The Stovebolt Site Forums The Shop Area The Electrical Bay Headlights and horn Relays

I’m going to be using relays on my H-4 halogen conversion reflectors and also converting to 12v with a total rewire in my ‘37. Let’s see if I have this correct. Fusebox power to the light switch, light switch then goes to the dimmer, which splits it two directions (high beam/low beam) then it goes to the relays, then to the headlights. I have a few questions. #1 does each headlight need its own relays ( high/low) for a total of four relays? Or just two relays, one for highbeam one for low beam and series wire to the two headlights? #2 Considering the H4 draws 65 W Should they be wired parallel with their own relays anyway? #3 would the single box with six relays all protected and mounted on the firewall recommended or all individually mounted relays better? #4 my simple mind is swirling with logic and the question, why are the 5 pin relays all SPDT when I am wanting it to control one circuit each? I’ve wired a lot of houses and understand KISS circuits such as power in, power out and grounds completing the circuit.

1: Depends on what relays you get. I recommend these 60A ones. They will handle anything you throw at them other than your starter. Jerry recommended them for a horn and they would work equally well for headlights.

2: You mention both series and parallel wiring. The headlights should always be in parallel or they will only get half the power.

3: Whatever method floats your boat. You only need 2 relays (one for both high beams and one for both low beams), so if you want to mount a pair of them somewhere you can. Or you can get a fuse block with both in it and mount it somewhere else. There is no functional difference.

4: The greatest majority of automotive relays are SPDT 5-pin because that lets the manufacturer (and parts house) stock a single relay that can be used in any of the potential configurations. It is purely for their convenience.

Clear as mud?

As a side note, your 65W bulb will draw ~5.5A. You are not talking about a huge amount of juice here.

100 watt 12 volt headlamps will pull 8.5 amps each on high beam, for a total of 17 amps. If your H4 lamps are DOT approved they are probably 65 watts and will pull about 6 amps each, 12 total. A single 40/30 amp cube relay for low beam and another for high beam will handle the lamps well.

You will find a lot of these relays will be rated at 40 amps on the normally open contact (pin 87, the one you'll be using) and 30 amps on the normally closed contact (pin 87a which you won't use).

Thanks guys, I misspoke about serial/parallel was intending “home runs”. yes Jerry recommended a 60 watt on another thread because of the gauge of the wire. Thanks for clarifying the SPDT. Seems the more I looked the more options I ran across.

12V and halogen don't need a relay. The switches will be OK if not broken or worn out.
You would be wasting time, wiring, money and possibly adding voltage drop.
But do what you wish.

Optional wiring:
Fuse box power to light switch, light switch to dimmer, dimmer to bulbs. Direct wiring from dimmer to H4 headlight plug, no connectors, no terminal boards.

Standard sealed beam 12V headlights are 65W high beam each= 130W = 10.8A total. Same as the H4 halogen and same as the original 1937 bulb. No new electrical stress will be introduced.

P.S. Larry...I see "horn" in your title but no mention in post. I think that means you decided on that from your other horn post?

Your 10.8 to my 12 amps isn't "way" high in my book. I have H4 lamps in my '52 GMC.

The headlight bulbs were used in a 1937 Chevy truck were GM part number 110185. They are not sealed beam. I wouldn't commit to using the original wiring until I knew how much current they drew. I can't find specs on them. Tung-Sol replacement is 2320L which is rated at 32 (high beam) and 21 (low beam) candle power. There is no converting candle power to watts unless you know the power factor (efficiency) of the bulb.

Somebody above mentioned "60 Watt". Watts and amps are not the same. Big wires and relays are just a common sense upgrade for most of the old rigs we work on. Material cost is a one time expense- - - -voltage drop and dim headlights as a result are forever. Mount the relays on the radiator core support as close as possible to the headlights, and run a voltage supply wire with 8 or 10 gauge wire to power the relays. Run a self-resetting circuit breaker (NOT A FUSE) on that wire. In case of a short, the lights will cycle on and off as the breaker resets, not go out altogether from a blown fuse.
Jerry

Bill, not wanting to create an argument, just an opinion for Larry like others are offering, and a Stovebolt record. I was comparing your 17 to my 10.8.
All three bulb ideas come to the same amp draw as I said. The bulb being sold for 1937 is 32W. 32W divided by 6V= 5.3A, the other two are 65W divided by 12V= 5.4A
The original used the stock switches and wire and no relay. Looks to me like a relay is not necessary and I wanted to mention that and support that opinion.That is the info I want to offer. Nothing else.

I did see bulbs listed at 32 CP. That makes NO SENSE when converted by any formula. So I decided the seller did not know what they were selling. There are many sellers calling it 32W.

Many folks on the net using 12V H4 halogen with no relay. Just info to ponder. Larry has new wiring. Larry did not ask if he should use relays, so I stated he should do what he thinks is best after all member input. That is what Stovebolt is for.

No argument from me. The 17 amps I was referring to in my first post were two of the 100 watt, not DOT approved for highway use, bulbs as stated in that post. I agree that the 65 watt DOT approved lamps would use 10.8 amps (where I said 12 - did the math in my head, which doesn't work as well as it used to.

You are right, he did not ask. He explicitly stated that he was going to use them.

Without knowing what gauge wire his new setup has for headlights and what the new switch is rated for, we don't have any insight into what is strictly necessary for him. There are universal kits available that call out in the instructions that relays should be used for the headlights and provide small gauge wires to trigger them.


I would agree that a lot of sellers don't know what they are selling. Original 2320 bulbs have the 32 - 21 C printed on them. By inspection of the contacts and base, 32W is far too much power for those bulbs.


A self resetting breaker is a very good idea. Some wiring kits come with them already, so it might be worth checking your documentation.

*I am assuming you are using a kit for the conversion. If not, then a lot of answers could be different.

Just for anyone looking/curious, here is an example of an auto-reset breaker that you can insert in place of a fuse:

https://a.co/d/b9mLZ6b

You have to pick the fuse base that matches your panel, but they are available in the common bases.

I found a box of 10 GM part number 110185 (correct bulbs for 37 Chevy trucks and cars according to 1929 - 1954 Chevrolet MASTER PARTS and ACCESSORIES CATALOG) on
ePay.

Both the end of the box with the GM part number and a close up of one of the bulbs are shown below. Note that the bulbs are marked 32 CP (candle power), not 32 watts.

Why 32 candlepower?? I have found references to, but have not found the actual law, in New York state that is supposedly still on the books that limits headlight brightness to 32 candlepower!

I agree with your statement about converting candlepower to watts because as I said before one would need to know the efficiency of the bulb (how much light vs. how much heat) in order to do so.

Mount self-resetting breakers as close to the power end of the wire as practical. I have two in my truck, both just inches from the power lug on the stomp starter switch.

I didn't know about the NY law, that is hilarious.

VTL 375 2(a)4(c):

I decided like Jerry suggested, that relays would just be a common sense and relatively inexpensive upgrade (though possibly not necessary) for the DOT 55/65W H4's and horn. The wiring harness I am using i think has 10-gauge wire for the headlight circuit, I'll have to check, could be 12. It only had one relay (meant for AC) of the new fuse box, so I have already ordered a marine box for additional relays. Jerry's suggestion of self-setting breaker sounds intriguing, so I'll look into that. New dimmer switch, the original headlight switch with built in fuse fell apart in my hands so I have GM 62-66 era switch with repo '37 knob mounted. It will be an upgrade over '37 with a dimmer capability. ignition switch when I got the truck was the ugly aftermarket universal one and didn't even have a key. Purchased a GM 62-66 switch and turned the stainless-steel face nut down and polished to look pretty close to the original look of the ring. The diameter is same as original. New GM octagon keys. I won't need the SOL terminal, or start position, but can gain the upgrade of an ACC position. Appreciate all the help and advice as I string wire! Plan, plan, cut and solder once! Optimism! Larry

Try to avoid solder whenever possible, as it is more apt to fail than a properly crimped connection- this is why aircraft generally do not use soldered connections in their wiring systems… the terminals are all crimped. Vibration is not kind to soldered joints.

Bill what size breaker are you using? What would you recommend for a 12v system?

I have two breakers. A 20 amp breaker feeds power to my relay controlled headlights. The rest of my home-made fuse/relay panel gets it power through a 50 amp breaker. Overkill, but who knows what you might want to add.

Both breakers are mounted near the starter and get their power from the battery cable lug on the stomp starter switch. They look like this.

Before I put headlight relays on a truck I had I turned on the headlights on high beam and touched a jumper wire from the bat. to the socket wire for the high beams.
The lights got noticeably brighter. That convinced me to install relays.

George