Headlamp relays.

[John Larkin]

Can anyone tell me what current is drawn through the control or primary circuit on each of the Elan's two headlamp relays? The original factory loom cables for the primary feed to these relays are 2 square mm cable (28 copper strands each 0.3mm diameter). This size cable has a capacity of 17.5 amps. Is a cable this big really needed? I have read through my various car electrics books and only one indicates a typical current for a four pole relay --- 175 milliamps.

I have a new loom in which the equivalent cables are 14 strand (1 square mm, 8.75 amps capacity) and I feel that these are acceptable for the task, but I am looking for a re-assuring arm around my shoulder. The supplier of the loom is willing to replace it, but I have tagged and numbered every cable termination and I'd prefer not to have to do that again with a replacement loom. I also want to get my car back on the road ASAP.

Yours currently,

John Larkin

[paddy]

I don't have the wiring diagram in front of me so I'm going from memory here. I'm familiar with the S1/S2 loom and don't know if yours is different.

The original wiring arrangement arrangement for the relays is unusual in that the relay is not used to divert current away from the lighting switch or the dip switch. The relay is just there so that the microswitches activated by the popups can turn the lights on and off. Therefore, there is only a single switched supply to each relay (one for main and one for dipped) which powers the lamps and also powers the relay itself. (The microswitches provide the earth path for the relay coil.)

So I think the cables you're talking about (one is blue/white and one is blue/yellow from memory) carry the full current for the lamps, not just the current for the relays. If this is the case then you do need 10 amps capacity (assuming you have 60W main beam each side).

If you can confirm that the wires only supply power to the relay and not the lights then, as you say, the current will be negligible.

Paddy

[paddy]

I've just had a look at the wiring diagram and my memory wasn't quite correct - the two relays are for "normal" and "flashing" operation (on S1/S1 anyway) - but the principle is the same. In the standard arrangement there aren't separate supplies for the lights and relay.

If your new loom is designed with specific provision for relays that do actually divert the main light current away from the switches, then you would have the situation where the wiring to the relay coil doesn't need to have the same rating as the "equivalent" wire in the original loom.

Did you get a wiring diagram with the new loom? If it's a direct copy of one of the stock looms, which model do you have and what's the colour of the wire in question?

Paddy

[mikealdren]

Simplistically (I know the load is not purely resistive), two 55w bulbs draw 2*55/12 amps, approx 10 amps. Bear in mind that you will want some headroom and at 14V running voltage, the current will be higher.

Mike

[alan71]

If you have a series 3 or 4 the cables I have marked in red are the ones that carry the high current to the head lamps.

Alan

[John Larkin]

Thanks for the replies; I really appreciate the support. Alan's picture of the headlamp power circuit tallies with one that I did myself last night. Two of my three "problem" cables, the blue/black and blue/brown, are the high beam and dipped beam control cables respectively.
The third cable (blue/yellow) supplies power to one pole on the headlamp flasher relay, which is shown as a three pole item on the factory wiring diagram. A second pole connects to a flasher unit which then connects to earth via a microswitch on a headlamp pod. The third pole on that flasher relay supplies power to the headlamp main beam filaments. So it seems that the blue/yellow cable carries full current to the headlamps and needs to be a 28 strand cable as per the original loom, and not the 14 strand as fitted in the new loom. I have all three relays (high beam, dipped beam, and flasher relays) in my car but not the flasher unit. Threre are no headlamp microswitches on my car. I have never had a flasher facility, and I can live without it.
The blue/black and blue/brown cables only need to carry a control current to the main beam and dipped relays. An automotive electrical engineering company here in Dublin confirmed that the control current would be 350 milliamps at most, and that the 14 strand cable is way bigger than needed. Problem solved. Thanks again. This is a great forum.

John Larkin.

[Dennis 45/9760]

Don't forget the black ground return (earth) from the headlamp unit itself. The higher current must pass through that path also.

I am new to the Elan net and have a Series 4 that I am doing some "cleaning up" of the engine compartment wiring.