Can you get SPDT center-off relays?

[TomoHawk]

If you wanted to wire a relay into the headlamps, could you get a SPDT relay with center off?  Otherwise, You'll need to double-gang relays like an interlock system to cut the power to the whole headlamp circuit when not in use.

 

All the wiring diagrams I've seen so far use a SPDT relay for the low-high  beam switching, but the circuit stays always HOT, which would make me nervous when not in or around the car.  What's there to cut the power so you don't drain the battery or start a fire?  A center-off relay would make things really simple.

Edited May 23, 2015 by TomoHawk

[djwarner]

I checked Digikey and found none.

 

I would suggest that if this concerns you, put a SPST relay ahead of your DPDT relay.

[TomoHawk]

that is what I mean by a double-ganged relay, with the SPST.

[Captain Obvious]

I've never seen a center off relay like you described.

 

The relay upgrades I've seen floating around have one side of the relay contacts always hot, but I don't see that as a problem. None of the three wires that go out to the headlights are hot unless the relay is closed. I guess I don't see much difference between one side of a relay always hot and one side of a switch on the stalk always hot. One side of something is always hot.

 

One question... You said the relay upgrades you've seen use SPDT relays? I don't think they have to be double throw. SPST relays would work just fine. Maybe it's just easier to find SPDT?

 

I'll draw up what I did. It's a little different than what most people do.

[TomoHawk]

Adding relays is so simple, but opening up the taped wiring and finding the wires, and even getting a good, clean/uncorroded bare wire is something different.

[djwarner]

Check out the wiring diagram in this thread. On the right side it says the 12 volt power should be fused before the relay. Also, the power should connect to the normally open contact, not the center pole of the SPDT switch. This ensures voltage will go no farther unless the relay is activated.

 

 

http://www.classiczcars.com/topic/48623-relay-question/?hl=%2Bheadlight+%2Brelay#entry442077

[TomoHawk]

I've been reading that the relay mounts are interlocking.  Is that correct?  Does it add any strength to a group of relays so the group can be mounted to a bracket  or bulkhead?

[djwarner]

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.

[TomoHawk]

This is a table I found.  What do you think of its information?

 

http://www.engineeringtoolbox.com/wire-gauges-d_419.html

[TomoHawk]

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.

This is not possible wt the 1978, because the headlamp connectors are not available.  the only possibility is to find some pins that fit the stock headlamp connector, which is more probable.

 

What is the AC circuit?

[TomoHawk]

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.  Also, if one fails, the other can still power the lamps, but you will lose the function of either the high or low beam, depending on which one fails.

[djwarner]

When I added the Vintage Air system, it was powered directly from the + terminal of the battery through a self resetting 30 amp circuit breaker mounted on the fire wall. This gave me a great place to tack on an inline fuse for the headlamps.