H4 Wiring

[GotaT]

Im soon to install a set of H4's and wanted to know the wiring schematic or breakdown of how to install the additional Relay.


Where do the wires come in from and go to?

[nesslar]

I haven't had the need for the relay yet myself, haven't installed one, but I do HAVE one, still in the package.....
Seriously, though, search for "headlight relay" and I imagine it'll come up. I know it's been written about, including pictures. Lar
Sunny today, and the Targa is on the road!

[Zeke]

Basically, what you are doing is routing the voltage from your fuse block thru the relay on into the back of the lamps. What triggers the relay is the original wiring up to and back from the stalk. So, you remove the wires from the stalk that go to the block and install on the relay. Then take fused power off the block to power the relay, or relays, as is the case, because you need two.

If you google relays and Markus Sucro, you will find some diagrams. I did. Good info on Daniel Stern Lighting.

[emcon5]

Originally Posted by nesslar

I haven't had the need for the relay yet myself,

If you have a 911, you have the need for one.

[old man neri]

This might help. I can't take credit for the write up though.

[Zeke]

That seems to be hard to read for the layman (me). It's really simple for the engineer types, but it can be a little daunting for the first timer who isn't an EE.

[GotaT]

Thanks for the link.. Ill get back if I cant figure it from the many posts.

[myamoto1]

here's a link to the plug wiring on the H4:
Headlight pinout question

I'd also recommend the headlight relay sold by our host (JWST-911-HLPR). You can see the relay in my pics from this post:
Pic needed of 85 Front Fuse Panel
Only took me about 10 minutes to install

[HarryD]

Gotta T

The unit sold by our host is designed for cars later than 1973. It will work but you will need to think about where each wire goes. If you can follow a basic wiring schematic, the drawing below is what you want (ignore the wiring for the driving and fog lamps). The numbers are the connection numbers on the various devices. Your 1971 has the headlamp fuses in Box #2.

[jpahemi]

My only fault was using the stock wires on the relay output (to headlights). Power to the relays and ground are 12ga, but I need to swap the wires to the headlights. At least the stalk doesn't run all that juice through it anymore.

[stevemfr]

Using 4 relays for 2 high and 2 low beams as in imcarthur's diagram (the link provided by old man neri above) is absolute overkill. A H4 high beam uses 60w and, as such, draws a tad less than 5A per bulb. General automotive relays handle 30A - 3 times whats necessary - so one relay for the lows and one for the highs is plenty.

HarryD, your diagram is much more sensible as far as relay use goes - I'd have a couple of small points to critique, tho. First, the reason that L and R are fused separately from the factory (for parking lights as well as headlights, btw) is that if you have a small fender-bender that happens to short one of the lights, you're not left in the dark as only the fuse for that side will blow. Second, running the trigger current from the light switch to the relay spool through a fuse is overkill IMO (tho having said that, I once picked up a relay at Radio Shack on a Sunday - the spool melted and caused a large release of magic smoke in my eng compartment...). Third, I'm not a huge fan of in-line fuses for important systems in cars - especially where an oem alternative would be avail. The solution would be to put the relays for hi and low beams ahead of the fuse box and use the original fuses. The fogs and driving lights could be fused in-line. This is all just a suggestion, BTW.

[HarryD]

Originally Posted by stevemfr

Using 4 relays for 2 high and 2 low beams as in imcarthur's diagram (the link provided by old man neri above) is absolute overkill. A H4 high beam uses 60w and, as such, draws a tad less than 5A per bulb. General automotive relays handle 30A - 3 times whats necessary - so one relay for the lows and one for the highs is plenty.

HarryD, your diagram is much more sensible as far as relay use goes - I'd have a couple of small points to critique, tho. First, the reason that L and R are fused separately from the factory (for parking lights as well as headlights, btw) is that if you have a small fender-bender that happens to short one of the lights, you're not left in the dark as only the fuse for that side will blow. Second, running the trigger current from the light switch to the relay spool through a fuse is overkill IMO (tho having said that, I once picked up a relay at Radio Shack on a Sunday - the spool melted and caused a large release of magic smoke in my eng compartment...). Third, I'm not a huge fan of in-line fuses for important systems in cars - especially where an oem alternative would be avail. The solution would be to put the relays for hi and low beams ahead of the fuse box and use the original fuses. The fogs and driving lights could be fused in-line. This is all just a suggestion, BTW.

Thanks for the thoughts. I did consider that in my design.

I placed the relays after the fuses so that 1) I would have a fused circuit on the control side ofthe relay and 2) The wiring was easier for me.

I ran separate fuses for the headlamps so I could also upsize them if I wanted to go to higher wattage lamps.

[Canada Kev]

You might also want to use fused relays to simplify your installation. Here is a pic of one that I got from the rallylights.com web site. They have lots of stuff for the automotive electrician. I think that ideally one could use a relay holder that had an integrated fuse. I remember seeing one somewhere before, but can't seem to find it now. I think it may have been made by Hella.