Headlight relay

[jimj]

I`ve finally got around to replacing my headlight relay on the S3 which might or might not be faulty. What a fiddly job to get to it. The new one doesn`t have any markings and is slightly different but I`m assuming the wires just go where they look they should in comparison. Can anyone confirm this? Thanks.
Jim

[lotusfan]

Jim

That could be a dangerous assumption!!!! given that the old relay has 3 connections and the new one has 4. You may need to add a wire for the 4th connection to get it to work.

What is the story on the new relay? How do you know it's the correct one? No markings you say, none at all?

Does the old relay have any markings?

See viewtopic.php?f=38&t=45688 and look at the 3rd post down from Craven.

It could be that your old relay is a SRB501 and your new one is a SRB111 but we neeed a few more clues.

I am not familiar with the headlight wiring on the S3 is it the same as the S4?

[The Veg]

I think I see a faint 'W2' marking by the terminal nearest the camera and a 'W1' by the one at the opposite corner. My car came with some spare relays that have this problem- the terminal markings are embossed into the soft material around the terminals, and they seem to fade over time. I have no idea how old my spares are but some are barely legible and even then some good light is required to see them.

[jimj]

Mike, the new relay is a 6RA from SJS, the old one has 3 terminals, the right one is double with a single blue and grey wire marked C1, the middle has another blue and grey wire but no marking. The left one is marked W1 and has a yellow and blue wire. The new one says Lucas and has 38/18 written in black, it has a double and a single connection on the right, single ones in the middle and on the left but no markings.
Jim

[steve lyle]

All the 3 terminal Lucas relays I've seen have one of the terminals being a ground. None of your 3 wires are black, which is a bit confusing. Was there a ground wire to the case? A relay isn't going to work without a ground....

In any event, you should be able to run a few tests and determine how to wire up your new one.

One caveat, first. I've not seen a Lucas relay that "switches off" - that when the relay is activated, the continuity between two of the terminals is broken (Bosch makes relays of this type). So you likely don't have to worry about this, but I'd first use a voltmeter and determine if you have 0 ohms between any 2 of your 4 terminals. If you do, don't include the those terminals in the following test - or if you do, expect a short. That said...

A relay is simply an electric switch. Two of the terminals (usually labeled W1 and W2, W1 being ground and W2 being switched +12v) on your new relay activate the switch (a solenoid inside the relay), and two are what gets switched. The activation terminals are a ground (W1), and a wire from your headlight switch, which goes to +12v when you turn on your lights on W2. I'm guessing that's the blue/yellow, but you can figure that out easily enough with a voltmeter - turn on your headlight switch, and see which of your 3 wires goes to +12v, and then goes to 0v when the switch is off. To do that test, put one of your voltmeter leads on a good ground, and the other on the terminal of the wire you're testing, and flip the switch.

To determine which terminals on the relay do the switching, get a ground wire and a +12v wire, and start trying out terminals - put the ground on any one of the four, then the +12v on each of the remaining 3. When you hear a "click", you'll know you've got the right terminals - the ground is on W1 and the +12v is on W2. If none of them "click", move the ground to another candidate terminal, and repeat.

Your more than halfway done. The other two terminals are what gets switched, usually labeled C1 and C2. One will be a constant +12V, the other will go to the headlights. Polarity doesn't matter, so hook up your constant +12v to one, and the wire to the headlight to the other. Test your lights, they should work.

[lotusfan]

Jim

Steve is right, it won't work without a ground, (or earth as we say over here.) Was there another wire connected to one of the relay case mounting screws? It should be a black/red wire from one of the headlight microswitches.

I have looked at the WSM and the S3 and S4 wiring for headlights is identical.

If SJS have sold you a 6RA as a headlight relay lets assume it is this one:

While I have been researching this Steve has given you more useful infomation.

[oldelanman]

Jim,
Looking at the wiring diagrams in the Workshop Manual it appears that you may have S2 wiring in your S3 ... perhaps it's an early S3 ?

The S2 diagram shows a single relay with 3 terminals controlling the headlights, which is what you appear to have, while the S3 and S4's have two 4 terminal relays controlling main and dip separately.

The 3 terminal relay is no longer obtainable and the 4 terminal is used in it's place. All you need to do is connect terminals C2 and W2 with a jumper wire (on the 3 terminal relay this connection is made internally) and then connect the 3 wires from your loom the C1, C2 and W1 as before.

S2 wiring diagram

[steve lyle]

Jim,

If oldelanman's diagram is correct, and I have no reason to doubt it, your blue/yellow goes to W1 (the single terminal on top/right of the diagram).

He's no doubt correct about the jumper and the C2 connection as well, but it will work, and be a bit easier, to put the "hot" blue/gray on C1, since it's a double terminal, and then make up a jumper from C1 to W2, taking advantage of the 2 connection points on C1. In other words, take your current C2 from your old relay (the middle wire), and put it on C1 on the new one, and vice versa. This works because C1 and C2 are switched terminals - it really doesn't matter which one is the 'supply', and which goes to the 'load'.

BTW - what threw me off was not realizing that the headlight switch is switching to ground, not +12v - that's why your old relay doesn't have a black wire - the blue/yellow provides the ground when the switch is "on".

[jimj]

Thankyou gentlemen, wired up and working now.
Jim