Lotus Elan

Ballast resistor?

PostPost by: Robbie693 » Sun Jun 06, 2010 4:28 pm


I seem to have a missfire at high rpm when the engine is hot so I'm suspecting the coil as everything else is new.

First off, the rad lower hose is touching the coil - would this cause the coil to get too hot and develop a missfire? When cold, all is fine..

Second question, I seem to have the wiring in place for a Ballast resistor set-up but can't find one anywhere. Also the coil, a Lucas sport DLB105, is for non ballast systems. There are two wires to the coil, a white and a white/yellow. The white/yellow goes to the solenoid and the white goes to the tach as far as I can tell (they disappear into the loom). So, what are the chances of a ballast resistor being in the loom? Also, if there is no ballast, does the extra wiring do anything else? Should I remove it or leave as is? Starting has never been a problem by the way.

Any ideas please? Pics of wiring:
coil.jpg and
solenoid.jpg and
tach.jpg and
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PostPost by: Higs » Sun Jun 06, 2010 5:23 pm

Yes - your wiring is for a ballast resistor. When the ignition key is turned for starting, the solonoid for the starter motor is "fired" with the white/red wire from the ignition switch. At the same time, the white/yellow wire (effectively connected to the white/red wire at the solonoid) energises the coil. Hence the coil "sees" the maximum voltage that the battery can give when cranking the engine (8-9volts). The coil is therefore sized to work with this voltage.

When the engine fires and the ignition key is released, the white wire from the ignition switch is energised and this goes to the coil via the tacho and the ballast resistor. Because the white wire will have 12+ volts on it now, the ballast resisror is required to reduce the voltage at the coil to the same 8-9v.

The intermittent current through the white wire (as the points make and break the circuit) is "read" by the tacho and hence it measures the rpm.

It looks like you do not have a ballast resistor coil - in which case the only thing that happens is that you lose out on the "cold start" performance of such a system.

[Some british cars did not use a ballast resistor but resistive wire instead in which case your coil would only ever see 8-9v and would give a low energy spark when running. To my knowledge, Lotus did not use this.]

However, do not remove any wiring as your ignition switch is set up for this circuit. If you were to remove the white/yellow, the engine would turn over but not fire; if you removed the white wire, it would turn over and fire up but immediately stop when you release the ignition key!

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PostPost by: pereirac » Sun Jun 06, 2010 7:12 pm

When I fitted a similar Lucas coil I took the ballast resistor off. Never had problems since... One less bit of wiring to go wrong


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PostPost by: bob_rich » Sun Jun 06, 2010 7:13 pm

Hi Robbie

As the coil gets hot its resistance will increase and so the available current will decrease. Copper wire,with which the coil primary ( and secondary) is wound changes its resistance by around 30% from room temperature (20C) to 100C. This will result in less current build up during the points closed period and so less energy and thus reduced spark voltage. At higher engine output with wider throttle opening and more revs the density of the charge in the cylinder will increase so the required voltage for a spark goes up. Could be this is the problem? Other possibility is the rev limiter in the distributor is coming in a bit earlier because the mechanism is a bit worn.

I would try and keep the coil as cool as possible

hope this helps best of luck

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PostPost by: alaric » Sun Jun 06, 2010 7:48 pm


If I remember correctly if you have a coil that doesn't need a ballast resistor then the extra wire - the one that's switched to 12V only while cranking - isn't needed. It can be left connected if you like but shouldn't make much difference. The other wire is permanently connected once the ignition is turned on - at least mine is, and the car starts no trouble at all. I've left the starting wire hanging loose at the solenoid end for now. So, following the other posts I would suspect the coil has developed a short that shows up once hot. It may be worth checking whether the coil that you have should have a ballast resistor or not - maybe that was removed by someone in the past.

Hope you solve it soon.

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PostPost by: billwill » Sun Jun 06, 2010 10:10 pm

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PostPost by: neilsjuke » Mon Jun 07, 2010 7:47 am

Wiring fine, something I do like to recommend on elan coils using a earth braided strap to the chassis in case a coil fault allows a ht spark to jump to the carbs / fuel line or your hand a bit of HT is good for waking you up up some will have it for radio suppression for the suppressor earth.
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PostPost by: Robbie693 » Mon Jun 07, 2010 2:27 pm

Great information and advice - thanks all.

I think I'll get a new coil to be on the safe side and try to move it away from the water hose. I don't want to start cutting holes in the body so maybe i can make a bracket so I can rotate the coil around the front bolt (the one with the earth strap).

And I'll leave the wiring as it is!

Thanks again

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