voltage to coil measurements--too low?

[Forden]

I've been battling rich conditions so I thought I'd check if weak spark was in the picture--coil resistances tested ok on both, but after reading another thread here I checked the voltage to the coils, here's what I got:

12.3V at battery (resting since yesterday)
10.2 at positive lead to coils at rest (orange/white wire connection to neg. battery terminal)
8.3 at positive lead to coils while cranking

Is this drop in voltage enough to be a potential cause for weak spark and rich plug readings? If so, any suggestions beyond weak connection somewhere else in the circuit? The connection joining the coils to the positive wire is new, and I'm getting the same reading at the coils themselves.

Many thanks! Learning every day but electrical was never my strong suit...

 

[Redman]

Cause of week spark, yes. (And let me quess, the engine doesn?t start until let off the starter button).

cuase of ?rich symptoms?, I don?t know, I don?t have that sort of experience to say one way or other.

as a test you could run a temporary jumper wire of main power to the coils. (Which is somewhat what the ?coil relay mod? would do for you, but only when key is on).

 

[Brendan W]

I had almost the same voltages at the coil.
I cleaned the fusebox, the ignition switch, the kill switch and the sneaky one was the coil supply plugs.
That got me back to about half a volt below battery and it fired up first push of the button.
90% of all carburettor problems are electrical

 

[Guest]

Brendan hit on a major point, there. There are MANY connections between the battery and the coils, you need to check/clean them ALL.

Also remember that anywhere you see a "connector", there are actually THREE interface points, each of which can be corroded and cause problems:
1 where the wire meets the brass terminal
2 where the brass terminal meets another brass terminal (the other side of the "connector")
3 where the brass terminal meets the next wire

I'll probably miss a connection, but here is where you can start:
- main + wire at battery
- connector between battery and fuse box
- junction of wire to input of main fuse
- junction of output of main fuse to wire
- connector between fuse box and main harness
- connector near ignition key
- contacts inside ignition switch
- connector near ignition key
- connector to fuse box
- junction of wire to ignition fuse clip
- junction of fuse clip to fuse input
- junction of fuse output to fuse clip
- junction of fuse clip to wire
- connector from fuse box to wire leading to connector under tank
- connector under tank to right handlebar sub-harness
- connection to kill switch
- connector from sub-harness to coil assembly
- finally, the terminal on the coil

Remember, at each 'connector', there are actually three interface points.
Considering all these interface points, Brendan's half-volt loss is darn-near perfect.

 

[allojohn]

Being the '78 750 does not have a fuse box and only one fuse, cleaning up connections isn't half as bad as it sounds...

 

[Forden]

Thanks everyone. I haven't experienced the symptom of starting only when the starter is released (that I've noticed). The bike starts and runs pretty readily, though needs choke for a bit longer than it should.

I'll do some more cleaning / restoring connections and repost new results.

 

[Rijko]

Welcome to the latest game, called "The hunt for missing volts".
You're not just missing 2.3 volts, you have a much higher current in the system than it was designed for.
Which can cause malfunctions and even kill electrical components.

10 volt is really low, for me that is reason to start cleaning.
8.3 volt sets off alarm bells for me.

The reason i start from 'endpoints' is i find it easier to trace back towards the battery.
And if the 'endpoint' is ok, i know the whole subcircuit is ok.

The coils and the starter relais are powered by the RH handlebar switch.
So if you have low power on the coils you probably also have that on the green/yellow
wire that operates the starter relais.

Next step would be to check out the RH handlebar switch.
Those corrode inside and usually require cleaning.

Be careful - there's springs in there that like to jump out.

 

[Gorminrider]

12.3V at battery (resting since yesterday)

Your battery is getting old. Try any decent car battery and see if that helps or Unplugging (or turning off) your headlamp will also make more current available...

Turning a four-cylinder bike over might be a bit of a strain for tan old battery but if it's spinning well...well it's always worth looking at the ignition electrical connections. Including the kill switch .Try testing voltage "across" the switch's connections when on ("voltage drop") It should be 0! Any good connection will test 0.

I don't know what bike this is but if it has an ignitor,("TCI") this will drop voltage at the coils a lttle too.

 

[Who Dat?]

Being the '78 750 does not have a fuse box and only one fuse, cleaning up connections isn't half as bad as it sounds...

Maybe not, but the principle is the same, ... check EVERYTHING.

I don't know what bike this is but if it has an ignitor,("TCI") this will drop voltage at the coils a lttle too.

His signature says "1978 GS 750 E".

 

[Grimly]

Just an aside - you can keep the engine running in the presence of a weak spark by throttling it more than normal, that leads to rich running.
Not saying it's your running problem, but the voltage on cranking over is far too low, as is anything under 12V at the coils when running.

 

[Gorminrider]

.......
His signature says "1978 GS 750 E".

Ah yes, that reminds me: Weirdly, I can't see his signature when logged in. I can't see any one's signature unless I'm NOT logged in.

 

[Guest]

Ah yes, that reminds me: Weirdly, I can't see his signature when logged in. I can't see any one's signature unless I'm NOT logged in.

At the top of the page, click on "Forum", then go toward the center, click on "Forum Actions", then "General Settings". Scroll down to the "Thread Display Options" section, verify your settings there.

While you are right there, make sure the next two items are adjusted correctly, too. Set the "Thread Display Mode" to "Linear - Oldest First", then set the "Number of Posts to Show Per Page" to the maximum of "40 per page". That makes it easier to read longer threads without all the page-flipping.

 

[Gorminrider]

ahaha Yes.. thanks. I had " signatures visible" unchecked. Some signatures here are awfully long and big and in a peevish mood, I was annoyed scrolling through them.

 

[Forden]

Thanks all, incredibly helpful. I'll start digging in this afternoon. Time to bust out the glasses for the wiring diagram...

Try testing voltage "across" the switch's connections when on ("voltage drop") It should be 0! Any good connection will test 0.

Didn't know this. So in the case of the kill switch, my touch points would be would be where orange and yellow/green connect to the switch? Test with key and switch both on?

 

[pdqford]

To measure voltage drop between two points of a circuit, the circuit must not only be “on”, it must have current actually flowing through it. So your meter actually reads the voltage that the circuit between the probes CAN’T handle.

 

[Gorminrider]

Thanks all, incredibly helpful. I'll start digging in this afternoon. Time to bust out the glasses for the wiring diagram...

Didn't know this. So in the case of the kill switch, my touch points would be would be where orange and yellow/green connect to the switch? Test with key and switch both on?

Yes, and since I now know your bike has points, rotate the engine so that points are closed.

...So your meter actually reads the voltage that the circuit between the probes CAN’T handle.

That's an interesting explanation!
I explain "voltage drop" to myself as:Since the multimeter's "ohms" reading is really derived from a voltage drop using the meter's tiny battery anyways, the tiny meter's battery cannot always supply enough current to make a voltage drop measureable.

 

[Brendan W]

Thanks all, incredibly helpful. I'll start digging in this afternoon. Time to bust out the glasses for the wiring diagram...

Didn't know this. So in the case of the kill switch, my touch points would be would be where orange and yellow/green connect to the switch? Test with key and switch both on?

I would open it up and clean it anyway. Most likely there is a lot of dirt in there.
My meter is not top of the range and I am suspicious of it's resistance measurements.
I would be more confident of voltage readings so I clean until the volts are good.

 

[Forden]

Yes, and since I now know your bike has points, rotate the engine so that points are closed.

It has Dyna S ingnition now, actually. Does it matter whether the magnet is lined up a particular way?

Making good progress, got a bunch of contacts cleaned or respliced, replacing some more connections today. The kill switch assembly was indeed full of crud, and the 3-way plug connector basically crumbled when I pulled it apart.

Taking the opportunity to wire back in the original turn signals, too--for some reason the PO swapped them out for some crappy plastic aftermarket ones. Go figure.

 

[pdqford]

That's an interesting explanation!
I explain "voltage drop" to myself as:Since the multimeter's "ohms" reading is really derived from a voltage drop using the meter's tiny battery anyways, the tiny meter's battery cannot always supply enough current to make a voltage drop measureable.

Interesting thing is, the more current a circuit draws, the bigger the voltage drop will be.

As an example, take a multiple stranded battery cable with only one strand of wire still in tact.
The ohm meter’s tiny battery puts out so little current that it will show no resistance over that one good strand of wire.

But switch to the voltage scale and engage the starter so that the circuit is drawing say 100 amps, and the voltage drop over that one good strand of wire will be nearly full battery voltage!!

Returning now to the discussion at hand..........

 

[Forden]

Ok, things seem to be improving--I'm still losing voltage, but less.

What I did:
-disassembled and cleaned both kill switch and ignition switch assemblys
-all new connections for both switches at junctions behind headlight
-cleaned all battery connections and grounds, new positive 6 gauge wire and terminals
-cleaned starter relay connections
-new connections both sides of main fuse

Results:
12.6 - 13V at battery (trickle charger overnight)
11.5 at positive lead to coils at rest (orange/white wire connection to neg. battery terminal)
10.5 at positive lead to coils while cranking

Voltage drop tests:
Ignition switch seems ok: Orange to Red .01, Gr to Br .00
O/W wire, from kill switch junction to coil junction: .15
G/Y wire, from starter relay to kill switch junction: .00
Kill Switch, both key and switch on (need help here):
O - O/W: .46 - .9 (got a few different readings)
O - Y/G: 12.3
Y/G - O/W: 11.85

Seems like the kill switch is a likely culprit, but I cleaned it thoroughly and respliced all three wires, so not sure what else to do there. Or did I miss something else?

Thanks so much for the help.
Mike