Headlight relay verification

[JSabah]

Can someone please have a look at my wiring diagram (below as it relates to the headlights only and a relay I’d like to install. Is the job really as simple as (1) running a new wire to the relay from the battery (with an in-line fuse) (2) putting a relay between the foot dimmer switch and the headlight terminal block and (3) grounding the relay to the car?

 

Other questions (1) it doesn’t look like the existing 30amp breaker will really be doing anything... will it? (2) will the ammeter still be functioning as the draw will be only minor from the coil? (3) what size fuse should be used between the battery and the relay (headlights I believe are 65W/50W and system is 12v)?

 

Now that I’ve asked the questions I’m thinking maybe the battery feed should actually come from the 30 amp breaker so the ammeter can detect the draw. I had been hoping not to run a new wire all the way from the dash as the battery is close to the headlights. Please advise.
 

Obviously, this will be done for both driving and high-beams .  Thank you 

[Jeff I indu]

I just went through this same exercise  today with my 49 Chrysler. What is the point with the relay? From the high beam/low beam switch the hot wire should go to the junction block. Though my is 6 volt I dont understand what the relay will be used for

 

Jeff

[Sniper]

All loads need to be ont eh alternator/generator side of the ammeter.  Move your relay feed from the battery + terminal to the output stud of your alt/gen. 

 

The only load that should be going thru the ammeter is the charge to the battery.  I would take this opportunity to clean and tighten the connections at the ammeter as well. 

[keithb7]

@Jeff I induThe purpose of a relay:

 

In the old days, the lights were wired up so that all the amperage needed from the ammeter, to light up the head lights, (hi & low beam) went from the ammeter to the head light switch. Then from there all the way out to the headlights. This is not ideal. Heavier amp loads are travelling through the headlamp switch, and also farther unnecessary distances. In time, as the old cars age, the lights get dimmer, mainly due to poor condition of old corroded wire and rusty/loose poor ground connections. This coupled with old headlight switches that also are boreline usable, can be dangerous. Things get hot and could ignite. This indeed happens. So always mount a fire extinguisher in your old car somewhere. 

 

What a relay does is relieve the switch, and the wires with long travel distances, from the heavy amp load that is required to illuminate headlights. A smaller amp load signal is sent through the headlight switch and from there on to the relay. The relay is another switch, that then switches on the heavy amp load from the battery as shown above, directly to the headlamps. This is safer, and often the headlights actually glow brighter.

[Jeff I indu]

Ok, I get it. The wiring in these old cars is indeed scary. I just finished tracing everything out today to get my headlights to work and followed the schematics in my repair manual. No relay so I did not understand. But I am trying to learn

 

jeff

[keithb7]

I just installed a pair of headlight relays in my '38 Plymouth last week. All new wires and cleaned up grounds. They certainly were improved as they are much brighter. I used a relay for each circuit, (hi & low beam). I also have each circuit wired up with it's own 15A fuse.

[Plymouthy Adams]

the relay will keep the high current off the H/L switch and I approve of their use.   Per your drawing, you need to activate the switching contacts of relay with the headlight switch and feed the high current contacts of  your relay from the battery and the output to the dimmer switch.  The dimmer is designed for the higher current, will feed both low and high beam from the relay.  This way you only mess with the wiring harness with splicing in the relay coil from the H/L switch that normally goes to the dimmer input and leave all other wiring stock.

Edited November 25, 2019 by Plymouthy Adams

[greg g]

Are you wiring the relay to the low beams or the high beams?  If both you need two relays. One for each circuit.  You can use the lines from the switch to the junction blocks to power each relay.  If you think of the relay as having two sides,it makes it easier to understand.  The relay comprises two circuits. The switch side and the power side. The switch side is over in and ground.  When powered it turns on a magnetic powered contacts on the power side. When connected it supplies battery power through the load to ground.  The position of the relay should shorten the path from the battery to the light lessening resistance to the load.  The longer path from the relay back to the dimmer switch is now a lesser current traveling a similar path but with a lower flow so again less resistance.  While you are messing with the circuits, clean or renew the lamp to ground wires.  And if there isn't a ground between body and engine or frame install one.  

[Plymouthy Adams]

there is absolutely no need for two relays...you can always install two if  you wish but the added wiring and cost is just not going to be any better than a single relay feeding the dimmer.  I agree that your existing harness from the dimmer to the bulbs including your grounds need to be clean and tight and up for the job.

[keithb7]

8 hours ago, Plymouthy Adams said:

there is absolutely no need for two relays...you can always install two if  you wish but the added wiring and cost is just not going to be any better than a single relay feeding the dimmer.

 

Searching the internet it seems all the info about headlight relays, covers dual relay installations.  I am interested in seeing a schematic for a single relay used for both high and low beams. I have not been able to locate that schematic. Does the high amp wire go to the 6V relay, from there then to the high/low beam foot switch? The hi beam foot switch sending the high amp signal to either hi or low beam circuit?  I can see how one relay would work, however then the high amp supply is going to and through the hi/low beam footswitch. Correct? I agree you can save $10 on a relay, and another $4 on a relay pigtail. Wiring savings seems negligible. Are there any other benefits to a single relay?

 

As mentioned I wired up two relays. A 15A fuse for each hi/low beam circuit.  Doing simple math if the headlamp high beam is 35W in a 6V system,  35/6 = 5.83A per light. Two lights on a circuit, equates to 11.6 amp total load on one of my fuses.  Seem about right?

[Plymouthy Adams]

majority of the folks utilizing the dual relay are taking the cue from the modern car that has the dimmer switch on the turn signal stalk and yes they utilized two relays and they also fuse both low and high beam circuits.  With our era heavier components, even the stock light switch is heavy in our old cars, BUT they are getting old and showing contact wear same as we are.  This wear is thinning contacts and with higher current and thin contacts this is a failure in the making thus the relay to remove the current and continue using the switch just to energize the coil of the relay.  Just feed the dimmer switch input from a relay and leave all other wiring in place as stock.  The wires will more than handle the current and your only task here is ensuring they are not frayed, nicked and have clean contacts at all junctions and clean grounds, star washer work wonders on grounds.    

[greg g]

I guess the question of doing two relays or one depends on a couple of factors.  First being the age condition of the dimmer switch itself as they can be a significant source of resistance in the circuit ase videnced by the number of them that won't pass any current after they have sat for a while.  The other is, how often you operate with high beams on.  If most of your after dark operation is around town, then maybe the relay for low beams only is an option.  If you spend a lot of time at night on country 2 lane roads maybe only the high beams need a relay.  Lots of options depending on you use and current condition of the components of your lighting circuits..

I found a significant increase in headlamp performance a few years back by cleaning the area where the ground from the lamp goes to the body.  On the p15 the ground screw goes to the part of the headlight bucket the receives all the water, mud, and other stuff thrown up by the front tires.  I drilled a new hole and attached a new ground wire to a spot that is out of the direct path of tire spray.  My 6v head lamps put out as much light as my 12v setup in my Studebaker.

And of course no one will mention the elephant in the room... At my age, I can't see at night like I did ten years ago, regardless of how bright my lights are.

[James_Douglas]

" The wiring in these old cars is indeed scary "

 

The wiring in these old cars is not scary. It is in many cases just old.  Good grounds and good connections are important when the voltage is low at 6V. When you have 12v or 24v systems, they are more forgiving of such things.

 

I do agree that "load shedding" high current items like the head lights using a relay can be a good idea if one has wiring that is getting brittle with age. However, if you put in a new wiring harness and are using 6 volts, then the system will work for 50 years as expected and is not in any way scary.

 

On the 1947 Desoto, I cut all the wires 18 years ago at the bulkhead and replaced it with new wire for the engine bay and the headlights. I still have the factory wires in the dash and to the rear. However, it is starting to have insulation issues. So when I rebuild it next year it will get all new wiring.

 

On the 1949 Desoto I used an all new stock harness. I added several grounds in the engine bay, two at the rear frame to body. One thing I do on all my cars is the following...

 

I take out the headlight bucket and make sure it is clean and perfect. I then either bolt or use a brass rivet and run a #10 wire from the bucket directly up to the battery for the headlight ground path. I do not rely on the body for that. Doing so makes sure that over time that the resistance in the fender bolts, the fender to frame bolts and the like do not degrade the voltage.

 

I can tell you that I have been driving the '47 for 18 years daily and nothing has burned up yet. But, time is showing itself....

 

[Plymouthy Adams]

James, it is not really the question of the wiring though as said a couple times already, it needs to be up for the task as far as insulation and good connections...it is about  the switch itself.  These are getting borderline at this age if they have not already failed and changed in the past.  The contacts get grooves worn in them and solid positive contact becomes the issue and poor contacts lead to higher current drain that will lead to more current till pop goes your fuse/breaker as you scramble to get stopped safely off the road.  Safety is the driving issue for the relay.   Finding replacement stock switches is really not that hard, many are offered used at meets and e-bay but it is often a roll of the dice if the one you getting is not as bad as the one you have.  Not everything NOS arrives NOS, often NOS can be stated as "nasty old stuff".  Removing the high current from the dash mounted switch is the key operation with the use of the relay.  As for dimmer switches....these are a dime a dozen and available new about everywhere you go for minimal cost.   

[T120]

When it becomes necessary to replace an unsatisfactory headlight switch it is a good time to install a headlight relay(s) for all the reasons mentioned.If I hadn't found a suitable dual headlight relay on ebay I would probably have used two separate relays.

 

 

 

 

[keithb7]

I was not aware of a Dual Circuit relay. Thanks for filling me in.

 

Here is where I placed my dual relays. There were holes already drilled in the inner fender well. Instead of drilling more, I utilized those holes. All mounted with stainless steel hardware as the hardware is exposed to road conditions on the inner of the wheel fender well.  That wheel well will probably not see much rain, mud, or a dirt roads going forward. My work is not done here. Later I will heat shrink all connections, clean up and anchor the wires better, and cover all in plastic loom.

 

 

[James_Douglas]

3 hours ago, Plymouthy Adams said:

James, it is not really the question of the wiring though as said a couple times already, it needs to be up for the task as far as insulation and good connections...it is about  the switch itself.  These are getting borderline at this age if they have not already failed and changed in the past.  The contacts get grooves worn in them and solid positive contact becomes the issue and poor contacts lead to higher current drain that will lead to more current till pop goes your fuse/breaker as you scramble to get stopped safely off the road.  Safety is the driving issue for the relay.   Finding replacement stock switches is really not that hard, many are offered used at meets and e-bay but it is often a roll of the dice if the one you getting is not as bad as the one you have.  Not everything NOS arrives NOS, often NOS can be stated as "nasty old stuff".  Removing the high current from the dash mounted switch is the key operation with the use of the relay.  As for dimmer switches....these are a dime a dozen and available new about everywhere you go for minimal cost.   

 

I have had just the opposite experience. My original and the three sets of NOS headlight switches I have all pass every test I put on them. I clean them good and spray the inside with an expensive contact cleaner/conditioner and then when I test them for resistance and current at 5 to 7 volts they test out just fine.

 

Perhaps it is just dumb luck that some of them are good and some of them are not. I would have to run through 200 of them to get a 95% +/- 10% on them.

 

That said, when I switch the car over to a new custom harness and 12 volts, I will be running everything over 7 or 8 amps off of a bank of relay's.

 

My 1949 Desoto however, is all bone stock with NOS parts and new wire.  I suspect it will out live us all.

 

James

 

[Sniper]

Why are you folks telling him to use the battery as the voltage source?  You do not want anything other than battery charging current running thru the ammeter and if you use the battery as the source you will be running eh headlight current thru the ammeter as well as the charging current. 

 

Poor choice there.

[Plymouthy Adams]

battery is often a term used in place of saying B+ as that would just lead to more confusion also...as in, where on the headlight switch is the terminal marked B+ 

[Sniper]

When words are all you really have to explain things it pays to be more precise. 

[greg g]

Ok, guess I need some electrical splainin done.  For brake lamps, I am running 55watt halogen bulbs in my rear taillight fixtures as well as the standard 6 volt single elemant center lamp. I am powering the brake light switch from the battery side of the starter solinoid.through a 20 amp inline fuse.  System has been working fine for a dozen years or so. With this wiring with the car running, the ammeter shows a positive charge when I apply the brakes.it remains positive till the brakes are relaesed.  This is pretty handy as I can tell from the driver's seat if the brake lights are working, but is seems that the load of the lights triggers the VR to call for a charge equal to their load instead of show a discharge or drain.  What gives.

[Merle Coggins]

You stated it yourself. Since the brake light load isn't going through the ammeter it doesn't sense the current demand. However, the voltage regulator does see the voltage drop in the battery and increases output of the generator, thus showing a positive charge on your ammeter. If the brake light circuit was pulling through the ammeter it would read a negative number for the current draw of the lights, and the positive charge from the generator would offset that. The ammeter would then only show the difference.

[kencombs]

31 minutes ago, Merle Coggins said:

You stated it yourself. Since the brake light load isn't going through the ammeter it doesn't sense the current demand. However, the voltage regulator does see the voltage drop in the battery and increases output of the generator, thus showing a positive charge on your ammeter. If the brake light circuit was pulling through the ammeter it would read a negative number for the current draw of the lights, and the positive charge from the generator would offset that. The ammeter would then only show the difference.

IMO, that's the way an ammeter should be wired.  It is intended to show the net of load and charge.  Otherwise you can be mislead by loads that are not wired from the ammeter.  Let's say the underhood or trunk convenience lights stay on due to a bad switch, but are wired battery direct.  No discharge shown but battery discharges. 

 

As far as diagnosing or providing charge system health, a voltage meter is superior.  And, no high current path into the car's dash area is a plus.

[JSabah]

On 11/25/2019 at 5:22 PM, Sniper said:

Why are you folks telling him to use the battery as the voltage source?  You do not want anything other than battery charging current running thru the ammeter and if you use the battery as the source you will be running eh headlight current thru the ammeter as well as the charging current. 

 

Poor choice there.

 

Thank you all for the reply's.  Just getting back to this after getting sidetracked and Thanksgiving.  All new wiring harness and every terminal cleaned.  Please confirm that picking up the power from the "Battery" terminal of the voltage regulator will accomplish what needs to be done.  Thanks again.

[Sniper]

I don't believe I said anything about the " "Battery" terminal of the voltage regulator ".    I believe I said the output stud of the alternator is where I would source the 12V for the headlights if converting to relays.  Make sure you use a fusible link or a fuse where ever you source your voltage.