Headlight Relay Question

[RedTR3]

I am planning to add a relay to the TR3 Headlights circuit(blue wire). Should I also add a fuse to this circuit? The original is unfused of course.

 

[vivdownunder]

Can't hurt, but suggest going well overspec with the fuse amperage. Not ideal for the fuse to blow at the first hint of trouble and no headlights.

Fusing the headlight circuit cuts the risk of the wiring catching on fire in the event of a major short circuit.

Viv.

 

[tomshobby]

The low beams burned out shortly after I installed the relays in my TR6. Found out the charging system was over charging. Just did not notice it until I improved the wiring.

 

[TR3driver]

I have had a headlight fuse fail while driving at 80 mph down a mountain, on a dark night. I would definitely suggest either using a fusible link instead, or having separate fuses for each side. My current plan is to actually use a separate circuit breaker for each side.

The fuse failure was eventually traced to some invisible corrosion on the fuse clip. Just enough to get hot after extended operation, and melt the solder inside the fuse.

 

[dklawson]

I have headlight relays in the Mini and the Spitfire. I used the readily available 30A Bosch/ISO relays you will find at the parts store (and cheaper on eBay). I have two fuses, one for the low beams, one for the high beams. Each is a blade type 15 Amp fuse. After more than a decade of use I have not had brighter headlights without any problems.

 

[hondo402000]

well they say not to fuse head lights, but either way if you have a short the wire will turn in to a fuseable link at the place of highest resistance, and you will burn the wire into and then have a live wire dangling and with the chance of shorting and causing a fire, I would much rather have a fuse to blow. just add some driving lights so if you loose your head lights you will have driving lights to get you to a safe pull over spot.

I put relays on my headlights and have had no issues

Hondo

 

[Gliderman8]

I added relays to my headlights a few months ago. So far everything is good; the lights appear brighter and I know the load is finally off of the switch.
I also added 4 fuses 2 for each low beam and 2 for each high beam. There really is no down side to fusing the circuit.

 

[DNK]

Some new relays have replaceable fuses built onto them

 

[TR3driver]

but either way if you have a short the wire will turn in to a fuseable link at the place of highest resistance, and you will burn the wire into and then have a live wire dangling

A proper fusible link is designed so that does not happen. The ones that look like a length of wire use special wire and insulation; so the insulation does not burn even after the wire has melted.

It's basically a very slow-blow fuse. It has to be replaced after a short, of course, but if properly sized, it will fail before the rest of the harness is damaged. Without a fusible link, the entire length of the wire gets hot enough to melt the insulation, long before the wire actually melts.

As John Herrera used to say, I have experimented with this. In fact, the short in the high beam wiring was what led to installing the relays on my TR3A, as I had to abandon those wires in the harness.

 

[Geo Hahn]

FWIW - both pics are of the TR4 but the TR3 is similar...

Both were contrived not to cut the existing harness for an easy return to original.

<span style="font-style: italic">Oh, I should mention that the second photo is a fusible link in case that isn't obvious.</span>

 

[dklawson]

well they say not to fuse head lights,

While I know our old British cars typically do not have fused headlights, I have to ask "why". Why are headlights typically not fused and out of curiosity, who is "they" who say not to do it?

I chose to fuse my headlights when adding the relays as the headlight relay kits I've seen (the one sold by VB and the one from Advance Auto Wire (Dan Masters design)) include fuses. What is the argument against fuses?

 

[TR3driver]

What is the argument against fuses?

I believe it is basically the same as I outlined above : fuses sometimes fail for reasons other than a short, leaving you without headlights. <span style="font-weight: bold">Very</span> disconcerting, especially if you happen to be on a dark winding road next to a cliff when it happens.

There is an additional reason, which I believe is the real reason TRs don't have them, which is because they add cost and complexity. Triumph Stags, for example, do have 4 headlight fuses from the factory.

Most American cars (since at least 1960 or so) use a thermal self-resetting circuit breaker incorporated in the headlight switch.

My 1970 Audi had 4 headlight fuses, and they did fail occasionally for no apparent reason. (In retrospect, I believe it was similar to the problem I had on the TR3A, where a marginal contact at the fuse holder was heating the fuse up until it failed.)

The wife's Toyota uses a single fusible link for the headlights.

 

[dklawson]

I see your point and I too have had fuses fail (in other circuits) without an apparent short. Perhaps I'll switch to the circuit breakers instead of the blade fuses I'm using. But if fuses (without moving parts) fail without a short... what is the expected life and reliability of a circuit breaker? Regardless, as it is now I have separate fuses for the high and low beams. If I lose one fuse and its lights, at least I can switch to the other beam.

 

[TR3driver]

what is the expected life and reliability of a circuit breaker?

All I have is anecdotal evidence, which of course proves nothing. But I have never seen a circuit breaker fail, unless there really was an overload.

The TR3 is still using the circuit breaker I bought in perhaps 1985 (which plugs into the fuse block in place of a fuse) and it worked as designed when I had a short last year.

The original circuit breaker for the Stag's power windows failed, but that was because the switch stuck on (and the motor stalled when the window hit the stop). In other words, it was supposed to open (and did), it just didn't close afterwards. Not bad for a nearly 40 year old Lucas part !

 

[sail]

in case that isn't obvious.

Nothing about wiring is obvious to me. Geo please correct me on the basics of your setup, I'm trying to get this straight in my head.

1. New power wire w/ fuse from solenoid to relay?
2. New wires from relay to both headlights. Also parking and turn?
3. Existing switch wires in loom to relay?

 

[Geo Hahn]

...I'm trying to get this straight in my head.

1. New power wire w/ fuse from solenoid to relay?
2. New wires from relay to both headlights. Also parking and turn?
3. Existing switch wires in loom to relay?

I did it and I have to ponder how it was done.

1. Yes, new (blue) wire from Control Box (aka Voltage Regulator) to relays.

2. No, used existing (blue/white & blue/red) wires from relays to headlights.

3. Yes, existing switch wires to relay.

Basically I just made a couple of jumpers (out of Bl/R and Bl/W wire from an old harness) and a couple of ground wires (Bk) for the relays. Changed a couple of single bullet connectors to doubles to accomodate the extra wires.

Must be many ways to do relays and other places to locate them, this is just one way.

 

[sail]

Thanks, that seems like a good unobtrusive way to add relays keeping the existing wiring intact. I am going through wiring now. Nice to work on something that is clean, light and doesn't require crawling under the car for a change.

 

[RedTR3]

As the original poster, I wanted to extend Thanks for all the advice....I ended up fusing the circuit with a 35 amp fuse just to prevent a fire from a direct short.