Author Topic: Headlight Ground Circuit Modification ( ST1100 ) *  (Read 9078 times)

Offline KoTAOW

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Headlight Ground Circuit Modification ( ST1100 ) *
« on: April 14, 2009, 06:56:33 PM »
The original headlight ground circuit mod thread:

written by Norm Keller.


Headlight Ground Circuit Upgrade

The headlights are powered through the headlight relays and grounded through the (Green) common ground circuit wire. A voltage drop test of my 1994 ST1100A with 59 amp (rated 40 amp) alternator indicated that this circuit could stand improvement.


Measuring the ground circuit voltage drop as follows:

Headlight ground circuit voltage drop = 0.385 Volts @ alternator voltage 14.3 volts.

A rule of thumb for 12 volt circuits is that voltage drop should be less than 0.2 volts for any switch or wire.

For those who are not familiar with the concept of voltage drop, voltage drop in a wire or switch is undesirable. Voltage drop is an indicator that power is being consumed to overcome resistance, which should not be present. We gain nothing by heating wires or switches rather than using all available power to operate the headlights

In order to remove the voltage drop it is necessary to reduce the resistance in the ground circuit (wires leading to the battery /alternator negative. 14 gauge wires (green) were connected from the headlight ground spade connectors in the headlight plugs. The two new (additional) ground wires were connected to a 10/12 gauge ¼” (6mm) connector which is bolted to ground (the frame) at the fairing brace in the front of the steering head.

Photo: Voltage Drop 1:
In order to connect the voltmeter, a headlight plug was pulled out and a voltmeter + probe pushed into the
space between the ground terminal and the plug body.

Photo: Voltage Drop 2:
The voltmeter was grounded to a PAIR stud because it was handy.

Following the installation, the ground circuit voltage drop was measured at 0.029 volts!

For interest, the H4 bulbs are Sylvania Silver Star 50/65 Watt bulbs at alternator voltage 14.3 volts current flow through each bulb is as follows: Low beam = 4.6 amps. High beam = 5.2 amps.


Steps involved in this modification are as follows:

Photo #1: Pull the headlight plugs from the back of each bulb. This view looks through the space between the fairing and the fork tubes. My hand can be seen reaching to unplug the right hand plug.

Photo #2: Reach up from under the front wheel to remove the plugs. The complete procedure can easily be performed through this access without any other disassembly. It may be desirable to place a soft cover over the front fender to prevent scratches or burns.

Photo #3: Note the green ground wire located in the left side (installed position) of the headlight plug. It will be necessary to remove the wire and terminal from the body of the plug to allow the additional ground wire to be soldered to the ground connector. I prefer to avoid cutting original wires or modifying components so that they cannot be returned to stock. This is just one of my quirks. (VBG)

Photo #4: Fairing support bolt can be used as the attachment point for the new ground wires. Note that I have already mounted a cigarette lighter socket here to power my GPS. The GPS has a power converter in the form of the cigarette lighter plug and it seems simpler to retain the plug assembly as the GPS is sometimes used in other vehicles.

Photo #5: Removing the bolt. Note that I used a hole in the bracket to attach the ground connector so did not have to remove the bolt.

Photo #6: The ground wire and terminal removed from the plug body.

Photo #7: I hope that you can see the small tang, which stands out from the upper side of the terminal. This tang acts as a barb to retain the terminal in the body of the connector. This removal activity is an ideal first time project for learning to deal with terminals because the tang can be seen when looking into the space between the terminal “boxes” in the plug body. Look into the center to the body from the side away from the wires. The terminal needs be pushed farther into the connector body to move the end of the tang free of the body. Use a small tool such as the terminal removal tool in my hand to bend the tang back in line with the connector so that the tang will not bear against the body as the wire and terminal are withdrawn.

When the terminal is out of the body, use a small tool to bend the tang outward again to allow it to act as a barb to retain the terminal when reinstalled. The tang needs only be bend outward sufficiently to catch the ledge in the body after the terminal is pushed into position.

Photo #8: An alligator clip is used to hold the new wire against the top edge of the terminal during soldering. Allowing a solder joint to flex before the solder cools will fracture the solder and produce a weak joint. Refer to the other two wire terminals, which are still in the headlight plug. You will notice that the new wire must be attached across the top of the terminal so that it does not prevent the terminal from being pushed into place. Note that the solder joint is not shown clearly in the photo. I actually solder very well. (VBG)

Sorry about the quality of the photos. I was on my own and they looked OK on the camera viewer.

This is my first try at posting an article with photos. Even more work to resize and such than to write the procedure. Doing it took ½ hour but the article and post ……


« Last Edit: January 05, 2013, 04:47:02 PM by Tom Melnik »

Offline KoTAOW

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Re: Headlight Ground Circuit Modification ( ST1100 )
« Reply #1 on: April 15, 2009, 12:17:51 PM »
The following are questions and comments after this article was posted on

Question by Raouf Wilson

So do the lights appear brighter?

I don't notice a difference but the lights are pretty bright so it would take a big change for me to notice. I'll try to remember to put a 0.4 volt voltage drop in series with a switch so it can be taken in and out. That might allow me to see a difference.

If memory serves the Vd in the "hot" side (the positive side) of the headlight circuit had around 0.1 volt drop but can't find my notes so will have to recheck some time.

I'm always pleased when something I contribute helps someone as that's the reason I try. Others are so willing to help me when needed so it's payback/investment for me.

People often ignore the role of the ground side of a circuit because of the misconception that activity somehow takes place "before" the load. It is most important to recognize that the circuit must be considered as a complete loop of activity.

The funny part of this misconception is that current actually flows from the battery/alternator negative, through the ground circuit to the component, through the wiring to the switch or relay, back to the fuse box and then to the battery/alternator positive.

We troubleshoot or explain a circuit as though current flows from positive to negative (Conventional Theory) but it actually goes the other way (Electron Theory).

Dig out your test voltmeter and check out a circuit.

The sum of the voltage drops in a circuit must equal the source voltage.

This means that any voltage drop in a circuit subtracts from the source voltage which reduces the total current flow. Reducing the current flow reduces the Watts (work being done) in the component.

Clip a jumper to a good ground such as a frame or engine bolt and touch the other clip to the ground connector terminal for the device. Watch to see if the light becomes brighter or horn is louder or motor turns faster, etc. It can take come attention but can work well.

Try it with you headlight. The worst which will happen is that you will blow a fuse but a voltmeter works better. I've used this technique to help students to accept the effects of voltage drops.

A very useful experiment is to connect a voltmeter from the headlight ground terminal to a good ground and switch on. Read the voltage drop. Now clip a good sized jumper wire to the headlight ground terminal while leaving the voltmeter in place. Switch on and monitor the voltage drop and headlight brightness.

Touch the other end of the jumper to a good ground while continuing to monitor voltage drop and brightness. Repeat several times and note results. This should convince you of the value of voltage drop testing.


Additional comment by Norm Keller:

I just noticed that the dash indicator lights don't flicker in intensity when the signal lights are operating as they did before the headlight ground circuit modification.

Next things to Vd are the rear light assembly and the fuel pump. Other loads don't really concern me at this time.


Question by Raouf Wilson

By the way you do the voltage tests with the engine running?

It's not desirable to have the engine running when checking voltage drop. It just makes the process more complicated.

I should have clarified that in the post because it is desirable to have the engine running if checking voltage drop in circuits such as the alternator output. Thanks for pointing that out.

If the engine is running, the higher applied voltage will result in a bit more current flow in the headlight ground circuit which will be reflected in higher voltage drop readings. The voltage drop readings either way will still show whether attention is required.


Additional comment by Norm Keller

The Datel is measuring the voltage between the positive to which it is connected and ditto the ground. If the voltmeter is connected directly to the power and ground with no other devices on the same circuit (wires), then the voltmeter (Datel) will "see" the voltage at the points to which the wires are connected. That is, of course obvious.

Keep in mind that a voltmeter has no current flow through it, at least in theory. The better quality is the voltmeter, the smaller the current flow as well as how precision and repeatable are the readings.

If you look at the literature accompanying your test meter and sometimes accessory meters such as the Datel, you will notice there is a specification listed for the voltmeter. The specification will list something like 20,000 Ohms per volt or 2 Mega-ohms per volt. A simple Ohm's Law calculation will show that the amount of current flowing through a 20,000 Ohm per volt voltmeter is pretty tiny! A 2 meg is almost nothing at all.

The issue of current flow through the meter is that circuits with tiny current flows can be affected by the current flow through the meter. Some of the really old voltmeters were analog and had wire wound magnetic coils which created magnetism to move the meter needle to the corresponding reading. These meters were really a tiny ammeter which measured the current flow through the voltmeter but were calibrated to display the voltage required to cause the amount of current flow through the meter to be displayed.

Hook one of those old meters to a modern electronic curcuit and the meter may draw enough current (amps) to change the way that the circuit works or even to short it out and fry it. In the old days a high impedance voltmeter was extremely expensive so relegated to laboratory and the like.

To save possibly confusion as to whether I'm referring to a test voltmeter such as used in the shop or an accessory voltmeter such as a Datel which is mounted to the bike, I'll refer to them as a test voltmeter or Datel respectively.

Where things change is if there is enough current flow in the wiring to which your Datel is connected to create a voltage drop in the wiring. This is to which the Datel reacts when you connect and disconnect that extra ground wire.

When there is a current flow in the wires feeding the Datel, the current flow results in a voltage drop in the wires "before" the Datel "sees" the voltage. What the Datel is "seeing" is the source voltage less the voltage drop (loss of voltage) in the ground circuit, and less the voltage drop (loss of voltage) in the "hot side" of the circuit.

If there is 12.8 volts at the battery and the Datel is connected directly to the battery the Datel will "see" 12.8 volts. If you connect the Datel across the headlight's high beam, the Datel will "see" the voltage across the bulb but not the voltage at the battery.

Why not the voltage at the battery? The current flow in the wiring to and from the headlight has resistance even thought the resistance is (hopefully) very low, there is still resistance to the flow of current. The resistance to the flow of current requires work to be done to force the electrons through the resistance. Voltage is lost in overcoming this resistance.

Let's say that the Datel "sees" 12.8 volts when connected across the battery but "sees" 12.2 volts when connected across the headlight bulb because that was a handy place to connect the Datel. OK, you hook your new Datel to the bike to monitor voltage because someone told you that 28 amp ST1100 alternators are very weak and you need to monitor the voltage to make sure that the battery is being charged and so on.

Hook up the Datel and fire up the bike. "Gasp" only 12.8 volts! Disaster, destruction, loss of civilization!" Except that you notice that the bike starts well, the lights are bright enough and so on. How can this be?

"Oh well, the voltage is low and that can't be good. Better cough up $1,000.00 and get the 40 amp alternator installed."

$1k later you pick up the bike, fire it up and guess what? The Datel still shows 12.2 volts! Bet you're steamed at the shop because they must have screwed up, right?

I've seen this many times but the problem is simply the location from which the Datel is "seeing" the voltage.

Let's say that the bike is similar to mine. I checked the voltage between the ground terminal of the headlight bulbs and ground which showed about 0.4 volts. OK, what does that mean?

If the Datel is connected across the headlight, it will see 12.8 volts minus the 0.4 volts which is required to make the headlight current flow through the ground wiring. The voltage "seen" by the Datel will be 12.8 volts minus 0.4 volts which equals 12.4 volts. From this we can calculate that the voltage drop (loss) is also 0.4 volts in the wiring from power to the headlight. Voltage at the source minus voltage drop in one part of the circuit equals the voltage drop in the remainder of the circuit. 12.8 volts minus 12.4 volts equals 0.4 volts.

You "removed" the voltage drop in the ground part of the Datel's circuit by adding the extra ground wire to the headlight.

I don't know how much experience you have in electrical testing, Raouf, but you seem to be well on the way now. It simply requires some experience, combined with a few break-throughs in understanding to move one up to a point where they can do some meaningful work with meters.

You have also proved that adding the headlight ground circuit wire will improve the operation of your bike.

It might be useful to compare the Datel voltage to the battery voltage when the new ground has been added. If they are not identical, you have a test voltmeter and knowledge required to find the voltage drop which is lowering the Datel reading. The choice would be to remove the voltage drop or move the source point for the Datel, depending.

Regardless, it would be well to consider the ignition switch by-pass relay modification because it improves the "hot" side of the ST wiring harness in the same way.


Question by Raouf Wilson:

Do you think the 40amp alternators need the ignition switch by-pass relay modification?

I absolutely recommend the ignition relay for the 40 amp bike as higher current flow through the wiring harness makes the Vd even worse and consequently the 40 amp will likely benefit even more.

Keep in mind that my "40 amp" Nippon Denso will put out 59.6 amps which is more than double the smaller alternator. If the maximum output were taken from the fuse box with original wiring the Vd would be likely in the order of 2.5 or 3 volts!


Question by Raouf Wilson:

So where do you measure the current drop for this?

I wanted to know where to put the voltmeter leads so that I can see how much the voltage drop was (shouldn't have used the word current). I was using the same logic as for the headlight, so didn't know what to measure across.

The voltmeter leads are placed onto the left hand headlight terminal (that is the bike's left) and to a good ground such as an engine bolt. If you consult the first two photos (Titled "Photo 1" & "Photo 2" in my post which shows the ground circuit modification, you will see the two points to which my voltmeter was connected.

I did forget to mention that the headlight plug needs to be put back onto the bulb with the voltmeter connected in order to monitor the voltage drop. Of course the headlight needs to be powered on also because we are needing to see the voltage required to provide the current flow through the resistance of the ground circuit.

Voltage drop is another means of measuring resistance but is more valid than an ohmmeter reading because voltage drop gives a live indication of the performance of the circuit in question.

There is no need to remove plastic to do the ground modification because the headlight plug can be reached as illustrated above the front fender. There is ample slack in the wires to allow the plug to be lowered enough to work in view.
« Last Edit: April 15, 2009, 01:02:09 PM by KoTAOW »