Except for the Neanderthals that think twisting wires together is an acceptable switching method. Relays are great if you understand what you want them to do. Electrical current like flowing water can be blocked or diverted. A 5 post relay is great for diverting current. A few years ago, I used headlight switch [push pull type] to wire a self parking 2 speed marine type wiper motor. [the user had a hand injury and couldn't operate the dial type switch] I used 2 relays Off position had the self parking wire on the reverse post [87A] of the 1st relay. When the switch was pulled out 1/2 way to park position it activated that relay to post [87] post [87] was wired in series to the 2nd relay to post [30] , then on that relay post [87A] went to low speed and post [87] went to high speed on the motor. When the switch was pulled all the way out, it diverted the second relay to post [87] high speed. I looked everywhere for a 2 speed push pull wiper switch , but the problem was getting one that switches off the self park when in high speed [they always self park at low speed]
When I wired my 32 that has an electric fan and halogen headlights, I was aware that every ignition switch I looked at was rated at 40 amps or less. Add up your system load and you will see why relays are necessary. So if you have sealed beam lights, a mechanical fuel pump, an am radio and a belt driven fan, you probably won't use 40 amps at a time. If you have Vintage air, you will see that it comes with a relay because the blower motor can pull 30 amps. Everything I have has relays because of the safety factor. Not everbody's cup of tea. I understand. Did anybody ever do any research to see how many cars suffer from electrical fires????
A Holley red pump states 2 amp draw. Unless the run is long and voltage drop an issue, I wouldn't bother. The Carter I have on the '31 draws 5 amps and I went straight to the fuse block.
The LR-32 relay in post 60 is listed for non-continuous duty, according to the linked information page. There's this style that has an "Octal" base, they turn up in military surplus often but usually 24-28 volts instead of 12: This one is currently on ebay for less than ten bucks and is 12 volts, but you can find them after this sale is over by using the keywords "12 volt relay octal". I think they date back to at least the fifties. Of course you will also need a base.
Yep, it would appear that is the case according to those specs listed. However, in the very old (2004) Standard Engine Management Parts Guide page 613 it is listed as: "LR-32 Light Relay - 12 volt single circuit application. These units are used to relieve overloaded switches and eliminate voltage drop along wires on high current circuits. Originally, they were used primarily for headlights but now they find wide application on almost every type of heavy electrical load. All circuits are protected by SFE 20 fuses conveniently located under the metal cover where they are safe from accidental brakeage." The text in red refers to LR- 34 double headlamp relay. I suppose the description may have changed to reflect how new production relays are made today. It's shown now to be a starter relay which of course wouldn't be continuous duty. I do know that when I bought the one I installed in my car it was listed as continuous duty, and has been used that way and performed very well. You'd think they could have issued a new part number if the use and specs were to be changed to that extent.
Perhaps the listing was changed due to some new testing requirements for continuous duty relays, or laziness in updating the listing. If it was listed for continuous duty at one time, I would be fine with using it as such. If it couldn't take it I assume it would just damage itself, since it is fused. Of course, the headlights would go out.
There is a relay designed for this job, called a change-over or latching relay. The 87 and 87a type relays require that the coil is energized the whole time you want to select the optional output. Change-over relay coil only requires brief energizing to change from one output pole to the other. (Low/high beam)
These type relays are almost always limited to 10 amp max, 5 amp is common too. They're also generally multi-pole (2PDT or 3PDT is the most common). A bit light-duty for most automotive use, they can be handy if you desire an electrical interlock, are sequencing an operation, or switching multiple circuits off a single switch. They are 'old school', the smaller 'ice cube' relays superseded them and do the same things for less money.
These are expensive if you go looking for them by that name. These were pretty common on European cars from the '50s at least through the early '90s, although they were merely called a 'headlight relay'. And even these aren't particularly cheap; the least expensive and easiest to get relay is for the VW bug at about $20.
Am I missing something? The op stated his fuel pump draws 2 amps? That shouldn’t require a relay, unless there is a significant startup surge. Some relays take two amps to excite. Bones
At 2 amps running current, a 10 amp rated switch will be more than adequate to operate it. No relay needed....
The 87A terminal is normally closed with 30 and does not require power in that default position (other than for the circuit itself). That's why I designed that circuit the way I did. Plus the odds of me finding a latching relay in East Bumfuck are far less than a 5 pin relay. It's not just about the design..... it's about servicing as well.
I never looked into the current rating of these old style relays, but they have fair size silver contacts so I assumed they were fairly high. I realize that the modern cube ones are "better", but they don't look right on an old car.
Correct, Steve and Audis, etc. If the car has a bright dim switch as part of the turn signal, probably has a change-over relay that is activated by pulling the lever and grounding the relay input briefly.
Great info here. If I am converting my 41 to 12v negative ground, IMO I really do not need a relay because my amp draw would be less than original 6V, std sealed beam headlites. So put a fuse on 12v to switch(original headlight switch and floor dimmer switch) KISS method should be fine??? Or am I way off base? As always Thanks
I use relays for high draw components that can eventually kill switches. But many of the devices you are using relays for will not damage switches, so no need to further complicate things by adding them. I have a relay for my electric cooling fans, and electric fuel pump. Nothing else has a relay, and never had a failure.
I used a relay for ignition on an early Ford Column switch so all the juices wasn't routing through that bakelite contact. Worked well, the starter button merely grounded the relay, if I remember correctly.
One consideration for me will be where the battery is placed in the vehicle of choice.....and where the fuse panel is placed. On higher amp items I probably will look at the circuits and see just how long the wires will need to be if I direct wire or if I'm better off with using shorter runs to activate a relay with a short higher current path. I don't think you can say that there is only one correct way to wire all variations of the cars we build ....or that doing it your way is wrong......both work if done properly. We don't all do anything else on our cars only one way.
One of the very few electrical statements I understand. For the most part no TWO people will give the same answer to an electrical question , and I'll die in the dark.
Logically you should gett less voltage drop than on 6V with higher current. That's good. If potentially 80+ year old switches still are up for the task may be another question you want to ask yourself. You could wire it up w/o relays, measure voltage drop under load and decide if it's good enough, or if you want to add relays to lighten the load on the switches and probably get a bit more light out of the lamps.
Everything has been said already in this thread. Only thing I can add is if you need to run a relay or two you can get a non working gen regulator or similar and gut it to put in new cube relays. A friend did it for a non functioning o/d relay that's expensive to get a new one, just put a Bosch relay inside and soldered it to the terminals..looks stock on the outside.
Yuppers, That was my issue that I mentioned in post 38. I was frying dimmer switches with the amp load of the headlights. The 55/100 H-4 bulbs draw a lot of amps. I don't drive to work at 3 am anymore but one of those walk arounds got hit and killed 100 yards from my mailbox one night when someone didn't see him. They left a bunch of black plastic parts on the road where he ended up in the ditch. Still we as hot rodders or custom car folk put a lot more electrical items on most of our rides than the original switches were intended to handle amp wise. 5 amps for a fuel pump, 5 amps for a radio, The electric windows in the doors draw 15 amps when they run and we put them in because we couldn't find decent hand crank regulators for the rig we have. Oh and that amp for the subwoofer Those little numbers add up and keeping some of those numbers to a minimum at the switch saves the switch.
The concern I'd have about using antique switches, even after they test ok..........is if they do fail later, where do you get a replacement?
Even my old Lucas headlight switch handles the converted headlights on my Austin with higher amperage halogen sockets and lamps I adapted. If a Lucas can handle the headlights without a relay then most older style headlight switches will do so easily. Unless they're modern Chinese made copies.
This is absolutely terrible advice. Without knowing the condition of another vehicle's wiring/electrical system, nobody else can say what that particular vehicle is safe to use. Running full (increased in this case) electrical power through old, undersized wiring is a recipe for a fire. Good to hear it hasn't happened yet, but that's no guarantee it won't in the future. Relays are inexpensive and not difficult to install, and easy to hide if you mustn't see them. If you are running an item with a higher current draw, they are a great idea. Plenty of people never upgrade their vehicle's outdated and suspect wiring, as "it's fine!"...until the vehicle is engulfed in flames on the side of the highway. Please, don't be that person.
Interesting, you may be the first to admit the number one function of electricity is fire, thank you for informing everyone about the danger. I'll read the posts, but never run a wire, just not worth the risk.
As @Crazy Steve said earlier if the circuit is designed to handle the load, wire size, switches, etc that's all that needed. You don't need 0/1 battery cable to run a 5 Amp fuel pump as some makes it sound due to Amp draw and possible overheating and fire from a fuel pump. Proper wire size, a good quality 10 Amp toggle switch with a inline fuse should be plenty.... ...
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