Wiring a relay for a heated rear windscreen

[pdcornish]

I want to include a relay in the wiring of my traveller's heated rear windscreens. The relay has numbered terminals: 30 85 86 87.

85 and 86 seem to operate as a switch inside the relay. 30 and 87 seem to complete the circuit when switch operates. Have I got it right?

Which terminal goes to which part of the car?

Is there a wiring diagram somewhere?

Any advice would be welcome. Thanks

[bmcecosse]

Be sure to put a fuse in the supply to the relay.

[mogbob]

Search ( yahoo ) for "wiring a heated rear windscreen relay ".
Second hit....www.heatedwindscreen.com does a very good diagram, which includes the illuminated rocker switch.
As Roy says it will need fusing... it gobbles up electricity.Check the combined draw of the two screens and fit the correctly rated fuse.
Also make sure that the Relay can handle the amps requirement.Heavy duty relays are available as well as ones that will switch themselves off after a certain period ( in case you forget switching them on )
Bob

[bmcecosse]

I suggest if both front and rear heated screens are fitted - they should be individually switched/fused/relayed. And only use ONE at a time to give the alternator a chance.

[pdcornish]

Search ( yahoo ) for "wiring a heated rear windscreen relay ".
Second hit....www.heatedwindscreen.com does a very good diagram, which includes the illuminated rocker switch.
As Roy says it will need fusing... it gobbles up electricity.Check the combined draw of the two screens and fit the correctly rated fuse.
Also make sure that the Relay can handle the amps requirement.Heavy duty relays are available as well as ones that will switch themselves off after a certain period ( in case you forget switching them on )
Bob

THANKS VERY HELPFUL.
PDC

[beero]

These diagrams might help you wire your relays.

http://www.vehicle-wiring-products.eu/V ... relays.php

[bmcecosse]

Good heavy gauge 20 amp cable needed for the power circuits.......

[IslipMinor]

Most of the standard 4-pin relays are rated at 30A, so more than enough for either front or rear screens. There are some at 20A, but still enough.

Do you have front and rear heated screens, or just rear? What size alternator do you have fitted?

Assuming you have -ve earth, the wiring convention is that the +ve fused supply from the switch (which can be from the normal green fused 'A4' supply) goes to terminal 86, and terminal 85 to earth (-ve) using either a 9/0.3mm (0.65mm2) or 14/0.3mm (1.0mm2) cable.

The switched side of the relay needs a separately fused supply connected to terminal 30 (this can be directly from the starter solenoid via a fuse), and the feed to the screen(s) is taken from terminal 87. The feed to and from the relay will need a 28/0.3mm (2mm2) cable.

The diagram below shows the connections, with 'ON' warning light added:
[frame][/frame]

[rayofleamington]

Sorry if I'm teaching you to suck eggs - I've seen so many dangerous mistakes on car wiring modifications, and the principles are not so difficult...

The fuse needs to be big enough for the load (e.g. the screen rating in this case).
Bear in mind that the fuse is there to stop the wiring catching fire if the load (or somewhere in the circuit) goes short circuit.
The fuse should be the first item attaching the 'live' wire (not at the end of the wire, not in the ground wire etc..)

Every wire you add needs to be protected - if you wire from any existing circuits you need to check the existing fuse vs your new cable. If the fuse is much bigger than you want for the new cable, add an additional fuse for your smaller cable.

The cable rating is usually safe somewhere between 150% and 200% of your fuse rating*.
In very crude terms for a 20A fuse, you want 30A (or higher) cable rating.

Using an oversize fuse is not a good idea.
Using larger cable is safe (although if you have 1000+ cables to worry about like a modern car it adds weight and cost).

e.g. a 20A fuse will take a long time to blow at 27A.. (135%)
so, using a 20A fuse with 20A cable isn't great protection!
The fuse should normally blow quickly with a short circuit (i.e. people usually get away with it if there's any kind of fuse) however it won't blow quickly if the load is less than double the rating e.g. if the wiring is already very hot!

The other thing to bear in mind with large DC loads is voltage drop. For a rear screen you can end up with something like a 3m cable length or more. The voltage drop in the cable will be reduced with a larger cable, meaning the screen heater will work better.

Don't do a long return on the cable! I've seen them done with a 5m PAIR of cables. Using a long return cable will double the voltage drop (double the wasted energy) and adds no benefit. Take the ground connection back to body as near as possible via a good earth point.

Relays:
Relays can (and do) fail. Sometimes they fail closed (i.e. with the load powered).
If you just rely on the 'switch position' to the relay you will be blind to a relay failing closed.
For a load that you can easily overlook (like a rear heated screen) it is really useful to add a lamp or LED on the dash to show if it is on (or not).
When you're connecting a lamp / LED to the load side of a relay, the cable needs to be either:
1) equally rated to the other big cable, or
2) a smaller rated wire, with a suitable fuse.

* If you want to be more accurate for cable size:
If the cable sits in a bunch of other cables all carrying high load then you have to increase the cable current rating.
If the cable can dissipate heat freely to air, then 150% is a reasonable starting point.
It sounds complicated but there's normally a look up table for length / current / installation type.

[lambrettalad]

an excellent answer it should be made a sticky

[stag36587]

an excellent answer it should be made a sticky

Couldn't agree more - enormously helpful