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Ignition Relay Bypass Modification ( ST1100 ) *

Started by KoTAOW, April 14, 2009, 06:46:31 PM

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Original thread can be viewed here:

written by Norm Keller.

Ignition Relay Bypass Modification

The intention of the ignition relay is to remove the voltage drop as current flows first thru' the ignition switch. The small size of wire to and from the switch is a contributing factor here. Before the relay installation, my voltmeter (Kuryakan LED) showed about 11V during idling. After the installation it read 12V, and did not dip in voltage when the turn signals were in operation. From a power point of view, this small increase in available voltage should mean a bit less work for the alternator. With a higher voltage, less current is drawn to satisfy the power rating of the headlight, etc.

This modification has the same advantage for the later 40 amp models as for the 28 amp ones.

Instructions for Modification:

I used a 70-amp Cole Hersee relay because it was the same price and size as a 50-amp. I used a 50-amp fuse in the mistaken belief that the alternator output is 40-amps but measured the actual output as 59-amps.

1 ) Locate an area to mount the 70-amp relay and 50 amp fuse. I used the area where the VRR was located before I swapped the 28-amp for 40-amp.

2 ) Connect a 10 or 12 AWG wire from the battery side of the starter relay to the 50-amp fuse.

3 ) Connect a 10 or 12 AWG wire from the 50 amp fuse to one switched terminal of the 70-amp relay. (#30 or #87)

4 ) Connect a 16 AWG wire from one coil terminal of the relay to ground. (#85 or #86)

5 ) Unbolt the fuse box and pull it out far enough to gain access to the rear, wiring side.  If it is an ABS/TCS model the wires are the same but the fuse box is bigger.

6 ) Cut the two wires leading from the (ignition switched) wiring harness to the fuse box.

In the above photo the second wire down (between the two red ones we are using) has been added by Norm Willey to power accessories. Ditto the red wire in fourth place which has a heat shrink sleeve.

             *Note: If memory serves, these two are purple but it is easy to confirm. These two are the only wires which are
               switched on and off by the ignition switch. To confirm, pull the fuses to inactivate output circuits and use a
               voltmeter or test light to find the two wires which are switched on and off by the ignition switch. Note that
               there is at least one wire which is on at all times and this in not one of the wires you want. These two wires
               connect into buss bars (common connectors in the fuse box) which feed a number of fuses from each wire.

               The wires are best cut two to four inches from the fuse box to give enough wire to work with.

Wires cut from the fuse box. The right hand wire is heat-shrinked to insulate because the wire isn't required. The left hand wire is connected to the 16 gauge wire leading to the relay coil.

7 ) Connect one of the cut wires leading from the harness end to the other coil terminal of the relay. (#85 or #86)

8 ) Check the relay operation by switching the ignition switch on and off. The relay should "click" and the not connected relay's switched terminal should turn on and off. This can be confirmed by a voltmeter or test light operation.

9 )
Join the two wire leads from the fuse box end together to a 10 or 12 AWG wire and connect the other end of the wire to the unconnected switched terminal of the relay. (#30 or #87)

10 ) Re-install fuses and you're done.

11 ) Options could be to connect a voltmeter lead to the same switched relay terminal as the fuse box. The same connection can be used to supply additional fuses for other accessories.


Hope this is complete enough. If you wish additional info simply ask what needs to be filled in further.

This is a very simple procedure so no one should be intimidated.


Norm Keller


Julien D', STOC #7236 has contributed an excellent schematic for this modification.  

It is based on the original wiring diagram for a "standard, after 1995".

THANK YOU Julien !!!

Wiring Diagram ( PDF-950kb )