Article: Relays 101

By Pete Snidal © 2007

What A Relay Is, and Why You Need It

Let's take a look at one application.

With a Flx Clipper, all your control switches - lights, ignition, starter, etc. - are in the front. But many of the loads they control are 'way in the back - ie, the First-Reverse solenoid (if you still have the Spicer 5-speed Gearbox), the taillights and rear clearance lights, ignition coil (gas engines), or the starter solenoid, to name the main ones. So, can a relay help you? The answer is often yes, and here's why:

It allows you to run a really heavy (#12, say) wire from your battery directly to a place now serviced by old, long wire with too many old connections directly. Of course, you can do this without a relay, too. For example, run a wire directly from your rear main board - or even directly from the battery connection on your starter solenoid - to your taillights. But that leaves the obvious problem that your taillights will now be on all the time. So, we use a relay, triggered by a low-current trigger line - fed from up front by your light switch - to close a relatively higher-current and much shorter circuit between the battery and the lights. (You could also let this relay feed the rear clearance lights.) Thus you now have all your lights being fed by about 6 feet of fresh wire, with a 6 foot return to ground, of course, instead of about 35 feet of old wire, more connections, and a 35 foot return to ground. Furthermore, since the path is now so much shorter, you could even include a direct wired ground return instead of relying on body metal.

Can such a relay improve your headlight and front clearance light performance? No, it can't, since the 35-foot path is inevitable, unless you have a battery up front as well. So, for the front lights, the only choice is to clean up the connections - and possibly replace the switch. But the switch is fed by the #6 wire-fed busbar, so the current flow to the front is pretty good. (The rear lights are, too, but the path back from busbar to the lights is old, tired connections through #16 wire.)

How Do You Know If It Will Help?

There is of course no sense in relay-equipping your rear light connection unless you've tested your present one. (I almost said "current one!" Silly me!) This is best done with a DC Voltmeter. Just check your battery voltage (engine stopped), then turn on your lights and check the voltage at a tail light. If there's a difference, you know that a relay - or possibly some serious connection-cleaning - can help with tailligh brilliance. If you've added more tail lights, you'll find that any connection problems will have been exacerbated, (worsened) due to the increased current draw on the existing wiring.



A low-current electromagnet is used to draw down a spring-loaded bar to complete the high-current circuit. This allows low-current switches, such as your stock dip switch, to control relatively higher-current devices, such as a mess of rear lights. (The more lights, the higher the current draw - and the greater the power loss will be.)

To wire it in, you need only identify the low-current terminals - they will make the relay "click" when connected to power - and the high-current terminals - they will close when the "click" is heard from as the low-current connection closes the relay.


Voltage Drop Test

The really sophisticated way to check any circuitl, such as your taillight circuit is called a voltage drop test. It will save the need to subtract the taillight-on voltage from the battery voltage to see just what the loss is. You just check the voltage from the taillight connection to the battery "hot" terminal - with the voltmeter set to lowest DC range. This will give you a reading of just how much voltage the current path from the battery, up front to the switch, and back to the taillight is actually being eaten by bad connections and long wire. If it's less than a volt, forget it, if it's more, read on.

In either event, don't forget the return path - from the lights back to the battery - actually, when the engine's running, back to the voltage regulator ground connection. Since your existing one is all the way through all that 50-year-+-old metal, and since the distance is so short to the rear lights, you may want to augment it with a jumper from light to light back to the VR or battery ground. Meanwhile, if you've found a significant Vdrop to the lights' "hot" side, here's how to remedy that the fast way with a relay:

1. Tools and Materials

  • A Relay - Your local auto supply store will have a selection of small horn relays. Bosch make a nice one. So does Hella.
  • Wire. Pick up a couple rolls of #14 stranded automotive wire. Red for Hot side, Black for ground side.
  • Crimp connectors - some spades, for the relay, a large (3/8") ring, and a smaller (3/16") ring or two, for the battery/solenoid/rear terminal board connections.
  • Some shrink tubing and solder, (pro) or some butt crimp connectors (am).
  • Crimping tool for the connectors
  • Two fuse holders, with 20A fuses, for the tail and marker light circuits.
  • Soldering gun and BIC lighter (pro option)
  • The usual tools - screwdriver, pliers, wire strippers, soldering gun,

2. What Connects Where?

There is often a little teensy diagram on the relay itself, but if you use a small one you can barely see it. There will be 5 connections. So, here's a coping strategy. First, since you're working on a "hot" terminal board, you'll have to be very careful not to touch the chassis ground anywhere with a wrench connected to a hot terminal. The 12V won't hurt you, but the heat produced in the tool - or watchband - which has become a shorting bar - can give you a serious burn. So, you'll have to be realcareful, or disconnect the battery between each test and check, reconnecting when you're ready for the next one.

  • First, find the energizer circuit. Which two? We will use a "hot" wire from the battery, and a ground wire. Clip a jumper to each battery terminal. Attach the ground clip to any tab on the relay.
  • Try a hot wire to each of the others in turn, looking for a "click."
  • If you don't get a "click," move the ground clip to the next one. Repeat step 2. Keep doing this until you identify a pair which will make the click. (Close the relay) Polarity doesn't matter; either of these two may be grounded or made "hot" to energize the relay. Mark them. Or better still, make a ground wire - spade on one end for the relay, ring on the other for the chassis mount, and connect it to one side. Wire only needs to be about 6" long, since you'll be grounding it to the relay mounting screw.
  • 2. Now it's time to mount the relay. Drill a hole and use a 10-13 screw and nut to mount it to the firewall close to the terminal board. Fix the ground terminal of the energizer pair under the mounting screw - clean the firewall surface with sandpaper before mounting.
  • 3. The other side of this pair will want to connect to the "lights-on" terminal on the terminal board. Looks like No. 17, shown as yellow in the diagram The test will be that it goes "hot" when the lights are on, no power when they're off. Make up a wire to your relay - long enough to reach it - with a spade terminal for the relay, and a ring terminal for the "lights" terminal on the board Now, with battery connected,when you turn on your lights, the relay should "click" to show that it's energized.
  • 3. You now have 3 relay terminals left. Two of these are "always on," ie connected when the relay is not powered up, and one of them is part of the ""sometimes on" pair, connected when the relay is energized, in this case (now) by your light switch. Find the two terminals that are connected, using your ohmeter setting on your multimeter, and mark them. Then turn off the lights.One of these "sometimes on" terminals will be the "feed," or input terminal, and the other the "load," or output terminal. The Feed (either one) must connect to the battery hot (Terminal Board 3) and the other will go to all the rear lights.4. Using a large ring terminal for the #3 battery connection to the terminal board, and a spade for the one on the relay, connect the relay to it's "hot" feed terminal on the board.
  • 5. Now for the wire feed to all rear lights from the relay load terminal. Starting at the light furthest away. (ie, the left outside marker light) connect a fresh piece of #14 wire as closely as possible to the marker light. Run it to the center marker lights, and connect it to each of them, also as close as possible, and then to the right side marker light. Tee into this line and run it down to the relay, leaving the end open for the time being. Then, do the same for all tail lights - you may have more than the stock ones by now. Run this wire also to the relay. All the Tee connections necessary may be made with butt crimp connectors - two wires into an end for most of them - or by stripping, slipping on a piece of shrink tubing, soldering, pulling the tubing over the joint, and shrinking with heat from the BIC. The soldered joint is much more reliable over time - you can follow this up with tape, if you like (semi-pro), but the tape is subject to falling off after a time, so the shrink is a better idea.Now strip the two light wires - marker and tail - at the relay, and crimp or solder each of them into one end of a fuse holder. Twist the other ends of the fuse holder wires, and crimp them into a spade terminal. Plug this onto the relay's remaining "load" terminal, and you're ready to test.
  • 6. Connect the battery if necessary, and turn on the lights. Your full complement of rear lights should now be lit, and a Vdrop check should show little to no voltage drop between battery and any light wire.
  • 7. You might also want do do a Vdrop check between the frame of each light and the battery ground terminal. (the - one) If you find a drop of any significance, it may be eliminated by repeating the light-to-light connection with a piece of black wire, ending at the battery (-) terminal - or the alternator ground (engine).

You now have the best possible connection from source to lights for all your rear lights. All the 70 ft long path from battery to light switch and back now must handle is the current required by the relay. Your new tail and marker light circuits are now fuse protected, will operate independently, so if one fuse blows, the other circuit will still show some rear lights. Remember where the fuses are for a time in the distant future when this may happen. (Alternately, a fuse block may be mounted on the outside of the engine compartment, with the feed lines going through one fuse each on the block. This way, the block fuses can be marked, and other fuses may be used for any other circuits you may decide to relay.)

In the same way, you can eliminate any current-restriction complications for your solenoid circuits (starter or reverse-1st gate in the case of the Spicer Gearbox), or ignition. One relay in each case.

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