HOW TO CHECK YOUR COIL

[Gulfstream1]

Hi Guys

Some notes on how to check a coil and a stator. These notes are a bit long winded and primarily for the benefit of folk who have no electrical experience, although I do accept that most guys who are PPG minded are advantrous and will have some automotive electrical knowledge.

Firstly – the COIL

YOU NEED

1) a multi-meter – nothing fancy, a cheap Halfords 10 – 20Quid example will do and digital is always easier to understand and read if you have little or no previous experience.

2) multi-meter leads

You are measuring resistance – so ensure the leads are plugged into the appropriate connections on the m/meter for measuring resistance (Ohms – it is often marked up on m/meters using the little symbol you find on Omega watches)

There are essentially 2 tests that need to be undertaken – both are resistance tests, and in both cases the connections made with the m/meter leads on the m/meter side stay the same.

TEST 1

a) disconnect all wires from the coil (i.e. the 2 side wires and the central HT lead for the spark plug.

b) Set the m/meter to the lowest possible Ohms/resistance reading option

-c) Place the end of one of the m/meter leads on one of the coil outer terminals, and the other lead on the other outer coil terminal – which 2 connectors? Just make sure either of the terminals you are connecting to is NOT the HT central terminal connector, and you’ll be fine.

d) Make a note of the resistance reading.

……… move onto test 2

TEST 2

a) still, with all wires disconnected you now need to test/measure the resistance between each of these 2 coil outer terminals and the central HT lead terminal (high tension lead) – which is usually the central wire and is always substantially thicker than the other 2 – and still not sure? It is the terminal that takes the thick wire to your spark plug?

b) Turn the dial on the multi-meter to the kilo-ohm setting ( i.e. the setting that represents units of measurement in 1000 Ohm amounts)

c) Place one m/meter lead end on the terminal of one of the outer coil terminals (makes no difference which m/meter lead is connected to which coil outer terminal).

d) Make a note of the resistance.

e) Repeat the procedure now for the other outer lead to the HT lead.

FINDINGS & NOTES

With respect to TEST 1 you should got readings of anything between around 0.61 – 0.89 Ohms (give or take around 10% either side of these limits).

If you get a reading that runs into 10’s, 100’s, 1000’s of Ohms and/or higher you have a damaged coil – and the chances are it will be a break in the wire from which the coil is wound.

If you have a reading of 0 Ohms you have a damaged coil – and the chances are it will be enamel insulation on the coil that has melted and has resulted in a short circuit in the coil, which is reducing the winding ratio between the primary and secondary windings and reducing the high voltage output to an amount that is too low for effective sparking to take place and ignite the air/fuel mixture.

With respect to TEST 2, you should have a very high resistance reading, but not excessively high. A figure of between 9500 Ohms – 11500 ohms (0,95 Kilo Ohms – 1,15 Kilo Ohms) is typical of automotive type coils – and I would go so far as to say that differences between coil manufacturers may even give you a reading of as little as say around 8800 Ohms (0,88Kilo Ohms) or as high as 12300 Ohms (12,3Kilo Ohms).

These are extreme readings and anything above/below these figures is excessive, suspicious and to be treated as faulty and/or unreliable.

Fortuneatly, most readings taken from coils leave little doubt i.e. they tend to be within range or way way out. If you get a reading of 20000 Ohms (20 Kilo Ohms) or any other figure in the 10’s of thousands, or hundreds of thousands, it is a clear indicator that you have a break in the coil winding that feeds the HT lead.

If you have a reading that is very low (any low figure) below the limits I have described above, you have a short circuit between the secondary and primary windings in the coil.

In either case, it would be folly to attempt a repair (not impossible, but quite frankly pointless, and with little chance of success) – bin the coil and replace with a new one. There is nothing esoteric or magic about them – they are cheap standard automotive parts that can be sourced from any half decent automotive shop.

__________________________________________________________________________________________________________________________________________________________________________

Lastly – there is one other test that can be undertaken, but some care needs to be exercised as it could be shocking experience.

You will need a small 1,25v AA or AAA battery (use a small one!).

I’m not going to write it up detail – just the basics, if you cant grasp the idea from these basic notes, then I wouldn’t recommend you be doing this test – and if you wear/use any kind of pacemaker or other heart rhythm controller DO NOT UNDERTAKE THIS TEST – accidents happen because they are not expected - it could kill you.

a) Ensure the multi-meter leads are now plugged into the + and – connectors on the multi-meter

b) Place a 2watt - 3watt 10Kilo Ohm (or high value resistor if you wish - the higher the better) in series with the + lead and the the coil HT terminal

c) that the m/meter is now set for DC voltage readings and the dial turned to the highest voltage multi reading that s possible

d) that the end of the + multi-meter lead not conneted to the multi-meter is connected to a resistor in series and then to the coil HT terminal and

e) that the – m/meter lead is connected to earth.

If you can’t work all that out then you should not do this test.

f) Connect the – side of the 1,25volt battery to the negative coil terminal with a piece of insulated copper wire.

g) Now connect an insulated length of wire to the +terminal of the coil, but NOT the positive of the battery.

h) You want the end of the wire that is going to be connected to the positive side of the battery, stripped bare and ready to just touch the battery.

When all is ready and setup, briefly touch the positive wire to the positive battery terminal and make a note of the voltage reading on the multi-meter - no more than a second should be plenty sufficent time - you want to see the needle or the digital reading go up as high as the multi-meter can read, that is all

You should be getting a reading will be above what the average mutli-meter is able to read different, but the exact figure is not important - coils will give different readings and differences can be quite substantial, but if you are getting anything less than 1000 volts, its time to chase up on Google the voltage ratio of that coil (i.e. the ratio with 1,25 volts input – and NOT with 12volts input). If there is no voltage reading, this will be further confirmation of a break in one of windings inside the coil case - and again, time to replace the coil.

Keep your hands away form the HT side of the circuit and the multi-meter leads to earth and the coil – and again, if you use any kind of heart rhythm control devise I do not recommend you undertake this test if you are not familiar with electrics.

Please Note guys - 2 observations made by Paul that need to be emphasized

- Point b) is important do not skip this - most DIY multi-meters cannot read 1K volts - the use of a resistor will protect your m/meter from excessive current/voltage which could damage it so long as you do not keep the battery connetced for anything more than a brief moment or 2 - I would say 1second absolute max - you only want to see if the reading is below or exceeds what your meter can read (which is usualy around 1kvolts)- that is all that needs to be established.

As well, Paul below makes the following comment: A very quick touch and release of the wire to the 1.5V battery is all that is required to generate high voltage. Be careful not to touch the bare end of the wire you are applying to the battery or you could also get a nasty shock from the inductive kick back when the wire is removed. If you don't understand this believe me it can bite you.

This was another failing of mine - his comment is bang on (excuse pun): disconnect both wires from the battery to the coil once this test is completed without touching the bare ends (which makes using insulated crocadile clips a good idea) - and if not, just leave them a lone for 15secs - 30secs after the test before removing them.

Thanks Paul

I'll write up the stator tests/checks later.

If I have stated something here incorrectly, or anyone has anything to add that will help other members - please do

Edited September 29, 2009 by Guest

[togsie]

Gulfstream1 a nice post which I'm sure many members will find usefull.

This would sit very nicely in a "How To" section of the web site. I've already seem posts on the topic of how to straighten a bent cage and maybe someone could do a write up on how to balance a prop, adjust a carb, etc.

What do you think Simon?

Regards.

Togsie.

[alan_k]

If you can’t work all that out then you should not do this test.

When all is ready and setup, briefly touch the positive wire to the positive battery terminal and make a note of the voltage reading on the multi-meter. A second or so should be plenty sufficient time.

You should be getting a reading in the 10 of 10000’s of volts range

Hi Gulfstream,

I have taken the liberty of highlighting your warning in red. These voltages are DANGEROUS. I assume you also mean 10s of thousand volts, not 10s of ten thousand volts;

IMPORTANT The multimeter is unlikely to have a range beyond 1000V and will not take kindly to being subjected to tens of thousands. I speak from experience having damaged my Fluke multimeter (an expensive meter) when I overloaded it inadvertently. If you can include a high voltage resistor in the +ve multimeter lead it can offer protection.

A very quick touch and release of the wire to the 1.5V battery is all that is required to generate high voltage. Be careful not to touch the bare end of the wire you are applying to the battery or you could also get a nasty shock from the inductive kick back when the wire is removed. If you don't understand this believe me it can bite you.

Above all be careful folks.

Alan

[Gulfstream1]

Thank you Allan - damn careless of me - I forgot that its unlikely most folk are going to have a meter at home that reads much more than 1K V - well observed Sir, I've become to used to working with lab hardware.

I have corrected the comments

Togsie has suggested that the writeup is moved to another section more approriate for "How To" comments - please feel free to move.