Does anyone know where I can buy the 47 volt zener diode
inside the 6CU ECU for my 1984 XJ-S? One of the two zeners
is shorted and causes the B-bank injectors to stay open. I
would prefer one that will fit properly inside the heat sink
housing. Also, is it anode-to-case and what is the wattage?
Thanks, J.D.–
JDiii
Salt Lake City/UT, United States
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JDiii wrote:
Does anyone know where I can buy the 47 volt zener diode
inside the 6CU ECU for my 1984 XJ-S? One of the two zeners
is shorted and causes the B-bank injectors to stay open.
Ooooh! Does it hold the full-power circuit on, or the hold-open
circuit? If the full-power circuit, did you burn up some injectors?
I
would prefer one that will fit properly inside the heat sink
housing. Also, is it anode-to-case and what is the wattage?
When you work all this out, please tell us all about it so I can add
it into the Book!
– Kirbert
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In reply to a message from JDiii sent Sun 16 Aug 2009:
J.D.
First study the old part itself to obtain the part number if you
don’t already have it. Then try searching the electronic suppliers
such as Newark Electronics (http://www.newark.com/); Mouser
Electronics (http://www.mouser.com/); Digikey
(http://www.digikey.com/); or perhaps even Crestwood Technology
Group (fairly high minimum order (CTG123.com). All are reputable
suppliers.
What is the part number? Hopefully, it is a standard industry
number which will make short work of finding the answers to your
questions.
By the way, how did you determine you problem was with this diode?
Regards,–
The original message included these comments:
Does anyone know where I can buy the 47 volt zener diode
housing. Also, is it anode-to-case and what is the wattage?
–
Paul Fitzgerald 90 XJ-S Convertible
Palmdale/CA, United States
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In reply to a message from Kirbert sent Sun 16 Aug 2009:
The 47 volt zener is the one that protects the output
transistors. I do not think that any of the injectors were
burnt because they all have the same resistance between
their terminals. Fuel leaks from the exhaust manifold when
all six injectors are connected and doesn’t leak when they
all are disconnected. I haven’t done any other testing like
trying to supply 12 volts to each one to verify if I hear a
click.–
The original message included these comments:
Ooooh! Does it hold the full-power circuit on, or the hold-open
circuit? If the full-power circuit, did you burn up some injectors?
–
JDiii
Salt Lake City/UT, United States
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In reply to a message from Paul F sent Sun 16 Aug 2009:
The old zener does not have a part number. It looks like the
old silver can style transistors but without a lip and has a
slightly conical base with one terminal sticking out (the
other being the case). The diameter is between 9 and 10 mm.
None of the electronics suppliers seem to have anything
resembling this.
I compared the two zeners on the heat sink using a
multitester. This one was a short. I have not checked any of
the other components because the lacquer has to be scraped
off first and because this output protection zener was a
short and not an open.–
The original message included these comments:
First study the old part itself to obtain the part number if you
don’t already have it. Then try searching the electronic suppliers
By the way, how did you determine you problem was with this diode?
–
JDiii
Salt Lake City/UT, United States
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JDiii wrote:
I compared the two zeners on the heat sink using a
multitester. This one was a short. I have not checked any of
the other components because the lacquer has to be scraped
off first and because this output protection zener was a
short and not an open.
So: Dyke the wire to that shorted zener and start up the car and see
how it runs. If it’s just for output transistor protection, it
should run fine with it disconnected. You can take your time finding
a replacement.
– Kirbert
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In reply to a message from Kirbert sent Mon 17 Aug 2009:
Recently each problem that I’ve had with this car has seemed
to cause another problem. I’d prefer to replace the diode
before closing up and reconnecting the ECU. I wouldn’t be as
concerned if I had the circuit diagram for the ECU.–
The original message included these comments:
So: Dyke the wire to that shorted zener and start up the car and see
how it runs. If it’s just for output transistor protection, it
should run fine with it disconnected. You can take your time finding
a replacement.
–
JDiii
Salt Lake City/UT, United States
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I compared the two zeners on the heat sink using a
multitester. This one was a short. I have not checked any of
the other components because the lacquer has to be scraped
off first and because this output protection zener was a
short and not an open.So: Dyke the wire to that shorted zener and start up the car and see
how it runs. If it’s just for output transistor protection, it
should run fine with it disconnected. You can take your time finding
a replacement.– Kirbert----------------------------------------------------------------
That might work for a limited time.
Lucas was no charity, if they put a zener in there it was intended to
protect something.
If transistors are switching an inductive load such as an injector,
you can have voltage spikes.
Sometimes a zener or similar device is working all the time to limit
voltage spikes, sometimes it is designed only to take care of a
problem developing elsewhere in the circuit or to compensate for
operator negligence, or for unusual events such as nearby lightning strikes.
Zeners are not so popular anymore for limiting spikes. Since Lucas’s
time there is a whole new family of devices to do that. One is
varistors, the other transorbs. Transorbs are readily available from
electronic suppliers in voltage up to 200V, maybe more. They come in
two flavours, unipolar and bipolar. To mimic a zener you need unipolar.
They are way better at spike suppression.
We still buy small zeners for various applications, but we use many
more transorbs. High power zeners, above 1 watt, are probably hard to
find nowadays.
Richard Dowling, Melbourne, Australia.
1979 XJ-S coupe + HE V12 + 5 speed, 1988 XJ-S V12 convertible, 2003 XJ350
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At 20:09 2009-08-18 +1000, Richard Dowling wrote:
Lucas was no charity, if they put a zener in there it was intended to
protect something.
If transistors are switching an inductive load such as an injector,
you can have voltage spikes.
Of course, you’ve got to ask yourself: what caused the zener to short?
— '88 Jaguar XJ-SC 5.3L V12 (LHD) ‘Black Cat’
Sean Straw '85 Jaguar XJ-S 5.3L V12 (LHD) ‘Bad Kitty’
Sonoma County, California '91 Jaguar XJ40 4.0L (LHD) ‘Trevor’
http://jaguar.professional.org/ '69 Buick GranSport 455 V8
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In reply to a message from sean.straw%2BJaguar@mail.professional.org sent Tue 18 Aug 2009:
The typical failure mode for a zener is to short. In most
applications the failed shorted zener continues to protect
what it was intended to protect, so it’s usually a good
failure mode and the engineer designs the circuit with this
in mind.
Why does the zener short when it fails? I would presume it
is a breakdown of the P/N junction over time, from the
‘abuse’ of the back-emf spikes from the load.
Operation without the zener would be a bad idea.
The purpose of the zener is of course to limit the back-emf
voltage from the load from exceeding a level the transistors
can tolerate.
.–
The original message included these comments:
protect something.
–
87 XJ-S V12, vin 135xxx ---- PICS: www.myrq.com/jag
Evington, VA, United States
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In reply to a message from JDiii sent Sun 16 Aug 2009:
I replaced the zener diode with a TVS (Transient Voltage
Suppression) diode (also known as a transorb or Transil or
an avalanche diode), changed the engine oil, and drove the
car for over an hour to make sure that the diode’s power
rating was sufficient. At first smoke poured out of the left
tailpipe because of the fuel that had collected in the exhaust.
I used an 8 watt TVS diode from mouser.com with a breakdown
voltage of 47.8 volts. The part number is 576-5KP43. I later
noticed that the manufacturer has a minimum and maximum for
the breakdown voltage (47.8 and 52.8 volts respectively)
which may mean that this diode is not the most appropriate.
Can someone confirm this? As the diode is approximately 1 mm
smaller in diameter than the heat sink sleeve in the ECU, I
used part of a clamp-on heatsink as a filler. There is no
space inside the ECU for a separate heat sink unless one
removes the existing sleeve. This diode has two terminals
which means that one terminal has to be attached to the heat
sink.
Thanks to all those who replied to this thread.–
The original message included these comments:
Does anyone know where I can buy the 47 volt zener diode
inside the 6CU ECU for my 1984 XJ-S? One of the two zeners
is shorted and causes the B-bank injectors to stay open. I
would prefer one that will fit properly inside the heat sink
housing. Also, is it anode-to-case and what is the wattage?
–
JDiii
Salt Lake City/UT, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php –
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