The Flxbook Manual

- by Pete Snidal (C)2007

Relays 101

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

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 real careful, or disconnect the battery between each test and check, reconnecting when you're ready for the next one. 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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