single relay for high and low beam

chickenStripCharlie

Been Around the Block
Hi,

I'm designing a small relay panel for my CB750 and want to keep it as simple as possible. I've always done separate relays for high and low beam but i'm wondering if there is a way to accomplish this with a single relay somehow that i'm not thinking of.

I want a relay for the ignition, one for accessories and was hoping to just add in 1 relay for both high and low beam

Thanks!
 

trek97

No Custom Title
DTT BOTM WINNER
If your start button has a pair of N/C contacts to power head light. (I believe all my Hondas do, cept maybe my 72 CL100)

Using this power feed will still temporarily kill the headlight for startup.

Use that power lead to drive the relays' coil through the high beam switch.

Jump the power lead feeding coil to the N/C contacts for low beam and N/O for high.

You won't be using the hot wire from the low beam switch.

That way if the coil fails at least you'll get home w a working low beam.

However, if the N/C contacts were to weld. (seldom but it can happen) you will only have high beam.

This is as simple as it gets, still acting exactly as it did from the factory.
 

chickenStripCharlie

Been Around the Block
Thanks for taking the time, but I must admit I am not following you.

When you say N/O and N/C contacts, are you talking about pins 87 and 87a on the relay? .... because if so, I'm not seeing how you can avoid the low beam (via N/C (87a)) staying on all the time.

I guess I'm not really sure what you mean by driving the power to the relay coil through the high beam switch but then also saying to power the coil through the NC NO contacts for high beam and low beam

Could you draw up a diagram on a piece of paper and upload the pic?
 

trek97

No Custom Title
DTT BOTM WINNER
Upload a pic of the schematic for the relay you want to use.

Also, include a screenshot of the bikes start button factory schematic. So can see the N/C (normally closed) contacts and read wire colors.

What year is your bike?

Ill make a drawing this evening after work.
 

SONICJK

Reminds me of...me No, I'm sure of it. I hate him
If you want to run with headlight always on with the key its pretty simple.
Just run a SPDT relay and put the lows on the NC circuit and the highs on the NO circuit.

So highs to 87 and lows to 87a.
When the relay is not energized you have a low beam, when you flip the high beam switch (energize the relay) you have high beams.
 

pidjones

Over 1,000 Posts
SONIC. said:
If you want to run with headlight always on with the key its pretty simple.
Just run a SPDT relay and put the lows on the NC circuit and the highs on the NO circuit.

So highs to 87 and lows to 87a.
When the relay is not energized you have a low beam, when you flip the high beam switch (energize the relay) you have high beams.
This is the best way, and not using the relay coil except for when needing high beam, which should be very seldom overall. Simple and effective, just make sure the relay contacts can handle the load (even better with LEDs).
 

chickenStripCharlie

Been Around the Block
But the 87a pin is connected to pin 30 when relay coil is not energized ....which means my low beam would be ON even when everything on the bike is shut down!

I'm not sure how you guys get 87a to NOT stay on all the time. I have never seen that done before.
 

chickenStripCharlie

Been Around the Block
trek97 said:
Upload a pic of the schematic for the relay you want to use.

Also, include a screenshot of the bikes start button factory schematic. So can see the N/C (normally closed) contacts and read wire colors.

What year is your bike?

Ill make a drawing this evening after work.

I will take a look as soon as I'm out by the bike today ... it is a 83 honda cb750sc - nighthawk.

The relay I was hoping to use is just the standard Bosch style with pins 85, 86, 87 and 30 (+ 87a)

As mentioned the 87a pin has, in my experience, always stayed on, so not sure how some are getting that to work as it's by default connected to pin 30 when relay is not energized
 

chickenStripCharlie

Been Around the Block
On second thought ... I plan on having an aux relay as well so I guess I could use that relay (switched on with key) to power the lights relay and not have the lights relay directly connected to the battery, which would solve the 87a on-all-the-time issue.

The relays are all 30-40amp these days so with the right size wires should be no problem wiring the lights on top of aux (heated grips and phone charger)
 

Sonreir

Oregon
DTT SUPPORTER
A single relay will work if you connect pin 30 to a switched power source.

For instance, the black wires in your harness will have power only when the ignition switch is turned on and so pin 30 can connect here. Then 87a (which is on all the time) is for your low beam and 87 is for your high beam.
 

trek97

No Custom Title
DTT BOTM WINNER
Sheesh. ;D

Honda shuts the headlight off when the start button is pressed so headlights not sucking an extra 1.21 Jiggawatts while picking solenoid, powering starter motor and ignition coils.
Its just easier on the whole system.
 

chickenStripCharlie

Been Around the Block
Sonreir said:
A single relay will work if you connect pin 30 to a switched power source.

For instance, the black wires in your harness will have power only when the ignition switch is turned on and so pin 30 can connect here. Then 87a (which is on all the time) is for your low beam and 87 is for your high beam.

But connecting pin 30 to an existing switched source, with mediocre wire thickness kind of defeats the purpose of rewiring and using a relay.

Now the black wire would be carrying the additional burden of the power draw

Disclaimer: I havent looked at the black wire yet, it's possible it is an adequately thick wire.
 

trek97

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DTT BOTM WINNER
Though Im not really sure why you want to add a relay to control the headlight.

But thats none of my business. Its all good, and can be done.
 

Sonreir

Oregon
DTT SUPPORTER
trek97 said:
I imagine they're all 18awg. More than adequate.

Depending on the make/model/year, many of the black wires (especially those coming directly from the ignition switch) will be 14 gauge. Just fine, I figure.
 

crazypj

Split personality, I fake being smart
It's kinda normal to run headlight through relay if you fit higher wattage bulbs or even to make sure you get full battery voltage to headlight. (particularly if your doing a lot of nighttime riding, did my 550 in 1978)
chickenStripCharlie said:
But the 87a pin is connected to pin 30 when relay coil is not energized ....which means my low beam would be ON even when everything on the bike is shut down!

I'm not sure how you guys get 87a to NOT stay on all the time. I have never seen that done before.
You need a second relay to control the first one. I didn't bother and just cut the ground wire to headlight then ran it through a normal on/off switch. You can get better charging by running battery direct to field coil through a relay. Output to headlight at same point.
trek97 said:
Sheesh. ;D

Honda shuts the headlight off when the start button is pressed so headlights not sucking an extra 1.21 Jiggawatts while picking solenoid, powering starter motor and ignition coils.
Its just easier on the whole system.
When did they start doing that? I've never seen it on any 1970's bikes and Kawasaki were the only people doing it in the early 80's? (through to at least 2005 when everyone else caught up)
 

crazypj

Split personality, I fake being smart
Never seen extra wires in switch or harness? I'd heard rumors of some markets having the ability but not in Europe or USA
 

trek97

No Custom Title
DTT BOTM WINNER
crazypj said:
Never seen extra wires in switch or harness? I'd heard rumors of some markets having the ability but not in Europe or USA

Maybe 360 didn't come that way from factory.

But mine does now, w new control and homemade harness. ;D
 

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