Lucas AB14 ignition module: NOT normal failure

Was doing some work on my '87 today. Had driven it a couple of days before with no issues. After doing the work (new fuel pressure regulator) it wouldn’t start. Thought it needed priming, but multiple cycling of the ignition to build pressure didn’t allow it to start. Tried starting fluid to rule out ignition, not even a pop. Had another AB14 ignition amp, plugged it in and car started right up.

I had recently replaced the 4 PIN GM module inside the faulty AB14 with a new, genuine GM module. Thought it might have been infant mortality of that module, but tested the rest of the unit. Capacitor was good, resistors good. The Zener diode was shorted. Multimeter leads across it showed same 6 ohm resistance in both directions. Hmmm, that shorts the GM module to ground. Clipped the lead on the diode, retested the AB14 amp, and it worked.

Supposedly, new GM modules have built in protection, and no longer need the 350v zener diode. New diodes of that type are unavailable. Supposedly, new AB14 modules don’t include the zener; can anyone verify?

A page I found online, PDF below, says you can put lower voltage zeners in series as an equivalent replacement. I’ll try that unless someone can verify new AB14 amps don’t use zeners.


Jaguar Lucas CEI Amplifier repair-XJ6-1.pdf (434.8 KB)

1 Like

What I’m about to post doesn’t answer your question, is all my opinion, and is not backed by any firm knowledge. And I’m not an expert. That said…

I believe that the original GM HEI ignition didn’t use the Zener–it was unique to Jaguar’s implementation of the GM module into a separate package rather than in the dizzy itself. Back in those days (60s) the breakdown voltage of NPN transistors (BVceo) for use in driving ignition coils was of concern. The output transistors in the modules were specified to survive the environment–the “back EMF” of the coil primary when the output transistor was turned off–leaving the collector to receive a couple hundred volts. This voltage wa substantially more if the sparking plug (or plugs) were accidentally disconnected–so that the secondary voltage was not limited to what it was just as the spark commenced. So it was good British conservative design IMO to fit the Zener. It would limit the back EMF, particularly if the secondary was open circuit, thus saving the HEI module. And, also typically Jaguar, forget about sourcing the Zener should you need to replace it. :slightly_smiling_face:

Whilst on this subject, IMO the fitment to some (but not all) cars of a series resistor between the HEI output (the collector terminal of the transistor discussed above) and the Ignition coil (-) terminal is similarly explained. When the transistor switches off (like points opening) the back EMF transient can have a rapid rise time, particularly when the plugs are disconnected. That means a lot of energy in a short time–very high power. As mentioned, that can fry the Zener, or the output transistor if the Zener isn’t used. A small resistance (compared to that of the coil) added in series reduces only slightly the current flowing through the coil (and the energy of the spark). But after the coil is switched off by the transistor, the current driven by the back EMF into the Zener (or HEI) is reduced (and so is the peak power). The resistor slows and diminishes the effect of the back EMF. Perhaps not really necessary (and sometimes skipped) but it was common practice in the day to “filter” transients from pricey semiconductors in this way. I can just see some Brit deciding to add the resistor in exchange for peace of mind. Note that in this application the resistor is not ballast for the coil in the usual sense nor is it bypassed to supply more current during starting (that’s done in the design of the HEI chip).

It was Lucas, and it was a good improvement for the otherwise hard to beat HEI IC. They knew what they were doing at Lucas, and they had some very capable engineers.

1 Like

Transorbs ( or tranzorbs to some ) are now the electronic component more often used for transient suppression. They are readily available in a wide range of voltages up to hundreds of volts. They can be unipolar like a zener diode, or bipolar. You can solder lower voltage types in series to give higher voltage protection and more power absorption.
They are not packaged in a metal can like the Lucas zener.
It is possible that GM modules have a built in transorb.

1 Like