Voltage drop

Information relating to the Matchless G3 or AJS Model 16 350cc Heavyweight
Mick D
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Re: Voltage drop

Post by Mick D »

Hi

Does the plug spark when you disconnect the coil?

As an aside, I had an issue with the CB isolating washer becoming damaged - worth a look.
CB Fault.png
Regards Mick
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aobp11
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Re: Voltage drop

Post by aobp11 »

Mick: Mike said "i remember separating the points after the battery started to drop, and the voltage continued to drain.". So you can't assume the points were closed all the time.
Mike: Why didn't you look at the ammeter for discharge? It's so much easier to read the discharge current than to measure the battery voltage during a length of time. Now that you topped up the battery you might hardly notice a voltage drop (except a nearly immediate jump) when the ignition coil is drawing current.
It must be fairly easy to trace the fault. When there is a connector plug there you can disconnect the lead at the contact breaker side (not at the coil side) and decide whether the fault is an earth short in the wire somewhere. If not, there remain the insulation between contact spring binding post and earth (there can be a short to earth when fiber washers fail) and a short in the capacitor. Probably it's easiest to disconnect the capacitor body from earth. This tests the capacitor. If not the capacitor then you have to detach all stuff from the binding post and check the insulators and order in which these have to be assembled.

Albert

(Mick made some remarks will I was still typing...)
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Harry44
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Re: Voltage drop

Post by Harry44 »

If you leave the ignition switch on overnight and it flattens the battery that's normal when the points are closed. However if you do leave it like this the coil will probably overheat and burn out leaving you with no spark.

Does this fit what you are seeing or have I misread your post ?
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mikeydpippin
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Re: Voltage drop

Post by mikeydpippin »

Thanks for all the advise, to update with a battery showing 5.8v I have with switch turned ON 5.8v on the neg side of coil, so power going to coil of 5.8v and zero on positive side of coil with zero volts at points.
Also I have no volts at no.15 on the switch ( the cable supplying the CB side of the coil.
I have ordered a new switch.
Mike
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Harry44
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Re: Voltage drop

Post by Harry44 »

Those results would make me think there is nothing wrong. What are you trying to cure ?
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aobp11
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Re: Voltage drop

Post by aobp11 »

Hello Mike,

I think you are drawing the wrong conclusion. The coil "C.B." terminal is ALWAYS connected to pin 15 of the ignition switch. IIRC this is needed only for the EMG start circuit. When in IGN position, pin 15 is left open by switch.
Coil terminal "C.B." is also connected to the moving breaker contact point. The fault disappeared when you disconnect that lead from "C.B." to the contact breaker. I ASSUME* that leaves the wire from "C.B." to switch pin 15 intact . Hence the switch cannot be the culprit.
I suggest you repeat your measurement and notice the effect when you disconnect the wire from "C.B." to contact breaker. I bet in IGN position you will read minus 5.8 V then at both "C.B." and pin 15.

* When both wires to "C.B." use a common connector to the coil terminal then the story is different. Then maybe you disconnect both wires together. The "C.W." terminal will show minus 5.8 V in IGN position. The contact breaker and pin 15 will be "floating" when the points are open; between earth and there you find zero volts. But also between battery-live and there you will find zero volts. (Might be not exactly zero volts when you use a digital multimeter. The slightest leakage will show a deviating value.)

BTW there is an error in the "Wiring Diagram - Singles" 1957. There the switch is shown to have an internal connection between pins 14 and 15. That would always short the voltage across the coil! The error is corrected in the 1959 diagram. (I don't have a 1958 version.)

BTW Is your battery that bad, 5.8 V?
Albert
mikeydpippin
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Re: Voltage drop

Post by mikeydpippin »

Harry,
There is no voltage to the points, so presume this is why I am getting no spark....
Besides I undone the switch and I,ve lots of bits..
Points are good multimeter sees a continuity opening and closing.
Coil sees a voltage at the neg but not at the pos.
So voltage going from the switch to the coil neg but not from the switch pos, which supplies the points.
Mike
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Re: Voltage drop

Post by mikeydpippin »

Sorry Albert your response came after my reply, I will fit new switch when it arrives and let you know what I find,
Really sorry my electrics knowledge is minimal but improving thanks to you Harry and Mick D for your patience..
Mike
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Harry44
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Re: Voltage drop

Post by Harry44 »

When the points are shut a voltmeter connected from CB on the coil and earth will show zero because the points earth the CB terminal when they are shut. If you open them by hand or slowly turning over the engine the voltmeter still on CB will suddenly read battery voltage.

However if the wire from CB to the switch is shorted to earth any where the voltmeter will still show zero thus demonstrating a fault.

Cheers Harry
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Mick D
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Re: Voltage drop

Post by Mick D »

Hi
mikeydpippin wrote:Coil sees a voltage at the neg but not at the pos.
Sounds like you could have fried the coil - disconnect both terminals and measure the resistance across them, if it's open circuit you'll need a new one.

Or:

With the points closed making / breaking the connection to the coil should induce a spark at the plug.

Regards Mick
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