Ignition coils

[PaulGover]

This is mostly for amusement or information - my problem is solved.

My 1986 4.2 Series III was last running in early summer, but exhibited the problem of fuel getting pumped from the LH tank to the RH one. It's a well-known issue, and the cure is to replace some or all of the three solenoid-operated valves in the fuel system.

Come last week, I did just that, but the damned thing wouldn't start. I put the battery on charge and tried again, still no joy. Cleaned the plugs (all gapped a bit wide, but not a lot), checked for a spark - something, though a bit feeble, and for fuel flow (slightly loosening a pipe clip produced a weep of fuel, so there was something there).

So next to investigate that week spark. The coil has a nasty-looking appendage on one side to which two wires connect. It looks like the ballast resistor, except I thought that was switched out of circuit when operating the starter, and this was always in circuit. Investigating further, I have the electronic ignition and yes, it is a ballast resistor, though nothing like any of the pictures. This one looks typical Lucas "just good enough" engineering - metal pressings held together with rivets, and a 1 ohm wire-wound resistor, in a simple metal case. The rivet connecting the wires to the resistor looked wobbly, so I played with it a bit, whereupon it fell apart.

Reading more articles, the resistor isn't totally necessary, so I removed it and connected the wires directly to the coil. Whereupon, it starts on the switch first time, just like an EFI engine should! Better than before - it was reluctant to start the first time each year, needing a lot of turning, thereafter it would start on the switch. I suspect corrosion crept in during the lay-over, and needed a bit of usage to get a good connection.

For good measure, I'll replace the coil with the listed one (DAC3001 equivalent, around £20 from the usual suspects), which AFAIK has any required resistor internally.

Now onto the cold air-con - at a guess, the car has a non-functioning or possibly missing thermostat; I have some lying around...

[piman]

Hello Paul,

the ballst resistor start is a clever idea and does work.
If you have the correct ballast resistor coil fitted (I.e. it hasn't been replaced with a standard coil) it will burn out if the resistor is removed as it will permanently be fed too much voltage. A standard 12 v coil has a primary resistance of about 3 ohms i think, and the Ballast coil about half that. A ballast coil directly fed 12volts will draw twice the current it is designed for.
Basically the sytem is that the coil supply is connected to one side of the resistor and the other side of the resistor is connected to the coil +.
There shoud be a small terminal on the starter motor which is live when cranking and a wire connects this directly to the coil +, this gives boosted voltage to the coil as the resistor is by passed (At crank, say the voltage drops to 10volts this is directly fed to the 6volt coil so is actually a higher voltage than normal when the resistor is in circuit). and the sytem is deisigned to overcome the drop in voltage when cranking.
A failed resistor gives the characteristic symptom of it firing when cranking then stops again when the ignition switch is turned back to run.
I supect that at some time your car has a 12volt coil fitted instead of the correct 6 volt one. Confusingly I have seen ballast coils marked 12volts but a meter can easily differentiate the two.

Alec

[PaulGover]

Alec, thanks but no. This is an '86 car with electronic ignition. The ballast resistor and cranking bypass setup is for old-style contact breaker points ignition, where it does indeed work well. But that's unnecessary when there's electronics to control the current. In these later cars, there's no bypass wiring. In this case, the ballast resistor was permanently in series with the coil.

The 1983-onwards wiring diagram shows no ballast resistor or bypass circuit; the 1985 parts manual shows just a simple coil - the DAC3001 I mentioned. I read somewhere that the later coils (unspecified) had built-in ballast resistors as appropriate. The coil fitted to my car looks untouched, and the previous owner was meticulous in his maintenance (like, a full service, oil and filter changes, despite only doing about 1000 miles a year), so I suspect the Jaguar parts bin was a bit empty when this one was built, so they used a different coil.

I'm not too sure why there's a resistor at all, as the electronics should limit the current through the coil; it might be to do with interference suppression, in the same way modern HT leads are resistive, but I'm no expert.

[piman]

Hello Paul,

I was not aware of that.
I do not understand any need for a ballast unless there was the resistor by pass from the starter motor as a starting aid? However removing or by passing it will increase coil primary current at normal running.
Limiting imput current is puzzling as the coil can be wound for the corect current, nor does electronic limitation make sense. The secondary (HT) coil voltage is highest when the primary power is broken cleanly giving maximum field collapse.

It will be interesting to see what effect a new coil has.

Alec

[PaulGover]

I expect a new coil will have no visible effect. I've read the opinion elsewhere that the ballast resistor on electronic ignition systems reduces the heat generated in the electronics module. This makes sense; the module's purpose is to reach a specific current through the primary winding as quickly as possible, to provide the best spark at any engine speed. That means that once it's reached that current, the transistor in the module limits the current, and therefore acts as a voltage drop, and that means the transistor generates heat.

Using a ballast resistor dumps that heat outside the module. In the case of the old coil, the ballast resistor was a length of wire wrapped round an insulating rod, inside a ventilated tin case - i.e. a miniature 1-bar electric fire! Without that resistor, the module will have that heater inside; the transistor(s) can probably stand that for some time, but extended idling in hot (Abu Dhabi or California say) weather might be too stressful. I guess the new coil has higher resistance windings, and a case that can dissipate the heat generated directly.

[PaulGover]

Yup, just tested. The old coil was 1.6 Ohms plus 1 Ohm ballast resistor. The new DAC3001 coil is 2.4 Ohms, so a little more heat in the electronics, but the majority will be dissipated in the coil. (The old coil might have been a DBC1140, which IIUC was fitted 87 onwards, but it doesn't look like any of the pictures.)