Coil Wiring and Relay mod - Kawasaki Vulcan 750 Forum : Kawasaki VN750 Forums
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post #1 of 60 (permalink) Old 02-24-2011, 06:28 PM Thread Starter
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Coil Wiring and Relay mod

Direct wiring the coils to the battery using
a relay to bypass the wiring harness and
give more voltage to the coils.

This mod would be used generally for older bikes that have questionable wiring. A quick way to know if this will help your situation, is to measure voltage at the battery with ignition on, bike not running. Should be around 12.5 volts. Then measure voltage on the red coil power wire, again with ignition on and bike not running. If you have less than 12 volts here, you are losing voltage somewhere in your wiring, through the numerous switches and connectors before getting to the coils. I did this now on my 2002 just to test it and see if it would help the hot starting problem. So far it, combined with moving my regulator/rectifier, seems to have made all the difference in the world. Time will tell.

Parts needed:

Basic automotive relay. Can be found in the bling section of your auto parts store.
Inline fuse holder. I prefer the waterproof mini fulse holder for this.
10amp mini fuse
12-14 guage wire, red and black
Various spade connectors and ring connectors, crimp type for 12-14 guage wire
Soldering iron and solder
Heat Shrink

Figure 1.

I borrowed this diagram from folks at, where I am also a member. Note: Our VN750's have RED wires connecting the coils to power, not Red/Yellow as in the diagram. Another note, my relay had #30 instead of #36 on the Power connector.

Remove the front left side panel that is located above the left air filter. There are two phillips screws holding this panel on. It's the panel with the reflector on it, in front of the fuel tank. This side panel is covering the front coil.

Figure 2.

A. Automotive relay
B. Original rear coil power wire, going to relay as the trigger.
C. New power wire going to both coils. You can't see the junction where I soldered this wire to the same gauge wire going to each coil as it is under the large wire cluster that you can see at the bottom.
D. Inline fuse holder.
E. Ground wire

Fish a red 12-14 guage wire up from the battery box area to the front coil. (C Figure 2) Leave yourself extra wire at the battery box end, enough to be able to attach this end to your relay (A Figure 2). I put my relay in the right front corner of the battery box area, taped to my frame. You might have to loosen the fuel tank and raise it slightly to fish this wire through. Make sure it stays under the frame where it won't have the tank sitting on it. Strip the wire and tin the tip with solder. Crimp on a female spade connector. I used spade connectors that have a cover over the entire connector. Pull the red wire off the coil, put some dielectric grease on the new connector and connect the new wire onto the coil. Wrap the old connector in black electrical tape and push it under the frame. I didn't cut any existing wires to do this mod. It can be reversed easily if needed.

Reinstall the side cover over the coil. Measure from the power terminal of the back coil to a location where it can intersect with the new front wire you just installed and cut another piece of red wire. Mine was about 8" long. Strip both ends of the 8" wire. Taking the wire you attached to the front coil, find an area of this wire where the 8" wire can intersect with it, and cut the long wire, and strip the end. Now using the rest of the wire you just cut, strip it's end and join all three ends, (end from wire from front coil, end of 8" wire, and other end of wire you are going to use to go to the relay) and twist them together. Tin the twisted wire with plenty of solder, then shrink wrap and tape the connection. Tin the other end of the 8" wire and crimp another female spade connector to the end. Remove the existing red power wire from the rear coil, dab a little dielectric grease on the new connector and connect the spade connector from the 8" wire to the power terminal of the rear coil.

The remaining end of wire coming from that 3 way junction you soldered, (C Figure 2) needs to be stripped and tinned, and a female spade connector crimped on, DE greased and connected to the terminal on the relay labeled 87. This is your power to the coils from the relay.

Pull the tape back on the original red wire (from the rear coil) (B Figure 2) to give you enough slack to be able to reach the relay with the wire. Again, dab a little DE grease in the connector and connect this original red coil power wire to the post on the relay numbered 86. This original wire will now be used as the trigger to close the contacts in the relay when the key is turned on.

Tin the leads on both ends of the inline fuse holder (D Figure 2) and on one end crimp a ring connector that is large enough to fit the battery terminal screw through it. On the other end, crimp another female spade connector, dab some DE grease and connect the spade connector to the terminal on the relay labeled 36 or 30, depending on which relay you have. Insert your 10 amp fuse into the fuse holder and close the rubber top.

Figure 3.

Take a piece of 12-14 guage black wire and cut a piece long enough to reach the relay (E Figure 2), and go to the grounding point on the right side of the frame (A Figure 3), above the starter relay. Strip and tin the ends and on one end crimp another spade connector and dab DE grease into it and attach it to the terminal on the relay labeled 85. Remove the bolt holding the ground to the frame, clean the area with a wire brush (I used my dremmel with a wire brush tool--wear safety glasses as these little wires become missiles!) and attach the ground wire and the new black ground wire going to the relay, and tighten well.

Now take the other end of your inline fuse wire with the ring connector and put it over the positive battery terminal and connect the battery terminal bolt and tighten very tight.

Attach your battery ground wire and tighten well.

I wrapped black tape around the relay and all the wiring connections and then taped the relay to the right side frame in front of the battery.

Some side notes: While doing this mod, I noticed that the starter relay positive terminal was rusty (B Figure 3) so I pulled it apart and cleaned the connection well. There is also a two prong connector that plugs into the starter relay and I pulled it off and the terminals were corroded so I cleaned them up as well, used DE grease on them and put them back together. Just things you should check while you are in there doing this stuff.

That's it. Now your power to the coils comes directly from your battery so weak wiring no longer comes into play, and maybe it will have a positive impact on the hot starting issue as well.

Kyle, TX member #707 VROC#19556
2002 VN1500 Classic
Spline Lube Procedure, with photos, R/R Relocation and Coil Mod
Rusty Tank Cleaning!
Electrical Fault Finding Flowchart

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post #2 of 60 (permalink) Old 02-25-2011, 09:52 PM
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Once again... Thanks Bro!
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post #3 of 60 (permalink) Old 05-26-2011, 07:54 PM
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Thanks Sticky, I did this and it worked perfect! It is amazing the difference 1 volt on on the primary side does. the bike starts every time without trouble now.

2003 Vulcan 750
Relocated rectifier
Rewired coils
Iridium plugs
AGM Battery
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post #4 of 60 (permalink) Old 06-26-2011, 05:32 PM
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it works

i did this to my 85 vulcan and fires up every time now only i had to tap into the kill switch under the front of the gas tank in the harness but works like a dream now thanks guys for the tip.
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post #5 of 60 (permalink) Old 07-06-2011, 07:46 PM
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Fergy, nice mod and one that I am positive even I could handle

2003 VN750 de-goated
Kuryakyn ISO Grips
Kuryakyn LED Voltmeter
R/R relocated
Slipstreamer Hellfire windshield
Shindengen Mosfet R/R
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Fire & Steel Footpegs
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post #6 of 60 (permalink) Old 03-30-2012, 09:23 PM
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So I just tested my coils and I'm pulling .12 volts on the rear and .14-15 on the front, how is that possible? I almost 100% sure I'm not reading my meter wrong as I'm an electrician and I use it daily.

This bike so far has made me hate cruisers, and I know I shouldn't. I can't help it though, you have a bad experience with a doctor and you hate all doctors. I traded my car for the bike, and the guy lied about everything. I specifically asked if the stator was good, because the bike didn't like to start. He said yes he tested it, so I assumed the carbs just need to be cleaned, from sitting. Well I cleaned the carbs, 3 times, and tried riding it. Died on me 3 times, so they said the battery was bad, and I replaced it. Same thing, well I tested the stator, and it was shorting out on all 3 windings. So I embarked on the long and hard adventure of replacing the stator. While I was at it I tested the regulator and it seemed way off from spec, so I went ahead and replaced it with a mosfet. This is my only form of transportation and I haven't been able to drive it for a good 3 weeks now. Well I got the stator in, and wired up the regulator(forgot the headlight wire). I also noticed he had broken the coolant drain plug, and drove a screw threw it and grinded it off. So I had to replace that with a metal one, I'm going to put anti-seize on it tomorrow. He also broke the temperature switch that screws into the bottom of the radiator, and just decided to add a switch to turn the fan on and off. Needless to say while I had the bike running it had a problem overheating. So the stator is back in, had to tighten up a few things that were leaking coolant, but it seems all is well. I'd also like to note there's a tear in the front carb air duct, so the rpms surge, and hesitate coming down. I pulled the rear wheel and looked at the splines also, never in my life had I seen so much rust powder. The splines looked decent enough though, so I just threw some waterproof grease on em(no vehicle to go buy moly). Another thing, he apparently took the throttle spring off one of the carbs, and didn't wind it enough when he put it back on, so my throttle plate would get stuck, had to fix that. Lets see what else do I want to rant about. Front tire had a "slight" leak, which turned out to deflate the tire in about a day or 2 of sitting. I found some silvery crap around the pickup coils which looks like paint got in through the oil fill(forgot to mention I did the pickup coil mod before I put it all back together). Carb o-rings were leaking when I put it back together so I put on a layer of permatex #2, I have a feeling this is going to come back and bite me in the ass, but I have no other choice. Anyways, compression tested good on both cylinders, one of the throttle cable ends that was in the control on the handle bar was all bent up and crap so the throttle would stick(fixed that). I also removed the emissions stuff that was on it, and instead of putting coasters, I cut out cardboard cover gaskets to cover the reeds(I don't expect it to last long). Needless to say, this bike has just been a nightmare for me. Sorry about the wall of text, just really frustrated.

*EDIT* My meter isn't broken, my battery reads 12.84 volts, and my coils read .12, and .14-15. Looks like I'm doing another mod.

Last edited by Conradzz; 03-30-2012 at 09:58 PM.
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post #7 of 60 (permalink) Old 04-21-2012, 06:14 AM
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Awesome Mod ....... I honestly think any bike benifits from this mod.... I have done this on my 1978 Kawi KZ1000b, 1995 and 1998 Honda Magna, going to do it on my 2003 Vstar.... It really makes a difference..... especially starting cold. when that starter is working hard dropping battery voltage...... Alot of motorcycles dont have the best electrical connectors and some even run the power for the coils up to the on off switch on the handle bars...and we all know how well sealed that switch is from the elements to protect it from corrosion.... lol....... If your OEM electrical circuit for the coils is not popping fuses or really not giving you any trouble, you can actually retain the OEM power circuit to the coils and just splice in the new power wires from the relay as close to the coils as you can get it to give you that extra full voltage from the battery for maximum spark........ Also really clean your OEM connectors at the coil with Marine or Snap relay electrical cleaner in the spray can..... And the best stuff before you put any single connector together is to put a little GB Ox-Gard on the mating surfaces...... very very little as this stuff is conductive, die electric grease is non conductive but seals it, this stuff conducts and seals like die electric grease.... I wouldnt use OX-Gard in a connector with a bunch of circuits in the same plug, it will short circuit..... Also OX Gard works great for crimp style connectors, you put a dab on the stripped wire, slip into the connector and crimp, top off with some marine grade adhesive filled shrink tubing and you have one hell of good connection as close to solder as you can get........ Also you can use Ox on the grounds, take off your Neg battery cable where it grounds to the bike, clean it up really good, little dap of OX and reinstall...

You can get OX-Gard at any Ace Hardware.

Thanks for taking up the time for the write up and diagram... is actually where I ran across this mod many years ago


P R Customz

TuxedoSeven's Stator Mod

Tuxedo style Stator mount plate kits for sale.
$50.00 shipped! PM me and look at link below.

Last edited by Rags; 04-21-2012 at 06:50 AM.
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post #8 of 60 (permalink) Old 04-21-2012, 11:35 PM
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Hey Rags, thanks for the "heads up" about the GB Ox-Gard antioxidant paste. It sounds like a good product to improve reliability of the electrical connections when used judiciously. I have never heard of anything like this before.

First I learn about di-electric grease on this board, then Ox-Gard a few years later. Who knew!!


1991 VN 750 -"Cosmic Lady" or "Bad Girl"?
Purchased May 16, 2008
Approx.19,300km (12,000 miles)

H-D windshield
Relocated R/R
MF-AGM battery
Fiamm Freeway Blaster horns
F&S luggage rack and engine guard
Kury Offset Hiway pegs
July 13, 2016, Riding on the DARKSIDE now, Classic Radial 165/80-15

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post #9 of 60 (permalink) Old 04-22-2012, 03:02 PM
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I have yet to do this on my 85 but I am due. my bike has stranded me too many times. I hate these Vulcan electrics
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post #10 of 60 (permalink) Old 05-04-2012, 04:03 PM
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I'm having trouble with a rough idle, mostly the back cylinder. By putting on a spare plug and looking, it seems I'm losing consistent spark at idle. I already have some plug wire ends on order and will replace the plug wires. I checked my volts at the coils and get about 10.5, with 12.5 measured at the battery.
I'm hoping this mod will help me. My reasoning is the voltage at higher RPM would be higher, and allowing consistent spark, but at idle it isnt enough.

4/12/12 1st bike ever! 95 VN 750 13k
New plug wires
Coil relay mod
MCCT mod
EPA stuff removed
Rear signals shaved
Shinko 170-110 oversized tourmaster tires
Splines lubed
Front signals relocated.
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