The Flxible Clipper Handbook

- by Pete Snidal (C)2004

Relays 101

What A Relay Is, and Why You Need It

A Relay is a sneaky way of switching short and/or heavy lines carrying high current with thin, possibly long lines and skinny switches carrying very low current.
A low-current electromagnet is used to draw down a spring-loaded bar to complete the high-current circuit. This allows low-current switches and long, thin wires to control high-current devices, such as motors, light systems, etc. through shorter and heavier-wire paths.

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 the low-current connection. A diagram is usually supplied with the relay.

It allows you to run a really heavy (#12, say) wire from your battery via a short path and then to "trigger" this line with a low-current much less direct line from the "office."

Do You Need Any?

One of the features of our beloved Flx's is their length - the alternator, voltage regulator and batteries are in the back of the bus, and the light switches and front lights are 35 feet up in the front. The current path is even longer for the rear lights and other load devices at the rear - from the batteries up to the front panel, through the switch, and then all the way back again to the light(s), starter solenoid, demister motor, or whatever. This 70 feet of wire can eat up a fair amount of power, especially at higher amperages. The taillights/rear marker light circuit, for example. This is especially important since today, the expectation of following drivers is higher than it once was for lights on the rear, and for this reason, many flx owners have added to the complement of lights showing at the rear. In many cases, we may be going slower than much of the rest of the traffic, and this is an additional reason for some extra lights. And of course, extra lights just compound the problem of the voltage drop in the 70-ft circuit. Adding a relay can reduce the length to a matter of the width of the bus, rather than the length.

A second feature of our busses is their age - the wires in there aren't getting any younger, nor are the connections and joints along the way. But this current path is almost certainly adequate for the low draw required to energize a relay. Thus, for example, the 10 Amps or more required for the rear tail and marker lights can be carried by a short, fat current path directly from battery through relay to lights, and the much lower current required to energize the lights will easily be carried by the existing light circuit - ending now at the relay, instead of at the individual lights. Similarly, a relay may be used in case of difficulties with the starter solenoid - itself another, larger relay - or accessories such as a radiator mister pump.

Testing: Will A Relay Help?

A relay is only necessary in cases in which the standard wiring is overtaxed. To test whether this applies to your application, it is useful to do a Voltage Drop Test.

In this test, you look for a small voltage where there shouldn't be one - between the battery "hot" terminal (the one not grounded to the chassis) and the load device feed wire, as close to the device as possible.

When the switch is open, there should be full battery voltage between the battery and the device, - ie across the switch - and when the switch is closed, this voltage should fall to 0. If any voltage is indicated, this "drop" signifies that power is being wasted in the circuit from battery to load.

For example, let's check for voltage drop on our example, the rear lights. First, locate the terminal on the rear board which feeds them. This should be terminal 17 - you can check simply by finding the terminal which is "hot" when the marker/tail lamps are on, and isn't when they're off.

Let's look at the current path for this circuit. The power must travel from the batteries to (most likely) the terminal on the starter solenoid - to the hi-current (#6 Cable) terminal (probably 13) on the rear board, up the cable to the busbar (behind the switch panel), through the "Mark-Taillamp" switch, and back down to the rear terminal board 17, before it is distributed to the tail and marker lights. That this 70-foot path may be showing some resistance, meaning lights dimmer than necessary, should come as no surprise. To see just how bad the situation is, just set your multimeter to Low DC Volts (0-5 is good), and check the voltage between the "hot" starter solenoid terminal - or the Big Amps rearboard terminal 13 - and the taillight terminal (13). With the switch off, the voltage should read full battery voltage, since there is so little current draw by the DC Voltmeter. But with the lights on, there should be no voltage showing, ie no "voltage drop." If there is (and I'd be surprised if there isn't!), this can be corrected by "jumping" the two terminals together (which would result, of course, in the voltmeter reading dropping to 0). If this results in considerable brightening of your lights, you know that fitting a relay will be a big help!

(The purists among us might insist that, in an ideal world, there should be NO voltage drop in the 70-ft circuit, and that A Real Man would just correct the problems - by cleaning/replacing all terminal connectors, switches, and possibly conductor wires. Possibly true, but it's also possible that you still wouldn't completely eliminate the drop, and in any event, the relay solution is much simpler.

Doing The Job

First, ensure that you have the necessary parts:

Tools and Materials

Once the materials are on hand, it's just a matter of following these steps:

IMPORTANT! - First disconnect your battery from the system to avoid unpleasant surprises such as fire, etc when working with the connections.


The Rear Circuit Board, Showing Rear Light Relay - Click to Enlarge

  • 7. Disconnect the taillight feed wire from the board. This should be terminal 17 - it's the one that goes "hot" with the rear lights on.

  • 8. Using a butt connector (or, better still, shrink tube and solder) connect one of the relay's hi-current wires to the taillight wire - the one that came off of terminal 17. If length permits, just strip the taillight wire, crimp on a spade terminal, and attach it directly to the relay's other hi-output terminal.

  • 9. Connect the other low-current energizer terminal wire to the rear board terminal which previously fed the taillights. (probably 17). This will provide current to energize the relay from the MARK switch in the cockpit.

Trouble-Shooting List

If, after all this, you find you no longer have rear lights, it's time to do a systematic check of connections. Here are some suggestions:

Other Uses For Relays

In the same way, relays may be used for other purposes at the far reaches of your bus. For example, if you're using a radiator mister system, with a rear-mounted pump, you can wire yet another relay in the same fashion as described above. Thus, you'll require less current capacity for your trigger line.

Another use for a relay is to eliminate high-resistance problems in the starter trigger circuit - if you sometimes have to go back there and trigger the solenoid by hand, you can wire a relay to fire the solenoid - a relay for a relay, so to speak.

No Connection Diagram?

There is often a little teensy diagram on the relay itself, but your mileage may vary. If you have no diagram, you can still use the following coping strategy to determine connections:

There will be 5 connections: Two for the energizer circuit, and three for the main circuit - one common, or "input", and two "output," - one "normally closed," and one "normally open." We will use the "normally open" pair, meaning that it closes when the relay is energized by power from the light switch.

  • 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.

  • You now have 3 terminals left. One of these is a "feed," or input terminal, and the other two are "load," or outputs. We will only have to identify two of these remaining terminals - those which are connected when the relay is energized.

  • First, energize the relay by connecting the battery to the energizer terminals. then, using your multimeter on "ohms," find the two that are connected when the relay is energized. These are your new "high beam switch," or "Load" terminals. Mark them. Disconnect the energizer power, and check to make sure that they are now open.

How To Connect It

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