I did a few simple tests on my ignition last night and found a few surprises. First off the coil wire had 54K ohms of resistance and should be less than 5K ohms -- the engine runs but this can't be good.

Also I have the Ducellier type coil with the external ballast resistor. The primary resistance measured 1.7 ohms and should be between .8 and 1 ohm. I'm taking this to mean I should replace the coil as well as the coil wire.

On the good side, the ballast resistor was 0.8 ohm which is still within tolerance and the plug wires were also within normal range as per the Service Manual.

I have heard of people tossing the Ducellier and putting an aftermarket coil in its place while still keeping the ballast resistor in the circuit. I have an old Jacobs cylinder type coil lying around that will work. It's primary resistance is just .6 ohms so I wonder if it will overload the ignition module and cause it to fail sooner than it would if I put another Ducellier back in?

I also need a new coil wire. Just wondering if there are any thoughts on aftermarket wires like Magnacore, Accel, MSD etc. or should I just would with the carbon based stock wires?

 

Any coil with about 1.0 ohm primary resistance should be OK. I prefer the Lucas DLB170 but that's just me.

Many guys dump the resistor. I've yet to find anyone who can conclusively say why Jaguar decided to even use one. Many Ser IIIs didn't. I think they became standard fit at about vin 421xxx or so.

The old 4.2 doesn't care what plug wires you use. Any name brand is fine. I've used Bosch and Rists (OEM) with good results

Cheers
DD

 

My guess on the ballast resistor is that it was added in the later years (86-87 I believe) to prevent the ignition modules from burning out prematurely. The ballast resistor drops the voltage seen across the coil (and the voltage the module has to switch) from 13-14volts to about 9-10 volts.

These days however, there are coils with internal ballast resistors so it is sometime hard to tell what is installed.

 

Mu understanding of ballast resistors was that they protected the coil, and were switched out, (bypassed) on engine start when voltage is low, then switched back in when the engine starts and the ignition key is returned from the start position. The coils were designed to produce a good spark at lower volts than the normal 12 but would overheat at normal voltage, hence the ballast.

Was this altered on the Series 3s ?

 

Interesting theory Fraser,

I had the volt meter out today so I connected it on the positive post of the coil and the ground lug of the battery to observe the readings.

With a cold motor and the key in the 2 (run) position I got 11.5v. Since the battery voltage was almost 13.0v I assume the ballast resistor was in the circuit and accounted for the difference in voltage.

After starting and with the engine running, I got 9.6v. This value steadily dropped to a stable 8.5v as the engine warmed up.

At 2000rpm, the voltage ramped up to 9.6v but went right back to 8.5v at idle.

My conclusion?

- The ballast is in the circuit under all conditions because there was
always a voltage drop across the ballast.
- The ballast resistance increases as the ballast warms up which leads
to a lower voltage across the coil as compared to when the engine
first starts and the ballast is cold.

The ramping of voltage up and down with rpm was interesting but may have more to do with the duty cycle of current through the primary winding with the engine running and the fact I was using a meter that averages the reading over time. I may drag out the scope to observe this just to confirm this is what is going on.

 

What you're describing is a bypass resistor, yes. That's the usual configuration.

On the Ser III 4.2, though, the resistor is "in ine", so to speak, at all times

Cheers
DD