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Old 09-11-07, 11:29 PM   #21
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surely the coil is going OPEN CIRCUIT if the dc resistance is increasing. A short would reduce the dc resistance and also the inductance. I presume you measured it with the capacitor out of circuit?
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Old 10-11-07, 03:43 PM   #22
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A coil cannot be 'going' open circuit. It is either open circuit or not.

A coil which is open circuit will read infinity on an ohm meter but a shorted turn produces such a small change in DC resistance that all but the best laboratory instruments will be incapable of measuring it.

Inductance tests involve using a Wheatstone Bridge but are of no use unless the inductance in Henrys of a good coil is known. Impedance (AC resistance) requires special measuring equipment also.

The best test for a faulty coil is always 'diagnosis by substitution' which does mean another known good coil has to be to hand. Always, it's best to do a few basic tests first such as HT lead continuity, earth connection, contact breaker points etc.

Proper testing of a capacitor involves a capacitance tester but some indications can be had by using an analogue ohm meter. Digital resistance meters do not give useful test results. These tests are only good for capacitors of a few micro Farads and more. Smaller values accept such a small charge that it does not read on the meter.

Connect the meter, set to the lowest ohms range, across the capacitor. If the capacitor is short circuit the meter will read 0 (zero) ohms. If the capacitor will accept a charge the needle will flick over then back as the capacitor charges. In fact, the needle moves as the charge current flows into the capacitor from the meter's internal battery. When the capacitor is charged the needle drops back. A similar test could be made using a sensitive, low range (micro Amperes) ammeter and a battery connected in series connected across the capacitor under test. However, if a capacitor breaks down under voltage then it is most unlikely a low voltage ohm meter will show this. A better test would be one using an insulation tester such as a Megger. It is important to note the working voltage of the capacitor under test and to ensure this is not exceeded.

No movement of the needle will occur in the case of a capacitor which is open circuit (o/c).

It is always worth doing some tests on known good items and noting the results for future use.


David G
Briggs & Stratton 2 1/4 HP Stationary Engine - Restored 2007. JAP Model 0 - Restored 2009.
2 Suffolk 75G14 models 6A and 21A undergoing restoration.

Last edited by dgosden; 10-11-07 at 04:13 PM.
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Old 10-11-07, 06:44 PM   #23
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To test a mag, you are best armed with one of these.

This one goes up to 10H not sure if it's enough for a mag coil, others go to 20+H but are more expensive (70)
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Old 10-11-07, 06:55 PM   #24
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Hi Burtle,
Excuse me for contadicting you, but it look to me like you have put a link to a probe pack for the LCR meter, the Auto LCR meter, LCR40 meter itself is 70 ex VAT.
If I'm wrong then I would say the meter is a bargain.
hth Dave.

Last edited by djrm; 11-11-07 at 12:21 AM. Reason: removed reference to expired link
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Old 10-11-07, 09:26 PM   #25
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None of those links are behaving in a way that inspires even a tiny bit of confidence so I don't know what you folks are on about. However this looks like fun;-
or if my link is also sh12 ebay item no. 220170452557
"Mid-life crisis - what mid-life crisis?" :rolleyes:
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Old 27-10-10, 10:22 AM   #26
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Hi, Folks,

I have probably restored about 30 magnetos in the last 20 years (mainly Lucas K2Fs) and I have only once found a bad armature, where the stiff wire that goes from the wining to the slip ring had broken off. The problem was always the condenser. I used to remove the condenser from the brass end piece, unsolder the end of the metal condenser case and scrape out the paper and aluminium foil from inside. I then soldered a modern mylar capacitor, 0.01 uF, 400v ac in its place. The armature winding was usually filthy, often covered in mould which would absorb humidity and cause arcing, this was removed by gently wiping with alcohol, giving a "like new" result.
In the K2F, there are 2 pointed screws at the slip ring end to provide an overvoltage spark gap. These MUST be removed before trying to remove the armature, otherwise they break chunks out of the slip ring bakelite. I don't know if this problem exists in the Wico mags.
The bearings are insulated from the body of the mag with paper insulators. The ones you find at autojumbles are normally too thick and useless. They must be eplaced to prevent arcing in the bearings.
Magnetos are not magic, just fiddly to repair.

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Old 27-10-10, 11:28 AM   #27
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I have about 10 wico or wipac ones out in the shed, I have cannibalised some of them and now have 3 or 4 working spare ones, on the wipac ones with the silver metal cover on the end, I have often found that the stop button often plays up, I have had 2 that dont spark with the stop switch wired in, so I just leave them disconnected, when i try and use those buttons the engines always seem to keep going and i get a belt!
Matt H
1947 D & F pumpset, 1943 close couple Lister Pumpset, 1931 & 1929 D types, 1929 Bruston D, 1930 Crossley 1030, 1942 Fowler PA, 1943 Fairbanks ZD, 1945 WD2, 1946 Bamford EG1, 1930 Superbloc 3hp, 1934 Conord F2, 1942 Ruston PT + driven equipment
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Old 27-10-10, 11:59 AM   #28
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Originally Posted by Paul_Sterling View Post
hi dave, i have probelms with the magneto on my A1's the ones fitted with a Wico series A seem okay, only i havent fitted the magnetos on them yet (im a coward when it comes to electricity!) but the other one is a completely cast aluminium one, (i think its a wico but not sure) it sparks, albeit poorly and doesnt have an impulse, any ideas on that?

Get a copy of my Wico A maintenance manual, available on ebay Item number: 300485488054, soon to be added to the website.
Douglas VS25; Douglas SV (both under restoration)
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Old 27-10-10, 12:07 PM   #29
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Originally Posted by djrm View Post
What is an impulse?

The usual suspects on a poor magneto are the points, the armature, the magnet, and the condenser. The points need to be adjusted for the correct gap, perhaps 12 thou. The armature may have poor insulation or worse, shorted turns, and the condenser may be either leaky or completely open circuit.

On a typical motorcycle (K2F) magneto the condenser is on the end of the armature and quite tricky to replace I believe.

I have heard that it may be possible to fit an external condenser across the points if the old one is open circuit but I'm skeptical whether that would really work.

Sorry but I have had very little experience of the stationary engine kind since about 30 years ago on my old 600cc JAP engined Howard rotavator which had something similar, possibly a Wico.

An impulse unit is a device which helps give a clean and precise spark at low revs.
The practice of fitting a condenser across the original is not to be recommended on technical grounds and is unlikely to be practically possible due to the lack of space. Despite many people who have a little knowledge suggesting otherwise,the condenser has only one purpose and that is to protect the points from the effect of switching an inductive current through the coil, ie arcing, there are a number of weak spots in the manufacture of the condenser, so it could fail open circuit or short circuit. It should only be replaced by one of the same value and type (ie designed to absorb high energy pulses)
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Old 27-10-10, 05:08 PM   #30
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My 2 penn'th for what its worth.
Old mags cant be revitalised (or shouldn't be), for the following reasons.
If the condenser can be rejuvenated, its just not worth it.
If the magnetic flux can be somehow restored, its going to be slight and not worthwhile.
Coils which need warming to reduce leakage are useless anyway.

There is a good method to test out coils, and condenser testing is best done by substitution.

To test coils a simple points and known good condenser arrangement is connected up to the coil in its armature a spark gap of about 3/8" is arranged.
A variable voltage supply is conected up in series with a ammeter and the points activated by a motorised cam.
The votage starts low and increases until the spark jumps over. a note is made of the current at that point.

Notes can be made of different spark/current levels for different coils.

Basically a high current requirement to make spark denotes a dodgy coil.

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