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No license plate lamp after tapping for relay.

6K views 17 replies 6 participants last post by  GoMotor 
#1 · (Edited)
Until today, my heated grips were hard wired to my battery. Which was fine provided I forgot to turn them off.

I recently ordered some automotive relays and thought the area under the seat would be an ideal location to mount it.

Looking around for an appropriate wire to tap, the license plate lamp looked convenient and pretty benign should it not be a smooth undertaking.

Anyway, the relay and grips work as they should when everything's connected. However, the license plate lamp doesn't illuminate.

It's wired up as follows: Blue (#30 from battery), yellow ($87) to grips. Black (85) and white (86) wires are connected to the lamp wiring. The red wire, #87A, is unused (5 pin relay).

Reversing the white/black wires from the relay doesn't fix the problem.

When I connect the original wires together again, it the light works as it should.

So, there's switched voltage at the relay to close and power the grips, but nothing for the lights.

I'm left to think that there's too much of a voltage drop across the relay and not enough power left to light the bulb.

In the meantime, does anyone have any explanations? Is there a better switched wire to tap into that runs under the seat?
 
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#2 · (Edited)
I don't see the wiring diagram you are referring to so have no idea function those wires serve. Also, what year KLR is it.

You need to tap one relay holding coil wire to the tail/tag light wire to activate the relay and run the other holding coil wire to ground. Then run the main FUSED power wire from the battery to one of the normally open contact wires of the relay and the wire other from the other side of the normally open contacts goes out to the grips. Then the other side of the grips goes to ground.
 
#4 ·
Generally I think you'd want to wire it this way, using the tail light wire as the trigger and taking power straight off the battery.


If you wired it like this, taking power from the tail light wire, then the grips would consume enough power to prevent the tail light from illuminating.


Note: Tail Lamp, License Plate Lamp, same-same.

Tom
 
#5 ·
Generally I think you'd want to wire it this way, using the tail light wire as the trigger and taking power straight off the battery.




Note: Tail Lamp, License Plate Lamp, same-same.

Tom
Yes. The tail light is the "trigger".

Left and right is a trigger (85 and 86), top and bottom power the grips (30 and 87) just as it should be according to any number of schematics I've looked at. The center pin, #87A, is unused.

Interestingly, if I remove the downstream trigger (86) and connect the supposedly unused wire (Normally closed 87A) in it's place, the tail lamp is always light, even when the bike is turned off. So, correct wiring yields no downstream trigger voltage and a supposedly unused wire is always hot. Weird.

As I mentioned above, I think a simpler 4 pin relay is the next step.
 
#6 ·
Why are 85 and 86 both connected to the lamp wiring? One should be connect to ground. You are grounding the solenoid through the license plate light.

Tom
 
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#10 · (Edited)
You're on the right track. It's your relay ground that's the issue.

Wire 85 to lamp hot side of lamp. Fuse with 1 amp fuse if you want to. Then if relay or wire shorts, 1 amp blows and your light fuse does not. Locate fuse as close as you can to "tap in" location.

Wire 86 to chassis or battery ground. NOTE: I use a chassis so I don't have to remove it just to swap a battery.

Fuse relay term 30 at positive battery as close as is reasonably possible. Use fuse that is same rating as grip fuse. NOTE: I wire to starter solenoid hot side (NOT BATTERY) so I don't have to remove a bunch of wires just to swap a battery.

87 is hot feed for grips.

**************
SIDE NOTE: OXFORD BRAND HEATED GRIPS SHUT THEMSELVES OFF WHEN VOLTAGE GETS LOW.

FOOD FOR THOUGHT: Consider wiring 87 to the feed side of a aftermarket fuse block. That would leave room for future switched accessories independently fused from each other. You never know what the next doodad will be that you'll add.





 
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#11 ·
Nice. Thanks!

Again, "hole" flow seems to affect my understanding of DC circuits.

I tapped pin 85 on the battery side of the wire thinking that I needed a + source. Pin 86 continues to the lamp and, to my mind, completed the circuit.

It seems that I needed to understand that I could draw a trigger from the tail light directly to feed the relay and then ground from there.

I guess my question now is what do I do with the snipped tail light wire that connects to the bike's wiring harness? Tape it and it fold it out of the way?
 
#12 ·
Just wondering, why not tap the BROWN wire for the relay control ("trigger") voltage?

BROWN is switched + 12 VDC.

Nothing wrong with using the tail/license lamp lead, but . . . tapping BROWN (maybe with one of those little plastic connectors that folds over the connection) doesn't mess with any other circuits.

A "solid state" mindset interfered with this non-transistorized hook-up, I think! :)
 
#14 ·
Got it. Everything's soldered and heat shrunk back in place.

I was placing the relay in series with the tail lamp. Instead, Pin 85 or 86 needed to be spliced in parallel (trigger wire in a three way splice with the tail lamp and then the other pin ran to ground).

I'll never understand why the relay can't be placed in series and the trigger current is unable to get a complete a path to ground *through* the tail lamp, though.
 
#15 · (Edited)
Because, when things are wired in series, there is a voltage drop across each component in series. Current is constant across all the components. The voltage drop is proportional to the resistance of each component.

The resistance of the relay's coil and the resistance of the license plate lamp are of the same order of magnitude, so let's just say they are equal. That means the voltage drop across them is equal. Since there is only 12v available, the voltage drop across each one is 6 volts. By wiring the coil in series with the license plate lamp, you were giving the lamp 6 volts when it needs 12.

When things are wired in parallel the voltage is constant across all components, but the current at each component is proportional to the resistance.

This is true, and the forumulae are V=IR and P=IV.

These two concepts lead to Thévenin's theorem and Kirchhoff's Rules, a study which will make first year engineering students take to drink. At least, that's the excuse I used...

Tom
 
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