Magneto trouble

This forum is about the Fordson F, N and E27N Major.
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SouthSteam
Not Quite Blue Yet
Posts: 4
Joined: Thu Apr 13, 2017 9:38 am

Magneto trouble

Post by SouthSteam »

Hi all,

I have been experiencing trouble with my Lucas RF4 magneto on my E27n. It is a previously reconditioned magneto so all intervals are good. It gives off a spark but does not fire the engine. I tried a mates identical magneto on and the tractor started up first rev, so definitely a fault with my magneto.

I am interested in looking at it myself, and currently have it in bits. All internals are pristine as I know it is a previously reconditioned magneto. My thought is that the spark is too weak as it fires outside but not under compression, therefore I have resistance checked the primary and secondary coil windings. The primary coil has negligible resistance, which is good. However the secondary coil has a resistance of 4.6 ohms. I am pretty sure this figure is too low as generally secondary coils have an extremely high resistance (thousands of ohms) in order to step up the voltage.

I am therefore asking if anyone has any experience testing coils or what their houghts are here? Or if anyone knows or is able to test what a working magneto secondary coil resistance should be?

CalGG
True Blue
Posts: 83
Joined: Thu Oct 11, 2012 2:44 am

Re: Magneto trouble

Post by CalGG »

If I recall correctly,

A bad bobbin on my RF4 measured just less than 5K ohms

A replacement bobbin (that actually would start the tractor under good conditions) measured right around 7K ohms.
The RF4 is in a box on a high shelf in the shop, along with the many many dollars worth of repair and replacement parts I threw at it. The unit probably needed to be re magnetized, as it had been apart. When the rotor is pulled without magnet "keepers" placed correctly, about 1/3 of the magnet force just vanishes. Less magnet means less electricity. Physics and all that stuff are to blame.
I finally had enough of Lucas, and fitted a Wico Magneto. Now the tractor starts up and runs under ALL conditions, hot cold or rainy. ;-)

It sounds like you need a new bobbin.

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