Electrical question - resistance across loom connections

[Craig380]

I recently got a new multimeter, so I've been having fun measuring stuff on the bike

When checking the main power feed from the battery into the loom, I'm seeing 0.2 ohms total resistance from the battery positive wire to the loom side of the fuse (that is, a total of 0.2 ohms across the fuse, fuse connectors AND two bullet connectors).

Is this small resistance fairly normal across a series of connectors?

Thanks for any input

[jabcb]

What resistance do you get when you touch the two multimeter leads together? You would be measuring the resistance of nothing, which should be zero. But you probably get a reading of something like 0.2 ohms. That's what I get with mine & its just due to the multimeter's accuracy limits.

Try measuring the voltage drop for the section of wiring when the lights are on & engine is switched off via the kill switch. Could then do the math to determine the resistance if you want.

Also, to avoid damage to your multimeter its a good idea to disconnect the item you are measuring from the battery before measuring resistances.

[Craig380]

thanks -- I get zero to 0.1 ohms when touching the multimeter probes together, so I guess 0.2 ohms across the loom connectors is OK (battery was disconnected when testing)

I'll try the voltage drop test as you suggest.

cheers, Craig

[Zunspec4]

Hi craig,

trying to remember my Ohms Law for this measurement. I think it is I x R = V

Assuming that you are pulling 5 A the equation would be;

5x0.2 = 1V that 1 less volt going to parts that need them. What got me pondering was thinking 0.2 Ohm over such a short line was a fairly high resistance. There must be folk on the forum who have been to college more recently than 35 years ago to put me right .

I would check and clean all connections and change the cylinder type fuse/holder to an automotive "Blade" style fuse.

Cheers Zunspec

[Craig380]

Thanks Zunspec, it probably is a good idea to replace the OE old-type fuse with a modern type. More measuring needed

[rbond]

Depending on the length of wiring between test points, it could simply be normal resistance in the wire adding to you reading, or a little corrosion at any of the bullet connections.

[BRP Tourer]

I do a lot of auto electrical work in my job and find that 0.2 ohms resistance across most circuits is fine. A better test for resistance is, as previously posted, a voltage drop test, especially on older systems. Let me explain...
When using an ohm meter to check continuity in a circuit you are passing a very small current through the circuit to see if it can get through. If you have a partially broken wire or a marginal connection it will still show as good (low resistance) but may not be able to pass enough current to do the job. Even a single strand making contact in a broken wire will show as a complete circuit. When you power the circuit and do a voltage drop test you are actually reading how much power you are losing through the circuit. Also keep in mind, as a circuit or electrical component heats up, resistance will rise.
As an example... I have found that on most older bikes the wiring to the headlight uses wires that are much too small for the current they are passing. I have had feeble dim headlights and measured current with the bike running showing 14+ volts at the battery, and barely 10 at the light bulb itself. All connections were clean and tight, resistance readings were good, very little ohms. Adding a heavier power wire from the battery to the headlight bucket and adding a relay made a world of difference.
I hope my long winded explanation is clear enough. Good luck and have fun!

[H2RICK]

Thanks for the very clear explanation, BRP. You are "clearing the fog away" for those who have an less than perfect understanding of electrical problems and solutions.
My GT550A is basically brand new and I don't have the problems you mention with my headlight, even though I've been running a 55W/45W
H4 setup for years.
The relay setup, though is something I've been considering for a couple of years now, if for no other reason than to take that high wattage load off of those fairly flimsy contacts in the dimmer switch and the on/off switch.
I need to get motivated and get that conversion installed this spring....
and pull that L/H control assembly apart, clean everything and pack it all with some dielectric grease. That should makes everything last quite nicely until the NEXT century.

[Craig380]

I do a lot of auto electrical work in my job and find that 0.2 ohms resistance across most circuits is fine. A better test for resistance is, as previously posted, a voltage drop test, especially on older systems. Let me explain...

Thanks BRP -- good stuff.

So with battery connected and ignition on, I set the meter to the 20V range and then check the reading in volts across the same section of wire?


cheers, Craig

[jabcb]

So with battery connected and ignition on, I set the meter to the 20V range and then check the reading in volts across the same section of wire?

Yes.

The fuse has a small resistance, around 0.05 ohms. Its needs to have a resistance so that it gets hot & burns out when there's too much current.

Resistance in the wires depends on the length and thickness of the wire. The longer and thinner the wire, the more resistance it has.

As BRP Tourer wrote, vintage bikes tend to have thin wire and a fairly big voltage drop at the headlight.

[tz375]

Thanks for the explanation BRP. I hadn't thought about it quite that way before.

Interestingly I have a late model GSXR loom and the wires are generally the same or thinner than the equivalent wiring on a GT. Headlamp wires appear to be similar.

R1 Yamaha by comparison has tiny skinny little wires on almost all the circuits and uses relays for main and dipped beams.

Wires for idiot lights and tail lights etc which carry very little power are all very small on both modern looms where a GT tends to use the same thick cables for everything. That's where I came unstuck trying to wire in additional lights using GT type thick wiring, until I woke up and realized I could use thinner wire.

Then I added in a decent fuse panel to split the loads, and a pair of R1 relays for the headlamps and I got a couple of relays from Eastern Beaver for the ignition circuit and for the next project. My switches now carry very low loads.

Modern bikes do have many more circuits for 5v sensors which carry very low loads and can afford to be much lighter, and those add to the general bulk, but even coil (stick) wires are tiny. Intersting.

[BRP Tourer]

I don't know if the manufacturers are trying to save weight or money buy using smaller wires in some circuits. The bike I referred to in my previous post about losing voltage to the headlight was on a 92 FJ12. I had an 05 Triumph Tiger that seemed a little weak and found that the wires to the rectifier were too small to carry all the current the alternator would put out. So it is not just the old ones. I also see this sort of thing in the automotive industry too, generally not as much in the newer cars though.

[BRP Tourer]

yes Craig380, you should not see more than 1v drop across a circuit, less is better. Make sure the circuit is turned on when testing also.

[H2RICK]

I don't know if the manufacturers are trying to save weight or money buy using smaller wires in some circuits.

Both, BRP.....and this has been going on for as long as there are accountants wanting to maximize corporate profits along with engineers wanting to improve fuel economy and performance.

When they design/engineer ANY harness, they ASSUME that
AA) there will NEVER be any corrosion on the connector interfaces, and
BB) there will never be any broken strands in the wire itself, either inside the insulation or at the connector crimp, and
CC) there will be no improperly plugged in connectors.

Of course those conditions only prevail in a perfect world.....and none of us live there. The designs and calculations DO look good on paper, though, and the shareholders and the EPA are happy.

[jabcb]

They also saved money by using a 35/25w headlight and an alternator that's barely adequate. The T250/T350 alternator is so meager that they could save even more by not using a voltage regulator.

But I don't think we should complain. These bikes were designed for a certain cost. Seems to me they did a good job making most of the bike pretty rugged.