Headlight Voltage Drops

dps650rider

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Even though I seldom ride after dark (too many deer in my area) I was looking into increasing the brightness of my headlight on my 78 SE. Currently I have a headlight using a H4 halogen with a 60w/55w Sylvania Siverstar Ultra bulb. I thought I'd start with checking the voltage at the bulb because I have long suspected it was a bit low. When I checked I was surprised at what I found so I made a quick simplified schematic and measured the voltage at various points. Note for all of the measurements the high beam was on, the engine off and the negative probe of my meter was connected directly to the battery. I was hoping to find a problem at some point in the circuit but instead I found there were significant drops everywhere.

Anybody else ever check this?
 

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I would suspect a weak battery
I would start up getting charging 14 V ish and look at the lights if it gets Better.
Taking new measurements

Very fine Schematic ....!!!

You have 10.7 on the ignition fuse also.. Can lead to starting problems.
Lights on and trying the starter I give it small chance of starting and should the battery be old
It will drain on the starter and then not start with the Kick starter either
The long walk home..
 
Could be a battery playing games...

The surprising one is the .6V drop to the battery side of the main fuse (should be 0). Then it's expected to have another .5V drop through the key switch.....some drop at the OEM fusebox, etc, etc.

Have you cleaned and tightened all connections (including fuse clips)?
 
Battery is fine, 12.4 v with a 5 a load that ran for 5 minutes is ok and with the engine off. Starts just fine. But look at the voltage drops!
If the battery is OK
You have correct power in but the circuit -- the system has excess demands
Then you are getting into the fault finding on a Quantitative fault.

An art in itself especially if not have the bike beside you.

I would still check charging Voltage as a First it is simple to Do.
I would start with removing fuses see if the drop disappears Trying to figure out which circuit is causing the above

Dont forget the switches .. esp at the handlebar
Rotor
Wiring passage into the headlamp bucket
 
I fixed that on another bike, by installing a couple of relays. One for low and one for high beam. The original supply to the bulb was used for controlling the relays. The relays are supplied directly from the battery, via a 10 A fuse, using 1.5 mm2 wire. This bypasses a lot of potential voltage drop issues.
 
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Voltage drops under current are never 0, there is always some resistance. And yes I have started cleaning connectors and so far to no avail.
Theoretically perhaps...in EE land...here in shadetree land it should be practically zero given 14ga copper equivalent, carrying 60watts, perhaps a foot run from solenoid to main fuse, should be a voltage drop a few tenths of a percent (~0.02VDC total @ 60W) - less than most of us (and our meters) can measure.
 
I fixed that on another bike, by installing a couple of relays. One for low and one for high beam. The original supply to the bulk was used for controlling the relays. The relays are supplied directly from the battery, via a 10 A fuse, using 1.5 mm2 wire. This bypasses a lot of potential voltage drop issues.
I was thinking about doing something exactly like that, I ordered some relays yesterday. But I am very curious to know if this is a common problem and how much of a problem it is if everything is perfect.

Think about this, if the headlight is 60 watts at 12.4 volts the current would be 4.8 amps. That would make the resistance of the bulb 2.6 ohms. Now with only 7.7 volts on the headlight the current would be 3 amps and the power would be 23 watts assuming the resistance of the filament doesn't change with temperature (it does).
 
Even though I seldom ride after dark (too many deer in my area) I was looking into increasing the brightness of my headlight on my 78 SE. Currently I have a headlight using a H4 halogen with a 60w/55w Sylvania Siverstar Ultra bulb. I thought I'd start with checking the voltage at the bulb because I have long suspected it was a bit low. When I checked I was surprised at what I found so I made a quick simplified schematic and measured the voltage at various points. Note for all of the measurements the high beam was on, the engine off and the negative probe of my meter was connected directly to the battery. I was hoping to find a problem at some point in the circuit but instead I found there were significant drops everywhere.

Anybody else ever check this?

Voltage drops under current are never 0, there is always some resistance. And yes I have started cleaning connectors and so far to no avail.
Very nice diagram. Checking under load is the best way to find volt drop. You are correct there is nothing wrong with your battery. Just too much volt drop/resistance through the system.
Keep in mind volt drop/resistance doesn't doesn't just happen at the mating ends of connectors it also happens at the crimps of the connector to the wires.
My suggestion is to clean the switch gears and replace the connectors on the ends of the wires. But check when you strip the wire and make sure the copper wire are a nice bright copper color if not it would be best to replace the harness. Green copper wires will crimp ok but will have a lot of resistance.
 
Very nice diagram. Checking under load is the best way to find volt drop. You are correct there is nothing wrong with your battery. Just too much volt drop/resistance through the system.
Keep in mind volt drop/resistance doesn't doesn't just happen at the mating ends of connectors it also happens at the crimps of the connector to the wires.
My suggestion is to clean the switch gears and replace the connectors on the ends of the wires. But check when you strip the wire and make sure the copper wire are a nice bright copper color if not it would be best to replace the harness. Green copper wires will crimp ok but will have a lot of resistance.
Yep very familiar with that in wire, worked as a bench tech in electronics repair for many years before moving to engineering. Suspect the crimps are a significant part of the problem. Was wondering, does anyone know what gauge the wire is in the harness? Resistance for a gauge size is readily available on the web.
 
Yes, and you have very excessive drops at shorter runs within that loop - so it indicates multiple problem points.
 
So for 18 gauge and an estimated loop length of 14 feet according to the chart I found the resistance would be about 0.091 ohms for the wire alone. At 5 amps that would be a voltage drop of 0.46 volts. So the main problem is in the connectors, switches, etc.

https://www.engineeringtoolbox.com/copper-wire-d_1429.html

Correct in 95 % -- 98 % of the cases on these bikes
But there is the possibilty that a wire has a crack or wear heat affected.. and so on --- is Partially grounded at say for ex the rear light. Not having the area or insulation to spec.
Almost impossible to find without Voltmeter installed and so.

Such a partial ground can pull down a fresh battery.

But yes connector and switches is a first.
I Would not replace if not obviously needed .. .Grinding with sandpapper and applying contact grease as a first.
 
Not to complicate matters - complete info - the wire from solenoid to main fuse should be 2mm (14AWG) and continues to key switch (red), then brown out of that switch back to fuse box is 1.5mm (16AWGish), then on 1mm
 
The stock headlight is 40 watt / 50 watt
Your H4 halogen bulb uses significantly higher watts.
I like using LED H4 bulbs, good ones that really light up the road (with good low beam cut off) typically draw 20-30% less than the stock bulb.
 
I was thinking about doing something exactly like that, I ordered some relays yesterday. But I am very curious to know if this is a common problem and how much of a problem it is if everything is perfect.
Well, I guess a brand new XS had acceptable, if not perfect voltage at the headlight. But 40-55 years later, connectors, wires and switches will all have deteriorated to some degree.

On my 77, which sat in (dry, indoors) storage for over 5 years, I found no spark whatsoever when taking it out of storage. Basically same story as your headlight, a rather convoluted path between battery and coils. So I did a quick cleanup of connectors and switches, and installed a relay and a separate power wire and fuse. A quick, easy and cheap fix IMHO.

Actually, it would be interesting to see actual voltage at the headlight bulb connector at a 60W load with a relay setup, compared to a stock harness in good condition.

A total re-wire with a M-unit would be just as good, or better as relays for solving voltage drop issues, but way more expensive and more involved.

Also, whenever I replace wires on motorcycles, I try to use tinned marine grade wire, instead of the cheaper automotive plain copper.

And a last thought: The ground connection for the headlight is also part of the circuit, so that should also be checked or upgraded.
 
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And keep in mind, a 10% voltage drop will result in 19% less power (W).
This because P=U x I, and I=U/R (R in this case being the resistance of the bulb itself)
Combine those two, and you get:
P= U x U/R
So it is critical to keep voltage drop at an absolute minimum, in order to get a best possible light output from a H4 headlight.
 
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