Can you get SPDT center-off relays?

[Chickenman]

The common relay used in automotive after markets are not interlocking. When I did my installation, I screwed them to the inner fender under the gas filter. I bought connectors for the headlamps (the ones in front of the radiator mount).

I used the original wiring for the headlamps to drive the relays. I tapped +12v from the hot side of my AC circuit and grounded the new circuit to a chassis ground. With the new harness made with connectors, the OEM wiring harness is intact. Thus the upgrade is fully reversible by simply unplugging and re-plugging.

 

That's the method I used to wire my Rally 510. Disconnect the factory HL terminals from the back of the H/Lamps and use the feed off of those to drive the new Bosch style relays.

 

Run new universal HL wiring connectors from the Relays to the Head lamps. The relays can be wired to work with Positive switching or Negative switching.  Use two relays. One for High Beams and one for Low beams.

 

Put a  15 - 20 amp ATO Fuse, or a Circuit breaker, in the +12v line to the relays  if you're worried about having the hot side connected all of the time. Always a good idea with relays.

[Captain Obvious]

I like Sarah's use of SPST relays.  You will only need two, and they are wired parallel, instead of serially, so unless you turn the headlamps on, nothing gets power.

 

Right. You don't need SPDT relays. A pair of SPST's work just fine for the simplest approach.

 

I used that same sorta basic concept, but I used four relays instead of two. That way, if one side fails, I can still have a bulb lit on the other side. In other words, if I lose one low beam relay, I could still have the other side. Now, of course, the low side switching for both of low beam relays is a common fault, so if something goes wrong on my low side, I'll still lose both sides at the same time.

 

So with my four relays, the relay contacts and relay coils are not shared fault modes between sides, but the switch and primary side wiring still is.

 

Using four relays also means I don't run as much current through each pair of relay contacts so I don't dissipate as much power in the resistance of the relay contacts. That means I get more power delivered to the filament instead of heating up the relay contacts.

 

I took a bunch of measurements before and after all my mods. I gotta find my numbers...

[Captain Obvious]

I know there are already a bunch of different diagrams showing headlight relay upgrades, but here's my version of the traditional concept. This traditional version is the simplest and (assuming you can find the proper mating connectors) it's possible to do it "plug-n-play" with the original harnesses.

 

Advantages are:
Simplest and plug-n-play.

High side switched (doesn't have any hot wires out to the headlights unless the headlights are turned on).

Headlight current passes through just one set of relay contacts instead of multiple contacts in series.

 

Disadvantages are:

Have to run a new wire and fuse from the battery.

All the headlight current (both sides and both high and low beams) all goes through the same fuse. One failure point.

All the headlight current goes through the same set of relay contacts (maximizing power loss in the relay)

One of the original fuses in the fuse block becomes completely unused (tit's on a boar).

The other original fuse in the fuse block (and the fusible link) is now WAY oversized for the current new use.

The writing on the fuse block cover is now incorrect and misleading as is the fusible link size and listed use.

 

 

Edited May 26, 2015 by Captain Obvious

[TomoHawk]

That traditional diagram should be improved to run the current from the fusible link to the relays instead of to the switch-  it's overkill for what you need to turn on the relays. At least substitute a fuse for the fusible link, or split the wire just after the fusible link to supply both the lamps and the switch.

 

I keep wondering if there is a way to put in a fail-safe condition in case a relay fails, so it defaults to low beams.  I suppose you could keep a shunt handy to bypass the relay, as well as a spare relay.

Edited May 26, 2015 by TomoHawk

[Captain Obvious]

Problem is that as soon as you start making changes like that, it's no longer plug-n-play. It's just unfortunately one of the down-sides of the traditional approach. It's part of the price you pay for the plug-n-play ease of use compromise.

 

You could (and I would recommend) that you completely remove the unused fuse from the fuse block and also replace the remaining one with a much lower amperage value (like maybe 2A). The fusible link would still be way oversized for the application, but in theory, it should only be employed with a hard power to ground short in the wiring.

 

 

 

 

[TomoHawk]

Well, since there really is no "plug-and-play"  actuality for the  '78, you are modifying the stock wiring nonetheless, so you may as well do it right or in an improved way.

 

OTOH- if you are adding some lamps, like fog lamps or driving lamps, that second (unused) fuse would come in handy.

[Captain Obvious]

Great idea. Additional lighting would be a good use for the previously unused fuse.

 

The printing and documentation would be wrong, but at least the use of the fuse would be appropriate!

[TomoHawk]

If you did want to mount several relays for the lamps,  what is there that looks neat and ordered?  I thought you might be able to mount some to a piece of aluminium angle in the area where the carbon can used to be. I have a fuse/relay block from an Altima (it was the right size to fit inside the carbon canister mount) but it's a mess of 20ga. wires and the 4-pin relays are odd.

[TomoHawk]

I'm sure there is a way to make up some clear decals with white lettering to replace the old fuse labels.

[Captain Obvious]

Funny you should ask a question about that... I nabbed these relay/socket assys from Honda/Acuras. I found them to be very compact, orderly, and gangable. They also use a very ubiquitous relay style that you can find pretty much everywhere:

.

 

 

I just posted a bunch of other details and pics in thread about my headlight upgrade experience:

http://www.classiczcars.com/topic/51554-a-different-headlight-relay-upgrade-4-relays/
 

[TomoHawk]

Heh....  I will consider looking for them if I go to the JY.  But as I usually say, "If someone says it's easy to find 'in any car'..." it's usually impossible to find, unless that person is standing there with you, pointing to it.  

[Captain Obvious]

Yeah, I understand. I've found the easiest to harvest is from the 98-00 Civics, but they are on other Honda derivatives as well. Good luck with the hunt!