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Adding a Headlamp Relay

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Geo Hahn

Geo Hahn

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From the other headlight post:

TR3driver said:
...Not having the relay may be the reason why /bcforum/images/%%GRAEMLIN_URL%%/grin.gif ... they draw less power if they don't get full voltage. And IMO, more voltage to the headlights (which means more light on the road) is the main reason for adding relays.
Click to expand...

Okay, so where is the best place to add said relay? Down around the dipper switch would seem convenient with one relay controlling power to both headlamp circuits but I suspect that is too much going on after the relay to enjoy the full benefit.

The main reason I had previously considered a relay was that the TR4 headlamp switch seemed just barely up to the task (having noticed once that it got hot when the internal connections were poor). The thought of brighter lights is enough to tip the scales and get me to do something (though I did disassemble and clean the switch).
 
I connected mine where the front-harness connects to the main harness. I used one relay for low and one for high-beams. They are bolted through the tabs on top of each relay to the horn brackets.
 
On my TR3A, I used a single can with 2 relays inside (sourced from JC Whitney many years ago as an upgrade kit for a Ford Model A !) and mounted the can to the bulkhead near the old control box. It's fed directly from the tie point I created in the alternator output lead, and I added new wires from it to each of the 4 bullet connectors near the horns (keeping the original subharness from there to the lamps). Not necessarily the best for any reason, just where I chose to put it.

On one Stag, the relays are hidden under the dash, above the original relay panel. On the other, I'm going to locate them in the cavity behind the headlights, mounted to the panel adjacent to the radiator. Easier to get to than under the dash.
 
I see the dual headlamp relay that Randall mentioned on ebay once in a while on ebay. It is an enclosed box that is also fused and was a popluar add-on to make your lights brighter. Pretty neat setup, and also vintage. See it here https://cgi.ebay.com/ebaymotors/Headlight...1QQcmdZViewItem

I just use standard Bosch type relays in my set-up. The advantage of mounting mine up front is that is where the wires from the hi-lo switch to the headlight harness connect, so it's an easy splice between the maine harness and the headlight harness. I run single a 10ga. wire directly to the + on the started switch for power (i.e. direct to battery post), then use the wires from the hi-lo switch to switch the power through the relays to the lights.
 
Hello Geo,

just get it clear that a relay is not the answer to brighter lights, it is the cable size to and from the relay that matters. Certainly a relay eases the load on an old headlight switch.
I can't quote cable size as British and American systems use different standards, but I'm sure someone can advise on that.
The closer you run a cable to it's maximum current rating the greater the voltage lost in the cable (and so less for the light filament)

Alec
 
Correct.

A direct(=shorter length) wire of appropriate size (i.e. 10ga vs the TR's 14ga wire) that bypasses all the light switch wiring and which also connects directly to the battery (or + starter switch post) will yield full voltage to the headlights, with a good clean ground, of-course.

Full voltage is what gives you brighter lights as there is no resistance over the 6 foot wire length in this setup. The relays, in addition to taking the load off the harness and switch, offer the light the full opportunity of full voltage, when fed power from with ample sized wire.
 
Here's what I did, ans it was very successful, and there is no splicing. I bought 2 new headlight pigtails at a local parts store. They come with a length of 10ga wire. These pigtails go to the headlights from the 2 relays. Separate wires from one original headlamp socket feed the relays. All wires fit through the bucket grommets. Makes the wire run as short as possible. Uses the original wire as triggers. Very, very low draw through the original switch.

I hope that's clear, I'm choking down some VERY hot Thai food at the moment, and my eyes are tearing up!


Peter C.
 
For a wiring diagram for relayed lights this might be of interest.

relayfuselights.gif
 
There's also a good article, along with a simpler diagram (no spot or fog wiring) at
https://www.danielsternlighting.com/tech/relays/relays.html

One point where I (now) differ with Daniel though, is the use of fuses in the main feeds. After having a headlight fuse fail while driving down a mountain at 80+ mph at night with no moon; I'm going to use fusible links instead.
 
Hello Randall,

"I'm going to use fusible links instead."

I'm puzzled, what is the difference between a fusible link and a fuse?

I prefer the fusing as the diagram posted shows, that of a fuse per filament which helps give some protection without total loss in case of an earth fault.

Alec
 
piman said:
I'm puzzled, what is the difference between a fusible link and a fuse?
Click to expand...
Hmm, perhaps the parlance is different in the UK. Here, a "fusible link" is a length of wire with special high-temperature insulation, designed to fail harmlessly when overloaded rather than bursting into flame as ordinary PVC insulated wire is apt to do. A "fuse" typically has a glass or plastic envelope, and a soft alloy link with a lower melting point than wire. Also typically, a fusible link is a permanent part of the circuit (wire); while a fuse fits into a holder for easy replacement.

Operationally, the difference is that a fuse will blow much faster for a given overload than a fusible link will. But, that soft, low temperature alloy is also prone to being affected by external heat and vibration.

https://www.madelectrical.com/catalog/fusible-link.shtml

Having a fuse per filament is another way to go (so a single failure doesn't kill all the lights) but leaves unprotected the wiring on the battery side of the fuse. My Stag, for example, does have an original fuse for each headlight; but a recent short in the wiring to the main switch nearly caused a fire.
 
I once got to smell my (prior) TR3's wiring loom turn to smoke... that's a smell that stays with you awhile.

With that thought in mind I finally got around to adding fusible links to both TRs. Not an attractive addition perhaps but some peace of mind that the first thing that melts won't be the harness.

I used ready-made off-the-shelf 16 gauge links though one could buy a length of fusible link wire to fashion your own:

Figure%201.JPG


The ends were fitted with connectors so they could be inserted into the 'big brown wire' circuit w/o modifying the original harness (TR4 shown):

Figure%202.JPG


Thanks to Randall's advice, both were mounted in open air rather than bundled with the harness since they need to get hot and melt if they are ever called upon to do their job.

Fairly discreet when in place on the TR4:

Figure%203.JPG


Harder to hide on the TR3:

Figure%204.JPG


Like my insurance policies and Toyota airbags, I hope I never find out how well they work.
 
1. I bought the headlight upgrade wiring kit from VB. It has relays etc. I haven't installed it yet. nobody has mentioned it here, is it any good?

2. I replaced my main fuses with circuit breakers from Aircraft Spruce. When I put in the upgraded wiring, I will use breakers instead of fuses.
 
I haven't seen the reference to Dan Masters' book and his comments on not fusing headlights.

Anyone?
 
Safety feature. Bottom line, less likely to lose
your lights due to shorts, unless I didn't get something.
 
Man, was I wayyy out of the loop. Just realized
I came in at page two, ignoring all the heretofore.
It's all about experience.
 
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