An XJ-S owner recently purchased a brand new AB14 ignition amplifier, not
from a Jag source but from Advance or AutoZone or some similar auto parts
chain, and he opened it up and sent me a photo of the innards. Everything
looked essentially similar to the OEM amp with one notable exception: The
zener diode pressed into an opening in the top of the case was missing. In
fact, this being a new casting, there was no hole there – although the boss
was still there, you could see where to drill a hole if you chose to.
What was there instead was a wire that disappeared into a length of heat-
shrink tubing, and what emerged from the other end was an eyelet that was
screwed down to one of the HEI module hold-down screws to ground it.
From the shape of the heat-shrink it was evident that there were two
cylindrical devices hidden therein. The owner reports that, testing with his
VOM, this arrangement appeared electrically identical to the OEM setup.
My presumption is that the single 350V press-fit zener diode has been
replaced with two inline zener diodes, perhaps 175V each or perhaps one
200V and one 150V. I did a brief search for zener diodes online and I
couldn’t find ANY that were press-fit like the original used here. I found stud-
mount zener diodes, which are similar in configuration, but I couldn’t find any
rated higher than 200V.
This brings up several questions for the electrical gurus here: First, is it
kosher to connect zener diodes in series? Do the voltages add? Does the
cumulative current rating become the lowest current rating of the individual
zener diodes in the series?
Second, I had long presumed that the press-fit zener diode was pressed into
the case to serve as a heat sink. Is this not necessary? Is the current rating
of a zener diode independent of its cooling? If someone tries to start their
Jaguar with the HT lead disconnected from the coil, are these little inline
zeners likely to melt down the heat-shrink tubing surrounding them?
Does anyone know what amperage the original zener diode was rated at?
And what amperage rating this application truly calls for? Is the amount of
current drawn by the ignition system during normal operation a good
indication? Or would there be current spikes here that call for higher
amperage ratings?
I didn’t ask this owner to cut open the heat-shrink tubing, although it’d be
easy to do; after checking out the components, a new piece of heat-shrink
could be slid on and shrunk into place. But ideally I’d like to know what
readily-available zener diodes could be used for this job. If we can learn
from whatever this aftermarket manufacturer did, it might provide a scheme
by which we all could rebuild our own AB14 amps. The GM HEI module,
capacitor, resistors and wires are all easy enough to find, the ONLY difficult
item to source has been that pesky zener diode. True enough, it doesn’t
often fail, but anyone looking to renew everything would want to know what
can be used in its place. A pair of inline diodes wrapped in heatshrink would
be easy. The original zener could simply be disconnected and left to plug
the hole in the case, keeping the appearance original.
– Kirbert
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