Topic: CB400f voltage loss between battery and pretty much everything?

[Tenn_CB400f]

I searched, I swear. Lots of close issues but not quite this one.

My wife's CB400f is stock, and we've had it since around 1985. It always had kinda dim lights and slow flashers, but was definitely charging the battery.

Currently, with a brand new battery, I get 13.5-ish volts at the battery, revved above idle with the lights on (can't be turned off). At the headlight I am seeing about 9.9V, same with the rest of the components light the coils, etc. I think the low volts may be making a weak spark, but the bike starts energetically with the electric starter.

Any ideas where to start looking for the voltage loss? I can add a relay to get battery power directly to the headlight, but I am not sure that would solve the rest. I have a good multimeter for testing stuff.

Thanks in advance!

[bryanj]

Most common loss is at the fuse connections, then inside the ignition switch. A relay will cure the switch but not the fuses.
It is not uncommon for a bade fuse connection to get hot enough to melt the plastic fuse box, Hondaman does a blade fuse conversion that may help.

If you have a voltmeter you can check for voltage along the various points but turn kill switch off
E.G.
Neg probe on battery
Pos probe goes
Battery
Fuse in
Fuse out
Ignition switch in
Ignition switch out(switch on)
Black multi connection in shell
Etc
Etc

[MauiK3]

I've had really bad corrosion up inside the battery cables where you can't see.

[Deltarider]

I'd first check the negative route. See to it there's no corrosion. The indicators often have a bad ground.
Are you familar with the V-4 measurement?
Here it is.
It's just 4 steps with the same meter. When you look at the pics long enough, you'll get it. Each picture shows two measurements. So the measurement on the left is the first, the one on the right the second and in the second picture the measurement on the left is the third, the one on the right the fourth step. 
Let's say there's a problem, something is not working right: a somewhat dim lamp.
Step 1: you measure the potential over the battery terminals. Here it is 12,0 V. This is now your reference.
Write it down: V1 is 12,0 V. Then go to the right, which is step 2. You measure the potential over both sides of the part that does not perform 100%. In this example you measure a potential of 11,0 V. Write that down: V2 is: 11,0 V.
Hmm, one volt does not make it from Batt POS to Batt NEG, so to speak. We want all Volts to pass. Where can I look for that missing volt, in other words where is the resistance? For this we go to:
Step 3. Check if there's any potential over Battery POS and POS side of the lamp. Ideal outcome would be 0 (zero), indicating there's no resistance and all volts arrive. In our example 0,1 V is missing. This is acceptable. BTW, you can interpret that little square as things in between like a fuse and/or a switch.
Write down: V3 is: 0,1 V.
Now check the NEG side of the route: the potential over Batt Neg and the NEG side of the lamp. Also here the ideal outcome would be 0 (zero), indicating there's no resistance and all volts arrive.In this example we measure 0,9 V however. Write that also down: V4 is: 0,9 V.
Before we begin interpreting our results, let's first check we did the V4 measuring right.
You only have to remember one formula: V1= V2+V3+V4. Always.
So in our example: 12,0 = 11,0 + 0,1 + 0,9. Conclusion: we did it right. As already said, the 0,1 V missing in the POS route is not much of a problem. Possibly there is some minor resistance by a fuse and/or a switch. Not much to worry about. The 0,9 V in the NEG route is a reason for concern however. But at least we now know, where to look in order to locate it: the negative path. In our example, it's probably a rusty, dirty or loose connection. Could be the thick Batt NEG cable, connected to a recently powdercoated frame.
Ofcourse this is a simplistic representation. Depending on 'obstacles' like switches and/or fuses, we may need more steps to perform, but V1 must always be the sum of V2 and V--n. Also realise, I've limited myself here to describing a situation, where a component is not working a 100%. There's also the possibility, something is not working at all. I'd then start by isolating that part first and connect it directly to a known good battery to verify that part is OK.

[Tenn_CB400f]

Thanks for all the tips! I got some testing to do.

Things already checked off: fusebox is not melted, and no corrosion on any of the fuses or battery cables.

[bryanj]

Visiility means nothing, check voltages

[willbird]

Visiility means nothing, check voltages

Also check with a load applied too. Recently my Oil Pressure sender tested fine with an Ohm meter but would not carry enough current to light the OIL indicator :-). The Ohm meter is only applying a tiny load to the circuit.

Bill

[Bodi]

If the starter runs at normal (fast) speed the ground is ok. That affects the charging too, you say it's ok.
So the issue is with power. I would look at the key switch first. If it's been dropping those volts, it is probably visibly overheated. The connectors are another possible, chech the key switch one and any others on a black wire for overheating - the bullet sleeves going brown or black is a bad sign.
You can pull out the HEAD fuse to shut it off.

[Deltarider]

All electricity consumers have their own (green) ground connection. It's not uncommon to see corrosion at the blinkers ground connection. In the wiring diagram I've seen the blinkers NEG and the headlamp NEG share the same connector in the headlamp bucket. Make sure that connection is sound.
But the IGN key switch is also to be checked. The V-4 measurement is the appropiate disciplin.
Set your meter to 2V and see if you can detect any V-drop:
1) Between Batt + and where it connects to the fuse box. Usually there is no V-drop here, but... disciplin.
2) Over the fuse itself. This can be the case. Clean contacts and check again.
3) Between fuse and IGN key switch.
4) Between IGN key switch and black wiring in the headlamp bucket.
Checking is facting.
Realise that when IGN is ON and kill switch is in RUN, you cannot expect to see the full potential at the coils. The electricity is a dynamic system, so with live consumers and lack of charging (no running engine) you will not see more than 11V at the consumers.

[Ozzybud]

My 1975 CB400F has the exact voltages as yours when Static.
When I unplug the coils everything jumps up to near battery  voltage. My assumption was  the coils draw a lot of current(stock everything).
It runs great and charges fine.
I did have an issue right after the restoration this spring. As the battery was not fully charged and the short rides around the neighborhood was not enough to get a full charge. Had dim lights, slow blinkers,  and would bog out over 6000rpm. . After getting battery to full charge all symptoms cleared up.
There is even a blurb in the little owners manual about needing to ride above 2000rpm for a bit to obtain a full charge.
So at this point it's not a problem. Time to riide!!

[Deltarider]

Ozzybud, I think you're to the point. Many expect 12V static, as they're used to cars with all kinds of electric accessories, like heaters, fridges, seats, windows etc. It's a simple bike without all the nonsense. Ride it!

[Tenn_CB400f]

I printed out suggested tests and will have some time in the shop.

I wanted to note: All the symptoms in my first post are with the bike running, with a hot, brand new battery.

[Ozzybud]

Remember brand new batteries do not always come fully charged. Likely they are at 75%. They will test @12.3 to 12.6 volts but not have the amperage hours of fully charged.

[scottly]

Currently, with a brand new battery, I get 13.5-ish volts at the battery, revved above idle with the lights on (can't be turned off). At the headlight I am seeing about 9.9V,

Any ideas where to start looking for the voltage loss?

A 3.6-ish voltage drop is a lot! To measure the voltage drop directly, connect the + meter lead to the battery + terminal, and then check the voltage at the red wire on the ignition switch with the key on. If your meter reads like, 3 volts, then the problem is between the red wire and the battery, including the fuse box, which is the most likely suspect.
If the red wire at the switch tests good, move to the black wire on the switch. If there is no additional voltage drop, the switch is not at fault.

[Tenn_CB400f]

Remember brand new batteries do not always come fully charged. Likely they are at 75%. They will test @12.3 to 12.6 volts but not have the amperage hours of fully charged.

Thanks. I always measure and top off new batteries.

[bryanj]

3.6 volts is near enough a third of available power, no wonder the lights are dim!

[BenelliSEI]

Check the main batt. cables carefully. Give them a good tug. A few years ago I had a cb550 in the shop with symptoms similar to yours (plus the starter turned slowly like a bad battery). The main cable from the battery tothe solenoid looked perfect, but about an inch from the battery it was just dust inside…… The clue came when I pulled on it and that section stretched!

[scottly]

Check the main batt. cables carefully. Give them a good tug. A few years ago I had a cb550 in the shop with symptoms similar to yours (plus the starter turned slowly like a bad battery).

He said "the bike starts energetically with the electric starter" in the first post, so that rules out your WAG (Wild Assed Guess).

[BenelliSEI]

Check the main batt. cables carefully. Give them a good tug. A few years ago I had a cb550 in the shop with symptoms similar to yours (plus the starter turned slowly like a bad battery).

He said "the bike starts energetically with the electric starter" in the first post, so that rules out your WAG (Wild Assed Guess).

Good point…..

[MauiK3]

I know it's been checked but I'm back to a bad battery, even though it's new.
I'd try some other battery to rule it out. I don't think I saw where that was tried.

[Ozzybud]

I decided to do some investigating as my CB400F has the same symptoms as Tenn_CB400f . When static and running just about everything has less than 10 volts. first thing i checked was the fuse holder. with the key on both the "main" and head fuse holders got very hot(burn your finger hot).I took the fuse holder out and all the soldered connections had resistance thru them and were loose.I tried to re-solder and had no luck. I could not get it hot enough without melting the plastic holder. I ordered a new Honda fuse holder and it should be here Friday.
I will report back if that cures the low voltage issue.

[Tenn_CB400f]

Thanks again everybody for suggestions.

I first tried the V-4 measurement on the headlight, and I think I did it right and got the below results, which seem bad. I also tested the voltage at the various ignition switch connections and recorded them. I took the switch apart and inspected and cleaned everything and reassembled and got about the same, so I doubt it's the switch. It was in excellent shape inside.

(off ignition should be 12.9V)



I checked the wires and connections inside the headlight bucket for the running light/turn signals in the front and was chasing grounds, and the signal wires don't match the diagram (Clymer manual) in that both signals have two black wires with little tracer rings at the terminal.

There are quite a few green-wire female connectors in the headlight bucket with two empty connections, so I just need to trace a few. No visible signs of hackery (which would have been from me!).

One comment earlier suggested setting my meter on 2v to test for leakage:

"Set your meter to 2V and see if you can detect any V-drop:
1) Between Batt + and where it connects to the fuse box. Usually there is no V-drop here, but... disciplin.
2) Over the fuse itself. This can be the case. Clean contacts and check again.
3) Between fuse and IGN key switch.
4) Between IGN key switch and black wiring in the headlamp bucket."


Is there a way to set my meter to 2V?

Like I said, thank you everyone.

[Ozzybud]

The black wires for the signals with colored rings is original and correct. It's common to have extra ground(green )female connectors inside the headlight bucket.

[Ozzybud]

I was able to disassemble the fuse holder and solder everything back to normal.

It did not change much in the way of voltage to the headlight but the fuse holder is not getting hot.

Before  Bat 12.6v headlight 9.9v
After    Bat 12.6v headlight 10.3

High beam is 11.0v

Ozzybud

[scottly]

 connect the + meter lead to the battery + terminal, and then check the voltage with the other meter lead at the red wire on the ignition switch with the key on.

Do this test and report back.