The Coil Relay Mod

[mollusc]

The wiring on your Nighthawk is old.  It's tired.  It wasn't necessarily all that great to begin with.  Many people have been able to get a bit of pep back into their steeds using the following non-invasive mod.

First, find the three-wire plug that controls and powers the coils.  On (most?  all?) Hondas, the blue and yellow wires control the coils' firing, and the black and white wire provides power.  Notice that there is just one power wire for two coils – somewhere between the harness connector and the coils, this wire splits in two.
• Follow the black and white wires to the coils and unplug them.
• Cover one of these loose connectors with heat shrink tubing or electrical tape to prevent it from shorting as it will no longer be used.
• Cut a piece of wire to the appropriate length to be able to run it from the other loose connector to a relay that you will mount somewhere convenient – usually under the seat.  Plug one end of this wire into the loose connector with a male spade connector, and the other end onto your new relay at terminal 86.  This wire does not have to be a large gauge, as it is only a signal wire.
• Find the frame ground, usually near the coils.  Run another wire between a ring terminal on the frame ground and your relay at terminal 85.  Again, this doesn't have to be large gauge as it is only the ground for the signal wire.
• Run a 12 gauge wire from the hot side of the bike's main fuse to terminal 30 on your new relay.  This wire should have an inline fuse holder and 10 amp fuse.*
• Join two 12 gauge wires together in one spade connector.  It's probably a good idea to solder them into the terminal.  Connect this to terminal 87 on your new relay and then insulate the connector with heat shrink.
• Run the other ends of these joined wires to the terminals on the coils where you removed the black and white wires.  Connect with spade terminals and insulate with heat shrink.

The terminals:

• Relay 30:  Heavy gauge wire to main fuse on starter solenoid
• Relay 87:  Heavy gauge wires to coils
• Relay 86:  To wiring harness side of black/white wire
• Relay 85:  To frame ground on left side of frame right above coils

Once every connection has been made, give everything a once over and fire up the engine to ensure it runs properly before you put the bike back together.  If you wish, you can also flood the top of the relay with hot glue to keep water at bay.


* Terminal 87 on your new relay is now a strong, fuse-protected, key-switched power supply.  If you want to use an even larger gauge of supply wire, you can use this terminal to power more than just the coils.  For example, if you put three wires into the connector that connects to this terminal, you can run one wire to each coil and the third wire to power your headlight/horn/GPS/USB.

If you are wary about using the same relay to run your bike as to power your accessories, you can always replicate this whole layout using new wiring and another relay, but switching this second relay with the unused black and white wire.  Terminal 87 of this relay will be key-hot just like the coil relay.

[Willie]

Having joined the forum today, I thought I'd read through all the posts that I find of interest. This one is especially interesting to me because of my experience with performing the coil relay mod on many of the 70's and 80's Suzuki GS's I've owned. Having experienced, 1st hand, the difference in performance, I'm a BIG believer in this mod.
Based upon my experience, I'd like to add one step, a suggestion, if I may. That is to measure and compare the voltage at the coils (with the engine not running but the ignition on) and compare it with the voltage across the battery terminals. The amount of the difference will be the reason for performing this mod.
My experience has been that I've seen as much as a 2 volt difference between the two. Take into consideration that a coil, essentially, amplifies the input voltage, it doesn't take a rocket scientist to imagine the affect a 20% difference in input voltage would make. All in all, I've been amazed at the increase in mpg, ease of starting and idle smoothness after doing this relatively quick, simple and cheap mod.
The End

[Larry Fine]

Based upon my experience, I'd like to add one step, a suggestion, if I may. That is to measure and compare the voltage at the coils (with the engine not running but the ignition on) and compare it with the voltage across the battery terminals. The amount of the difference will be the reason for performing this mod.

Welcome to the forum.

You can also directly measure the voltage between the battery + and the coil +, performing what we electricians call a fall-of-potential measurement.

However, with the engine not running, the ignition module may not be conducting current, or the same current, as it would with the engine running.

[slodoug]

did this today. 2v difference

[mollusc]

Do you notice any difference in how the bike runs?

[slodoug]

it started easier, but havent ridden it yet, waiting on a new recitifier.

[slodoug]

test ride went fine. more power at lower rpm. i think one of my yellow wires off the stator is going bad so i'm replacing that next week. battery shows from 12v to 14.7v at 4k rpm and fluctuates.

[Bob H]

more power at lower rpm.

Yeah that was my result also from doing the coil mod. Subtle, but real.