General Tech Electrical Relays

When installing relays on a positive ground system, do we just split the power wire and insert the relay? Headlights, horns, wiper motor etc. Where does the relay get it's power from to operate the switch? do I have to install a third wire? I'm a hammer and chisel guy, not an electrician. I can follow a wiring schematic, but beyond that I'm lost. Any help on this? PJ

In short, the power that originally went to the (say) headlights goes to the relay coil. Then, you run another wire from the battery to the switch contacts of the relay, then the headlight wire to the other switch contact. Positive and negative ground are the same story.

I'm out of my office now, but I can scan a sketch for you when I get back. Meanwhile, here's what I did on my TD; I hope it helps:

https://www.nonlintec.com/mgtd/electrics/#wiring

good diagrams here

https://www.advanceautowire.com/

Oh and it isn't hard to do

Thanks fellas, Much appreciated! You've explained it very well! Steve, did you mount your relays under the dash on the firewall, or ? PJ

Durnit, Paul!! The relay is aught more than a load absorbing "thingie" to take the load off the switch. A "pre-switch switch"!

Here's a sketch I posted on another forum:

https://www.912bbs.org/vb/showthrea...n-switch-relay-same-thing&p=320834#post320834

It's down around the bottom of the page.

Actually, I put all the relays into a box and mounted it right behind the battery box. I also put a set of modern fuses on the outside of the box (so they're easily accessible) and have large connectors for wiring to the dash and harness. That way I can remove the box if I need to fix something. Perhaps a bit more than necessary, but I'm an electrogeek, so this kind of thing must be expected. You can see pictures of this if you use the link in my previous post and scroll down a bit.

If it's just a couple of relays, I think any convenient location should be fine. Probably on the back side of the dash is easiest, is out of sight, and is not likely to get kicked accidentally. You can get relays that have a nice mounting tab with a screw hole, and that makes mounting the relays infinitely easier.

DrEntropy said:

Durnit, Paul!! The relay is aught more than a load absorbing "thingie" to take the load off the switch. A "pre-switch switch"!

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Well Doc, I wasn't very good at math either! :highly_amused:

Steve, Thanks a bunch for your input along with the photos. Sure looks like your not an amateur in this type of work. Being up under the dash, I should be able to connect the original type harness to the relays in place of the switches and the non original wiring going to the switches should be well out of sight. Thanks again, PJ

PAUL161 said:

Well Doc, I wasn't very good at math either! :highly_amused:

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Relay relieves the load on a switch generally,

OK , I'll bite , at the risk of being pummeled by all you electrical experts out there. I've had numerous British cars with the original systems and none of them ever had a problem with the switches failing. Aren't you trying to fix a problem that doesn't exist?

Brinkerhoff said:

OK , I'll bite , at the risk of being pummeled by all you electrical experts out there. I've had numerous British cars with the original systems and none of them ever had a problem with the switches failing. Aren't you trying to fix a problem that doesn't exist?

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Only partly. There a few (I think) very good reasons for using relays.

1. 40 year old car = 40 year old wiring. These cars were never really intended to last 40 years. So, the wire (and really the covers) will degrade over time - I am still using the original wiring on my car and want to do everything I can to keep the current load light. Using a Relay effectively adds a few new circuits to the electrical system.

2. 40 year old cars = 40 year old switches. In many ways the real culprit is not the wiring, it is the switches. Over time oxidation increases the resistance of the switches, producing heat and lowering efficiency. Using the switch to switch the circuit but not power it is a good solution.

3. Relays = Brighter lights Lucas (and British Leyland) tended to cheap out on wiring. Thus most of our cars have lighter gauge wiring than they really need. Of course I can't find it just now, but have seen calculations of LBC's loosing as much as 1/4 of their voltage in the run from the battery to the headlights. So, instead of 12 volts, they are getting 8-10. With a relay you can run Halogens and be confident of a fuller voltage.

For me, Relays fall in the category of "sensible upgrades" - along with such things as seatbelts, and modern voltage regulators. They make the car safer and/or more reliable.

The last two posts kind of hit on the ultimate solution. The relays greatly reduce the load put on the switch (particularly for the headlights) Having a relay in the circuit, the current for the headlights through the switch is reduced from several amps to a few milli-amps, the relay then doing the actual switching of the higher current. This saves the switch, but does very little for increasing the amount of light from the headlights.

When redoing the lighting circuit in our early MGB, I ran a single 12 gauge wire from the main 12 volt source to a set of relay contacts and from the relay contacts to the lights. The relay was then controlled by the original 16 gauge (or the British equivalent) from the switch. By using the heavier wire going to the relay contacts and on to the lights, I achieved greater light output from the headlights.

Done correctly, installing relays, one protects the switch while also increasing the light output from the lights. It also allows the use of today's higher output headlights (which draw considerably more current than the OEM headlights.
Cheers,

Well put David.

On the other hand, a significant improvement in the existing lamp's brightness can be obtained by simply otaining your switched power to the lights from the starter solenoid and using a 16 AWG wire to the relay.

Dave, My harness is new, but you have me wondering if being new is enough? The wires are vinyl covered with only the outer covering to match the original. I don't have the original wiring to compare the new to the old, so I'm at a loss there. The original wiring was so bad and patched up, I just trashed all of it. So what do you think, just use the new harness as is and hook them up to the relays, or? PJ

As Steve suggested, the realys are best located under-dash, close to the switches. Less messing with running longer trigger wiring and "kinder" to your new harness.

PAUL161 said:

Well Doc, I wasn't very good at math either! :highly_amused:

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Ya know I wuz jus' pushin' yer laig, Paul!

A couple of points worth noting. First, a large part of the resistance is in the connections and switches, not just the wiring. Adding a relay can reduce that part of the resistance, which is why cars with relays usually have noticeably brighter lights. Second, these cars were not designed to be on the road 60 years later. In the 50s and 60s, it was accepted that the maximum lifetime of a car was about 10 years and 100,000 miles, so it was pointless to make the parts as good as they could be. All the electrical parts now are likely to be near the end of their lives, so the electrical contacts in switches are probably pretty bad. And, of course, anything that minimizes the load on them should increase reliability.

Today, 200,000 miles is nothing special, and the AVERAGE age of cars in the US is now about 10 years. I suspect that improved electrical systems are at least part of the reason for that.