Lotus Elan

Ignition coil has to be grounded?

PostPost by: pharriso » Mon Sep 28, 2020 11:32 pm

My Sprint has a 3 Ohm (internally ballasted) coil & I have electronic ignition. For the last 6 months or so, it has not been starting easily even though it cranks quite fast.

I have replaced the usual suspects, Battery, Coil, plugs, checked trunk ground & chassis to engine ground strap but still a bit hit & miss.... I was reading the Buckland Bible tonight & see a comment in the coil section that says "if there are problems check the following items.... #2 the earth to the casing clamp which is in fact one side of the secondary winding"

I also note this prior forum item https://lotuselan.net/forums/viewtopic.php?f=16&t=9144

Now I've seen a spare earth, tie-wrapped back to the harness with the white coild wire. Should this infact be connected to the coil at the clamp?

Was this the cause of my starting issues?
Last edited by pharriso on Tue Sep 29, 2020 12:19 am, edited 1 time in total.
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PostPost by: elancoupe » Mon Sep 28, 2020 11:56 pm

I’ve always found the coil ground to be very important - I had similar issues many years ago.
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PostPost by: 2cams70 » Tue Sep 29, 2020 1:24 am

This was a topic of some discussion here several months ago. The conclusion was that the ignition coil is an auto transformer construction and therefore does not require a ground for functionality. Grounding the metal can may help reduce RFI emissions however - i.e less radio interference. Not sure what you mean by "internally ballasted" coil. Never heard of such a thing.
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PostPost by: pharriso » Tue Sep 29, 2020 1:55 am

2cams70 wrote: Not sure what you mean by "internally ballasted" coil. Never heard of such a thing.

The coil has primary resistance of 3 Ohms, as opposed to a coil of resistance 1.5 Ohms that has to be used with an external 1.5 Ohm ballast resistor. This may be an American term...
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PostPost by: StressCraxx » Tue Sep 29, 2020 3:23 am

2cams70 wrote: Not sure what you mean by "internally ballasted" coil. Never heard of such a thing.


An example of an "internally ballasted" coil is the venerable Bosch "Blue" coil used on many European cars such as the BMW 1600/2002. An external ballast resistor is not used nor needed.
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PostPost by: 2cams70 » Tue Sep 29, 2020 3:56 am

My understanding of the difference between a coil that is a "ballast resistor" type and one that isn't a ballast resistor type is that the ballast resistor type is designed to run with around 8V on it's primary side rather than 12V for the non-ballast resistor type. There's no ballast resistor internally within the coil itself for the ballast resistor type.

The actual resistance of the coil primary is not related to whether the coil is "ballast resistor" or "non-ballast resistor type" Low resistance coils however produce more spark energy and are designed for use with electronic ignition systems specifically (OEM design electronic ignitions that is - only some aftermarket ignitions work reliably with low resistance coils). A low resistance coil will pass more current through the primary so if you are using a conventional points and condensor setup this will reduce the lifespan of these components.
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PostPost by: richardcox_lotus » Tue Sep 29, 2020 6:03 am

My Lucas coil db105 has a small black earth cable on one side of the clamp, and a large earth braid on the other side.

This is in the original mounting position in the wing. Car has always started OK

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PostPost by: Andy8421 » Tue Sep 29, 2020 6:22 am

2cams70 wrote:This was a topic of some discussion here several months ago. The conclusion was that the ignition coil is an auto transformer construction and therefore does not require a ground for functionality. Grounding the metal can may help reduce RFI emissions however - i.e less radio interference.


Yep, ignition coil is an auto transformer (windings are shared between primary and secondary). The case is isolated from the windings (you could always check with a multimeter if not convinced), and doesn't need to be earthed.

Only reason for earthing I can see is to reduce radio interference.

Edit:

Link to one of the excellent Lucas technical correspondence courses. This one focuses on ignition systems, and the bottom of page 10 explains the auto transformer design of the ignition coil. Worth noting there are no connections made to the can.

http://www.da7c.co.uk/History%20Section/LUCAS%20ACHIVE/lucas%20coil%20course%203.pdf
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PostPost by: pharriso » Tue Sep 29, 2020 5:08 pm

Andy8421 wrote:
2cams70 wrote:This was a topic of some discussion here several months ago. The conclusion was that the ignition coil is an auto transformer construction and therefore does not require a ground for functionality. Grounding the metal can may help reduce RFI emissions however - i.e less radio interference.


Yep, ignition coil is an auto transformer (windings are shared between primary and secondary). The case is isolated from the windings (you could always check with a multimeter if not convinced), and doesn't need to be earthed.

Only reason for earthing I can see is to reduce radio interference.

Edit:

Link to one of the excellent Lucas technical correspondence courses. This one focuses on ignition systems, and the bottom of page 10 explains the auto transformer design of the ignition coil. Worth noting there are no connections made to the can.

http://www.da7c.co.uk/History%20Section/LUCAS%20ACHIVE/lucas%20coil%20course%203.pdf


Faced with 2 different trains of thought #1 yes it's critical, I had the same issue #2 the coil is an autotransformer therefore not needed I checked resistances:

+ve to -ve terminals: 3 Ohms as expected
+ve or -ve to Centre Terminal: 8.48 KOhms
Case to any terminal: Infinite

So at least for the Lucas DLB105, I can form it's an auto transformer, the case is isolated from the terminals & the primary & secondary windings are in Series. As Andy & 2Cams70 have both said, the only reason to ground the case would be to reduce RFI.

p.s. The anodized finish on the Lucas Sports coil is one great insulator, with MOhm resistance between the coil bracket & the Coil case itself! :roll:
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PostPost by: pharriso » Tue Sep 29, 2020 5:20 pm

Faced with the above, I measured cranking voltage at the coil & at the Blue/White (Ballast resistor bypass) terminal.

At the coil I'm getting 8.6 to 9.2 volts while cranking
At the Cold Start Terminal (Ballast bypass) I get 9.9volts!

If I run an extra wire from the Cold Start Terminal to the Coil +ve terminal, the car fires up immediately!!

Reading reference material, Lucas specify 8.5v minimum for batteries cranking, so my voltages without the extra connection are marginal.

Where is the extra voltage drop at the coil coming from while cranking? :roll:
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PostPost by: ericbushby » Tue Sep 29, 2020 6:57 pm

Hi Phil
There will be considerable volt drop at the battery terminals due to the internal resistance of the battery.
Measure there first when cranking to see what you are starting with.
A battery usually reads 13 to 14 volts, but that is with no current flowing and no internal volt drop.
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PostPost by: pharriso » Tue Sep 29, 2020 8:28 pm

ericbushby wrote:Hi Phil
There will be considerable volt drop at the battery terminals due to the internal resistance of the battery.
Measure there first when cranking to see what you are starting with.


Eric, the new Bosch battery was measuring 11.45v at the terminals during cranking (coil disconnected so would not start.)
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PostPost by: gjz30075 » Tue Sep 29, 2020 9:11 pm

Phil, no coil ground on mine, either. Pictured is how it's wired (as per stock). Ignore the neg side of the
coil, the one with the green wire. White with blue stripe wire is straight from the solenoid to the coil. That's how it gets the 12v for cranking. Once started, the white wire takes over (key is now in the on / run postion) and the voltage gets routed through the ballast, knocking it down to 8v or so.

Maybe your solenoid is going bad.
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PostPost by: pharriso » Tue Sep 29, 2020 9:43 pm

gjz30075 wrote:Phil, no coil ground on mine, either. Pictured is how it's wired (as per stock). Ignore the neg side of the
coil, the one with the green wire. White with blue stripe wire is straight from the solenoid to the coil. That's how it gets the 12v for cranking. Once started, the white wire takes over (key is now in the on / run postion) and the voltage gets routed through the ballast, knocking it down to 8v or so.

Maybe your solenoid is going bad.


Greg, yours appears to be the original factory installation, with a low resistance coil (1.5 Ohm) & seperate ballast resistor.I have a 3 Ohm coil & no ballast resistor.

Because the voltage is dropping so much while cranking on the regular ignition feed (white) wire I am having to add the blue striped wire to the same terminal as the white wire on the coil, it is supplying 9.9v while the regular white wire is only supplying 8.6--9.2v.

At this point I have 3 choices:

#1 Figure out why the white ignition wire is dropping to 8.6v, rather than 9.9v & fix it. Looking at the wiring diagram, the battery voltage is fed to the ignition switch from the main lug on the starter solenoid, which supplies both the white ignition wire (thru Tacho & anti-theft switch) & the starter solenoid engage signal. Maybe there is a poor connection somewhere during cranking? When running I get full voltage at the coil.

#2 Live with my current wiring with the extra blue/white wire from the solenoid to the coil

#3 Stick with the same white & white/blue feeds as #2 & as original, but go back to a ballast resistor & low resistance coil.

Maybe this issue is the whole reason the ballast resistor/low resistance coil system was invented? But why does that centre terminal on the solenoid have 10volts when the white wire is only seeing 8.6-9.2v? :roll:
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PostPost by: 2cams70 » Wed Sep 30, 2020 3:25 am

Keep your voltmeter probes between the coil +ve (or -ve if it's a +ve earth vehicle) terminal and ground (check of course that the coil has been wired the correct way around according to battery polarity - i.e +ve or -ve ground)

Reading is around 9V with ignition key in the start position and cranking - check.
But what is the reading directly at the coil with the ignition key in the run position??
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