How to add a relay to your main beam circuit

[johng]

Fitted a relay into my main beam circuit today. A 30amp fused relay from Halfords did the job for £6, plus a few crimp connectors and bits of cable I had lying around (the power cable from the alternator is somewhat larger than necessary, but it was available). Even managed to get enough slack on the light green headlight wire to fit it into the relay without having to extend it, so nice easy job.

The wiring is pretty straightforward. There will be a diagram on the relay as below.
You need to cut the wire which goes from the dipswitch to the main beam bulbs (on my 78 it's a light green wire, not sure if it's the same colour for all years). The end of this wire which comes from the dipswitch connects to contact 85. The end which goes to the lights connects to contact 87. Then you need to run a power cable from the alternator stud to contact 30 and an earth to contact 86. The power cable should be at least 12AWG I used much bigger than this just because it's what I had.

You need to have a fuse as the power for the main beams will no longer be coming through the headlamp switch. You can either use a relay with a fuse in it (as I did) or fit a separate inline fuse. My main beams draw just under 20amps, so I've gone for a 30amp fuse.
This is the relay I got.
Halfords HEF553 Relay 12V 30A 4 PIN | Halfords UK

 

[Mad4slalom]

Fitted a relay into my main beam circuit today. A 30amp fused relay from Halfords did the job for £6, plus a few crimp connectors and bits of cable I had lying around (the power cable from the alternator is somewhat larger than necessary, but it was available). Even managed to get enough slack on the light green headlight wire to fit it into the relay without having to extend it, so nice easy job.

The wiring is pretty straightforward. There will be a diagram on the relay as below.
You need to cut the wire which goes from the dipswitch to the main beam bulbs (on my 78 it's a light green wire, not sure if it's the same colour for all years). The end of this wire which comes from the dipswitch connects to contact 85. The end which goes to the lights connects to contact 87. Then you need to run a power cable from the alternator stud to contact 30 and an earth to contact 86. The power cable should be at least 12AWG I used much bigger than this just because it's what I had.

You need to have a fuse as the power for the main beams will no longer be coming through the headlamp switch. You can either use a relay with a fuse in it (as I did) or fit a separate inline fuse. My main beams draw just under 20amps, so I've gone for a 30amp fuse.
This is the relay I got.
Halfords HEF553 Relay 12V 30A 4 PIN | Halfords UK

Thanks for reposting this johng, i got the same relay now so will have a go at the weekend.
the owners handbook mentions the thermal cut out will trigger flashing or cutting out of full beams .

 

[Derek Nicol]

The owners handbook mentions the thermal cut out will trigger flashing or cutting out of full beams.

Does the supply for both dipped and main beam run through the same thermal cut out?
or is the thermal cut out only on the main beam circuit?

 

[johng]

Both circuits run through the same thermal cut out.

 

[kentvette]

Thanks for this John! I'll bee digging out the wiring diagrams, peering under the bonnet and having a look in Norauto, (our equivalent to Halfords) for a suitable relay!

 

[Chuffer]

Does the supply for both dipped and main beam run through the same thermal cut out?
or is the thermal cut out only on the main beam circuit?

I read somewhere a while back that the thermal circuit breaker cuts out at not much above the stock rating of 140 - 150 Watts total for 4 sealed beam Halogen units during operation . That means it`s very marginal and even a modest increase in output on any of the dip or main beam units will cause an overheat and lights cutting out with all four lights on . I believe the stock outer units ( dipped beam ) are Type H4000 and the inner ones ( main beam ) are H4001 .
When I first got my C3 and encountered the dreaded on / off flashing when all 4 lights were on and learned about the thermal breaker , I took the headlight switch out of the facia and discovered what must have been long standing damage . The plastic body of switch where all the cables enter it was burned away completely in places ! A wonder there had not been a major electric fire under the facia .
I replaced the switch and only put full beam on for very brief periods on dark roads . Really need to fit a relay . A winter project methinks .

 

[kentvette]

I may be tempting fate now, but I haven't ever experienced a cut of lights. I changed the dip/main units some years ago (can't find the invoice, so don't know when exactly, or what rating the bulbs are!) but some months afterwards we returned from a long weekend and had to do a few hours night time running, most of which was on main beam (no traffic an no lights here in the countryside!). I was a bit worried, but had no issues.

But, I will be looking at relays!

 

[Derek Nicol]

Both circuits run through the same thermal cut out.

So i assume that as you are barreling along towards a country bend on full beam and are suddenly plunged into complete darkness that switching to dipped beam wont necessarily provide any immediate relief illumination (until the cut out cools and reconnects)... if so, how on earth did GM ever get away with designing in such a huge safety issue.

 

[Forrest Gump]

So i assume that as you are barreling along towards a country bend on full beam and are suddenly plunged into complete darkness that switching to dipped beam wont necessarily provide any immediate relief illumination (until the cut out cools and reconnects)... if so, how on earth did GM ever get away with designing in such a huge safety issue.

The dipped will come back on immediately when you switch off main beam. But you you do need to have the awareness of what has caused the black-out to know what instant action is needed! You still cover a bit of road with no idea what’s in front of you though. Definitely dangerous….but usually a self inflicted failure from using non stock bulbs at the end of the day.

 

[Derek Nicol]

The dipped will come back on immediately when you switch off main beam. But you you do need to have the awareness of what has caused the black-out to know what instant action is needed! You still cover a bit of road with no idea what’s in front of you though. Definitely dangerous….but usually a self inflicted failure from using non stock bulbs at the end of the day.

It cools and reacts that quickly?.. good to hear.

 

[Chuffer]

Not in my experience it doesn`t . It can take around a minute or two to cool sufficiently to allow low beam to stay permanently on . You can actually feel the stem of the pull out light switch getting warm if you hold your fingers round it . That`s a bloody long time on a dark country A road , trust me , I have been their !

 

[johng]

When my main beam cut out I switched back to dip beam and it came on pretty quickly, then cut out again for a moment, then came back on and stayed on.

 

[Chuffer]

When my main beam cut out I switched back to dip beam and it came on pretty quickly, then cut out again for a moment, then came back on and stayed on.

You were lucky then . Mine cut out several times on dip beam and I actually crawled along in the dark with the hazards on and the lights off for a just a few minutes trying to find somewher to pull off the road until the switch cooled enough to keep dip beam on . Those few minutes felt like eternity !