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MG MGB Technical - To relay or not to relay?

OK... Iv read plenty of archives about installing Relays for the headlights to get maximum output.

I am currently fitting my old loom (harness) back into the engine bay and am considering installing relays for the headlights (new halogen ones).

Based on the info I have read here, theres no point installing relays unless you upgrade the wires to a heavier gauge- including earth. After looking at the existing headlight wires, they appear to be 28 gauge (14 US) to the front of the car, then connect with smaller 28 gauge (18 US) wires on the individual headlamp harness. Is this not inefficient?

Instead of adding relays, why not just upgrade the lighter headlamp harness to the same larger gauge wires which are in the loom?

Also, has anyone got any photos of a nice tidy wiring installation in their engine bay?
Innes Bint

The wiring is perfectly adequate for typically higher output headlamps, it is the connectors and switches that cause the problem, and one half of those will be bypassed by fitting relays and the other connectors at the front of the car should be replaced with new, bullets polished, and assembled with Vaseline or similar. One exception is the ground wire for the headlights which I think is standard gauge back to the grounding bolt, and not the heavier gauge used for the feeds to the headlights. Obviously a design error when the grounding bolt moved from by the headlights to by the fusebox. Thinner gauge on the headlight tails isn't 'inefficient', the relatively short length compared to the run right back through the dip and main lighting switches to the solenoid means it makes little difference. I did this job for someone else and he reported the difference was amazing. Whereas before switching beams resulted an a dark period while the new beams slowly warmed up, after it was instantaneous as well as being much brighter. The new cabling from the solenoid, through the relays and fuses to the bullet connectors at the right-hand headlight was of course new, albeit the same gauge as the original.

Relays and individual filament fuses should *always* be installed if fitting uprated headlamps, as even if you ignore the loss of efficiency of older switches and connections these switches aren't designed to take the sort of loads you can get these days, and especially replacement switches. With the low losses of relay powered headlamps fuses are essential otherwise any damage to wiring will cause even more rapid and worse damage than normal.

I'm attaching a photo of an installation I did. The relays (two were for other circuits) are shown circled on the right and the fusebox on the left



Paul Hunt 2

Most of us who have installed relays for our headlamps (ANY type of head lamp) would not go back to stock. There is that much difference in their efficiency and their performance. Like Paul, I did my own installation years ago, primarily in my case to take the load of higher output lamps out of the OE wiring harness and use the OE harness wiring thru the switches to simply control the relays that would connect the lamp filaments more directly to the battery (thru a connection to the battery connection at the starter solenoid). Doing this not only netted brighter lights, but much reduced wear and tear on the somewhat marginal switches (dash and dipper), prolonging their lives.

I would mention that if you wish to do this and do not feel that you want to undertake an installation on your own, you can buy, from Advanced AutoWire, http://www.advanceautowire.com/ , a complete kit designed specifically for our LBCs. This set up is offered by Dan Masters, an acknowledged expert on LBC wiring and sometime contributor to this and other MG BBSs, and is very highly regarded. This is NOT a plug as I have no connection or interest in his offerings, but just a heads up that such an option exists, if this seems more interesting to you.
Bob Muenchausen

To follow up on one other note from above: ALL connections must be clean and snug, and properly crimped or soldered. A badly done-up connection can generate a LOT of heat - enough heat to melt plastic.

Regarding relays - it's definitely the way to go, for all the reasons you've read about.

R.
Rick Stevens

Headlight relays are the most important improvement that you can make. Unless you never drive at night. I had a headlight switch fail at night while driving. It is not good to pull all of the load through the switch. I installed my relays between the fuse box and the front slam panel. Just cut the headlight leads and added a new feed line from the starter soleniod. I did renew the grounds for the lights. I don't see much difference, except the lights do stay on at night.
Jim Lema

I just this past weekend installed headlight relays on my 69 MGB and wish to report that, as Bob Muenchausen said above, there is a HUGE difference now with the efficiency and output of my lights.

I got my kit from a vendor on another MG forum that I belong to.
Tom Fisher

At least 5 years ago I wanted better lighting and switched the lights to halogen one piece units. It has worked fine ever since with much improvement. I did not install relays and even ran an additonal wire to have daytime running headlites operated by the ignition key. I have had no problems since.
Bob Ekstrand

Innes-
You are fundamentally correct in your assessment; Paul's comments are generally right, except for the matter of wire sizes.

"... theres no point installing relays unless you upgrade the wires to a heavier gauge- including earth."

Relays will still help a lot, for all the reasons Paul cites, but...

" After looking at the existing headlight wires, they appear to be 28 gauge (14 US) to the front of the car, then connect with smaller 28 gauge (18 US) wires on the individual headlamp harness. Is this not inefficient?"

The main harness HL wiring is 28 strand/14ga US, but it is very long with many connections and switches. The headlamp pigtails are 14 strand/18ga US, or a bit less (for OE pigtails - some repros are worse). This is not adequate for the loads involved. I have measured dozens, and reliably find 0.35V drop for the pigtail complete, or 0.16-0.18V per leg (hot & ground), measured at near 13.5 system V, with standard 60/55W bulbs. That equates to about 8% drop in light output. Replacing the pigtails with 28str/14ga will reduce this drop to near zero.

I am sending you two Word.doc, which should answer most questions.

Precise details vary by year of car, but what you need to do is provide fused power feeds from solenoid to relays. If relays are mounted on the Rad support panel, then all connections can be made at the harness breakout in front, with no cutting of wires. Use the old pigtail leads to connect to the relay triggers. I install additional ground leads from the existing Black bullet connectors direct to wing mounting bolts right next to the bullet connectors, both sides.

For those who are paranoid about failure of headlamp fuses, I suggest separate relays for each filament, but I feel this is overkill if your work is decent - and unless you duplicate all switches and wiring, you still have single failure points in the light circuits.

If you have electric fans, install a relay for that (a few came with fan relays), and I suggest an Ign relay for earlier cars, to unload the IGN switch. This will also greatly improve things like the heater & wiper performance.

I install a 6 fuse block to feed all browN circuits; relay feeds come from that. That gives some disaster protection for all circuits in the car.

FRM
FR Millmore

A couple of questions for FRM if he doesn't mind. You appear to be very expert.

Do you need to have a fuse on the main power for the relay, or does the relay act as a fuse? Switching itself off when too hot. I am currently running a large fan with a 30 amp fuse placed prior to the relay and if it is unnecessary I would like to remove this.


On high beam I would like to run both the high and low beam filaments at the same time, as when you flash your lights. This vastly improves visibility at night and on dark poor roads would be a big plus. Currently the switching to high beam switches off the low beam filaments. I have a 100amp alternator so power isn't an issue. However the lights are going to get hotter with both filaments operating, too hot?
Peter

Peter-
You fuse the power to the relay; the relay is not a fuse or circuit breaker - we hope! There are a few relays with fuses incorporated, but they are not common.

Correctly wired good quality (Cibie, Hella, Bosch) lights will give all the illumination you need to over 100mph under any reasonable conditions, and some unreasonable. (I drive fast at night on very dark roads - and our 'roos jump on all 4 legs = deer) That's with standard 60/55 H4 bulbs. Voltage measured across the bulb terminals should be within 0.5V of full system voltage, which should be 14.0 - 14.3V (B+ to B-, or A+ to A-)hot and running at 2500rpm.

Running both filaments lit will overheat the lamps - IF they have correct voltage they are running at max temp with a single filament lit. Plastic lamps will melt, as will cheap connectors. Even correctly sized wires will overheat if more than one wire in the harness is carrying lighting loads. In the headlamp pigtails, you would have both hots carrying full current, and the ground lead carrying double = meltdown. If these are then incorporated in a single harness, it gets worse - don't!

I will send you the above referenced docs.

FRM
FR Millmore

On my 68, the OE wiring is such that when the dipper lever is pulled back to flash the headlamps (something done when this car was new in Europe and perhaps GB, to signal passing in traffic), both filaments light up for the length of time one holds the flasher back. Because I used the OE wiring pattern to switch on the relays for each beam, when I pull back on the lever, both filaments of both bulbs light up, giving a quick flash, OR a quick temporary burst of light brighter even than the high beams, which is especially helpful on some dark roads here in the Intermountain West's outback. I realize that lighting both filaments at once will cause the bulb to overheat, but since I do not run with them both lit for more than a very few seconds, my empirical experience has been that the bulbs still seem to last a very long time in their regular designed use with no apparent damage to the socket ~ IF one keeps the use of both filaments at once to a minimum.

With Halogen lamps, as FR mentions, turning both on and LEAVING them on will ultimately cause the glass envelope to melt and once air enters it, the bulb is done and perhaps the reflector. Anyone who has replaced a blown quartz halogen projector bulb has probably seen this same happen. Consider the melting point of (quartz) glass and then that of the other components in close proximity to it and you can see that trying to run both filaments at once as your regular "high beam" is beyond the design parameters for even our rather ordinary halogen lamps and their supporting equipment.
Bob Muenchausen

A few more points re Bob's comments:

Using the dip switch high beam flash as Bob says on a non-relayed system will rapidly kill at least the dipswitch, and maybe the headlamp switch & associated wiring.

The "flash" function was not included on US MGB in the early days, as it was illegal in the US. We used to replace the switch with the European one, as it had metal turn signal spring fingers, rather than the failure prone plastic ones on the US switch; getting the flash was a bonus. (Nowadays, I see that my Dodge truck has one - an "optical horn" by the owner's book!)

I seriously doubt that you are going to melt the fused quartz bulb envelope under any circumstance, but nothing else in the area will be happy. The usual failure point for the bulb will be the filament, followed by the connector.

You may notice, if you do a lot of night driving, that most cars with burnt out headlamps are less than two years old. This is because after two years, the wiring & switches are so degraded that delivered voltage drops enough to give nearly infinite bulb life. The design life of a standard H4 bulb is about 300 hrs - that's 30 nights of use. Most high output lamps have much shorter design life - as low as 50 hours. In all cases, the shorter life is a function of filament temperature, itself a function of voltage in conjunction with design parameters. If you have not replaced your bulbs recently, you can pretty well bet that the delivered voltage at your lamps is way below where it should be!

FRM
FR Millmore

Briefly, the relay in my 71 is connected to the flasher switch, so the multi-switch (and therefore beam-switch) only needs a coupla watts to flip the heavy duty relay connected directly to the solenoid lug, all mounted just behind the radiator shroud.

I'll also just briefly ignite (so to speak) the good ol' debate about fusing headlamps. Some say it's another place for a circuit to fail. Self resetting fuses are sometimes advocated (although at 100kms on a dark road...). Mine are fused, BTW, and it would seem that a left versus right fusing pattern would be safer, as this would provide protection against a fire, whilst in theory meaning if a fuse blew you'd still keep half your lights (without having to beam switch on the fly!). Of course if you fuse your main line, you're probably asking for trouble, IMHO, as the thick and short length of wire SHOULD be safe from a short.

...let the debate begin!
Curtis Walker

I agree, Curtis, which is why I didn't fuse the common supply to the relays as well as the individual filaments. If you *do* fuse there, it should be *at least* four times the filament fuses, probably six times if you are likely to use the main beam flasher while the dipped beams are on.

Circuit breakers are useful once the short has gone, but if a short comes on and stays on are no better as far as illumination goes than fuses.
Paul Hunt 2

Many thanks to you all, I thought that that might be the case (fuse and over heating eventually) however if you don't ask obvious questions every now and again, you'll never learn anything much.
I probably have got quite a voltage drop to the Headlights. On one occasion the high beam switch burned out and I had no option but to give the "flash" a pretty good workout for 6 or 7 hours driving through the hills in eastern Victoria. By inference I can't be getting the full voltage at the lights otherwise they woul have buned out.
As well as relatively lightweight kangaroos there are Wombats that like to spring out from the side of the road. Imagine a cube shaped animal made entirely of muscle and bone about as tall as your radiator. I once saw a car hit a wombat and the wombat got up and walked away. The car was not so mobile with a mangled radiator.
Peter

Thanks fellas.

Glad I live here in NZ... no 'roos, wombats or FLIES!! teehee.(sorry aussies)

Seriously though, thanks for all the advice. I am about to head to the shed and plan an assault on my harness. I like the idea of keeping the relays near the existing fuse block so will try and mount them on/behind the existing bracket on the RH guard. (i have a '69 so this bracket is not used unlike later variants).

I think I will upgrade the headlight pigtails to the heavier gauge wire as I'm doing a full restoration and everything is in bits anyway (thanks FRM and also for the 2 docs). I will also earth the relays directly to the guard rather than use the sole earth point on the harness (cockpit sidewall by wiper motor).

Have read Paul Hunts pages and will try to build his circuit to protect the overdrive I have just bought to replace my 4spd.

While Im at it, I will fuse the fuel pump, main leads from the solenoid and headlight circiuts.

Im trying to keep the harness as tidy and concealed as possible as I want a nice detailed engine bay.

Hope I dont get my wires crossed!! Thanks again.

Innes
Innes Bint

This thread was discussed between 11/11/2007 and 14/11/2007

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