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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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 Posted: Mon Mar 10, 2025 11:32 am Post subject: Trundles fails to start! |
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With some fresh petrol and fully charged battery, I had expected Trundles (my 1930 Austin Swallow) to start as usual...but nothing is happening. This is an Austin 7 engine with original CAV DS4 - 2 distributor and 'Rist' coil. The HT leads are copper core.
With the ignition on, I am not getting a spark from the HT king lead against a head stud.
I checked the 6 volt coil. The primary has 1.4 ohms but the secondary reads only 2.7 Kohms. I therefore assumed the problem to be with the coil.
I fitted my spare coil (a new 6 volt Remax) with a primary reading 1.7 ohms and a secondary reading 7.2 Kohms.
When the engine still failed to start I checked the col as follows. I established 6voilts to sw and took a fly lead from CB to earth. I expected the king lead to produce a spark when placed near a good earthing point.... but it didn't.
I am assuming a condenser is not necessary in this test as I only want to check the coil.?
Last edited by Ray White on Mon Mar 10, 2025 1:34 pm; edited 1 time in total |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4242
Location: South Cheshire
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| Posted: Mon Mar 10, 2025 11:51 am Post subject: Re: Trundles fails to start! |
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| Ray White wrote: | | I am assuming a condenser is not necessary in this test as I only want to check the coil.? |
Correct, if the coil is good you should get a spark even if the coil primary is wired the wrong way round, easily done when a coil is marked SW! |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Mon Mar 10, 2025 1:31 pm Post subject: |
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I checked SW on the coil for 6V and removed the wire from the distributor and checked for continuity (no frayed ends) and held it onto a head stud.
I checked the copper core HT lead from the coil for continuity.
The coil is unused and resistance is correct.
I get a good earth from the head stud.
I should get a spark holding the HT lead held close to an earth point but I don't see one. (even in shade).
I Just don't know why. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4242
Location: South Cheshire
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| Posted: Mon Mar 10, 2025 3:08 pm Post subject: |
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| I'm assuming you have checked that the points are closed? are you getting 6v across the coil? if yes you should get a spark when the power is removed. |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Mon Mar 10, 2025 4:53 pm Post subject: |
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| ukdave2002 wrote: | | I'm assuming you have checked that the points are closed? are you getting 6v across the coil? if yes you should get a spark when the power is removed. |
Yes 6 volts across the coil. With CB wire connected to the distributor I flicked the points with the HT lead close to an earth point ...but no spark.
I get Full battery voltage at the CB terminal on the coil with the points open but not dropping to zero when closed which implies the points are dirty. I have cleaned them with fine sand paper but to no avail. Perhaps there is a short in the distributor?
I am still not getting a spark from the coil to earth when it is detached from the distributor. Could I have two separate faults?
What I am wondering is ...would it be possible for a coil to show correct resistance but still not spark? The old coil has a 1.4 Ohm primary which is O.K. but reads a low 2.7 KOhm at the secondary. As this coil is very old I don't know if they all used to be like that many years ago? OR is it now a dud.?
The replacement coil is a few years old but unused...so although it has the right resistance it is an unknown quantity.
Last edited by Ray White on Mon Mar 10, 2025 5:10 pm; edited 2 times in total |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4242
Location: South Cheshire
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| Posted: Mon Mar 10, 2025 5:09 pm Post subject: |
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| Ray White wrote: | | ukdave2002 wrote: | | I'm assuming you have checked that the points are closed? are you getting 6v across the coil? if yes you should get a spark when the power is removed. |
Yes 6 volts across the coil. With CB wire connected I flicked the points with the HT lead close to an earth point ...but no spark.
What I am wondering is ...would it be possible for a coil to show correct resistance but still not spark? The old coil has a 1.4 Ohm primary which is O.K. but reads a low 2.7 KOhm at the secondary. As this coil is very old I don't know if they all used to be like that many years ago? OR is it now a dud.?
The replacement coil is a few years old but unused...so although it has the right resistance it is an unknown quantity. |
If you are getting 6v across the coil with the points closed, we know that the points are good and the coil is "charging" , if there is no spark when the points are opened, then the only possible culprits are either the king lead or the coil its self |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Mon Mar 10, 2025 5:13 pm Post subject: |
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| ukdave2002 wrote: | | Ray White wrote: | | ukdave2002 wrote: | | I'm assuming you have checked that the points are closed? are you getting 6v across the coil? if yes you should get a spark when the power is removed. |
Yes 6 volts across the coil. With CB wire connected I flicked the points with the HT lead close to an earth point ...but no spark.
What I am wondering is ...would it be possible for a coil to show correct resistance but still not spark? The old coil has a 1.4 Ohm primary which is O.K. but reads a low 2.7 KOhm at the secondary. As this coil is very old I don't know if they all used to be like that many years ago? OR is it now a dud.?
The replacement coil is a few years old but unused...so although it has the right resistance it is an unknown quantity. |
If you are getting 6v across the coil with the points closed, we know that the points are good and the coil is "charging" , if there is no spark when the points are opened, then the only possible culprits are either the king lead or the coil its self |
I have continuity of the king lead so I must have a coil problem. |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Mon Mar 10, 2025 7:57 pm Post subject: |
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I suppose a low resistance reading implies the a couple of the wires in the coil are shorting out and reducing the resistance/induction...resulting in a weak spark...or no spark at all.
Why, then should a coil with normal resistance readings also fail to spark?
I asked the Distributor Doctor about this but they had no answers apart from suggesting a faulty HT lead. |
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petelang
Joined: 21 May 2009
Posts: 475
Location: Nottingham
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| Posted: Mon Mar 10, 2025 9:51 pm Post subject: |
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Sprung brush in distributor cap or rotor arm? Are you certain king lead is contacting the terminal in the coil top?
_________________
Daimler Fifteen 1934 (now sold)
Armstrong Siddeley 15 Long 1933
Daimler V8 250 1969 |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Mon Mar 10, 2025 11:49 pm Post subject: |
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| petelang wrote: | | Sprung brush in distributor cap or rotor arm? Are you certain king lead is contacting the terminal in the coil top? |
Yes, there is a good connection in the coil. I have also removed any sign of corrosion there and in the distributor cap. The rotor is good; not shorting out and the spring is making good contact in the cap.
I will be going through it all again tomorrow. I should also be getting a new condenser from the distributor Doctor.
Two things concern me.
1) Not being able to get a spark from the coil (independent of the distributor).
2) Battery voltage not falling away at the CB coil terminal when the CB points are closed despite thorough cleaning. |
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Rusty
Joined: 10 Feb 2009
Posts: 280
Location: Bunbury, Western Australia
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| Posted: Tue Mar 11, 2025 12:58 am Post subject: |
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| Does it have a pigtail lead (short wire that connects the points base plate to the distributor body) sometimes they give issues and the points base plate doesn't earth, so no earth = no spark. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4242
Location: South Cheshire
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| Posted: Tue Mar 11, 2025 7:06 am Post subject: |
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| Ray White wrote: | | 2) Battery voltage not falling away at the CB coil terminal when the CB points are closed despite thorough cleaning. |
Voltage drop across the points should only be around 1/4 V, can be less if the points are in really good condition, so in your case this looks good.
| Ray White wrote: | The old coil has a 1.4 Ohm primary which is O.K. but reads a low 2.7 KOhm at the secondary. As this coil is very old I don't know if they all used to be like that many years ago? OR is it now a dud.?
The replacement coil is a few years old but unused...so although it has the right resistance it is an unknown quantity. |
Resistance looks correct for a 6V coil, 6v coils require a lower primary resistance than 12v coils in order they get a similar amount of energy into the coil when its charging, have you measured the current draw with the points closed? should be around 4 Amps.
If the coil is old, its internal insulation may have broken down, possibly why you are not getting a spark? |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Tue Mar 11, 2025 10:08 am Post subject: |
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| ukdave2002 wrote: | ........... have you measured the current draw with the points closed? should be around 4 Amps.??..............
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NO. That is something I can check. Thanks. |
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Ray White
Joined: 02 Dec 2014
Posts: 7139
Location: Derby
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| Posted: Sun Mar 16, 2025 7:12 pm Post subject: |
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The points show 3.5 amps when I open them.
I have just taken a reading from the LT terminal (ign. switched on) to the dist. body. I am getting 58 Ohms.
A similar test from the LT on the dist. to a head stud shows 68 Ohms.
I dunno.  |
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petelang
Joined: 21 May 2009
Posts: 475
Location: Nottingham
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| Posted: Mon Mar 17, 2025 9:50 am Post subject: |
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Ray, get a test lamp. Connect the clip on the cable of the test lamp to battery positive. Remove your ignition feed from SW on coil. Connect probe end of test lamp to SW. Ensure CB wire is connected to distributor. When Contacts are open, light should be off. When contacts are closed, the light should be on, bright.
If the test light only glows dimly, you're not getting an adequate earth on the distributor base plate. The small wire may be broken. Or the points contact faces may be oxidised.
Don't overthink it.
If dim, you can short out the contacts with a clip, and if that produces a brighter lamp then you know its resistance in points contact faces. If you short supply side contact to earth on distributor body light should be bright.
You can use a test lamp such as a 21watt bulb as drawing higher current is a good test of integrity of connections.
_________________
Daimler Fifteen 1934 (now sold)
Armstrong Siddeley 15 Long 1933
Daimler V8 250 1969 |
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