volt.com/ohm/page2.asp#current

 

[mjknight71]

i think i will upgrade dynamo to a 75A alternator

fit the windscreen first and find out :? .....sounds an expensive route...

 

[skin69]

To find the current draw, measure the resistance of leads (mine has two sets of leads).
Then it's a case of Current=Volts/Resistance

12 Volts
--------
0.3 Ohms = 40A

Hope thats clear as ????

Remeber though if you have 2 sets of leads total would be 80A....

 

[curly head]

thanks for all your help.

do you know how yours is set up skin69 and what size alternator you have? mine also has 2 sets of leads as there are 2 seperate circuits, left and right.

again cheers for help - i hate electrics!!!!!

 

[skin69]

I think my alternator is about 55A, but not wired up the screen yet as it's on my to do list!!!
Remember that you don't need the screen on full all the time.
A few mins should start to clear it then just turn it on/off as required, if you only use half once clear it'll be fine :roll: