Feedback on intermittant Astra engine C14NZ cutout fault

Firstly, appreciation to all who responded to my post just before
Christmas concerning an Astra fitted with the C14NZ type engine which
had an intermittent engine cutout fault

Anyhow, I think feedback on this problem would save a lot of stress
to others who might experience intermittent engine cutouts on this
model as this one drove me nuts despite all the test equipment
equipment to hand and caught me out. Just to recap here's the fault
now with the test equipment used and eventual remedy:-

As said, engine would intermittently cutout at random for no reason
during a journey. After approximately five - ten minutes after the
fault appeared, engine would then restart without problem and could
then run for minutes, hours or days before again re-appearing (nasty).
It was established during fault presence that no spark was to be seen
at plugs. Voltage measurements at ignition amp module and LT coil
showed 12 volt power supply presence as per Haynes circuit diagram.
DC resistance measurements with Digital Meter on resistance range both
with and without fault present showed no difference in ignition coil
resistance readings both on LT and HT side of ignition coil (so no s/c
in windings turns or o/c coils detected) even with the accuracy of the
DVM used. OK its a DC test which is meaningless for inductance testing
but it proves there's nothing abnormal with the coils in the fault
mode. Conclusion by myself together with other comments canvassed
from newsgroups suggested ignition amp module located beneath
ignition coil to be culprit which seemed logical at the time. Anyhow,
brand new amp unit was purchased and fitted but provided no cure as
fault still remained.

Oscilloscope waveform examination of the 12 volt pulse square wave
output from the distributor's Hall Effect I.C. pin which is not a
pulse coil as it's a chip which is an integral part of distributor.
Its job is to sense engine rotation and oscillagram showed that
square wave pulses were present on engine rotation with starter motor
(distributor cap to coil HT lead disconnected) both with fault
present and not. The distributor's Hall chip also requires a 12
volt power supply and a ground to its remaining two pins for its
operation which also proved to be correctly present with voltmeter
reading. Anyhow, due to difficulty in connecting oscilloscope probe
to the ignition amp module's input pin, which as already stated had
been replaced with a new unit, suspicion than began to fall on Engine
Management Unit (ECU) which lives in the drivers side foot well. The
ECU relies on the distributor's chip pulse output on engine rotation
whilst simultaneously reading other sensor inputs dotted around
engine. It then computes distributor's pulse input in correlation to
other engine sensory inputs to the ECU before sending a suitably timed
ignition pulses to trigger the amp module through the LT side of the
ignition coil to instigate a cylinder spark. Even probing with an
analogue multi meter set on voltage range between ignition amp module
and LT coil connection showed that meter needle correctly pulsed up
and down on engine starter motor rotation even with the fault present
suggesting the ignition amp module was correctly being triggered by
the ECU but still no spark - if your still with me, baffled (remember
the HT coil is not faulty)

The upshot is that the vehicles appeared to have no fault at all as
components appeared to be correctly testing out. However, with weird
faults like this, one begins to suspect the engine management unit
(ECU) which is costly item to replace if a s/h unit not available.
The ECU accepts pulses from the distributor before it in effect passes
them on to the ignition amp module to trigger LT inductance and
ultimately a spark. Not being convinced the ECU was at fault thinking
the output mark/space ratios were not of suitable duration to
correctly trigger the amp module, I sent vehicle to an Auto
Electrician for a second opinion. He then had a nice time and some
stress trying to find the fault over the next two days before
eventually he diagnosed problem.

The Auto Electrician explained that the Hall effect IC in the
distributor had gone faulty in such a way that wasn't immediately
obvious. It appears that you need a nice clean square wave voltage
drive from the distributor's Hall chip (i.e.+12v which then sharply
drops to 0v on oscilloscope waveform for ECU to recognise a valid
pulse input from distributor's chip as engine rotates). However, in
the fault condition the distributor's Hall switch was only going down
to 0.8v volts above ground level and not to 0v but its peak voltage
remained correct at 12 volts on oscilloscope. It appeared the ECU was
getting confused with this input and it subsequently provided an
output albeit at a source impedance (current drive) which is
sufficient to drive amp module input but not quick enough to sharply
collapse LT's built up magnetic field via amp. As mentioned above,
analogue meter readings will indicate pulse presence at the LT coil
via up/down needle movement giving the illusion that all is well on
starter motor rotation but appears not to be enough to cause
instantaneous collapse of the LT's magnetic field and energy coupling
to HT side of the coil. As the engine can only be rotated in fault
mode with the starter motor, it adds to the illusion that all
components are functioning correctly due to starter's relative slow
rotation of engine.

I am not a car mechanic but an Electronics Engineer by trade so
experienced mechanics will no doubt have knowledge of common faults
appearing on these models and no doubt prefer component substitution.
This can prove costly to us DIYers if the suspect component is not the
problem so one has to deal with the fault with sensible testing
procedures. A Gunson Fault Code Reader also indicated nothing wrong
with engine and flashes 12 for normal indicating no fault despite the
obvious fault condition as the computer does not log fault code
thinking there's nothing wrong.

Hope this info is of help to others out there with the same engine
type before we all die on madness.


regards
Dave Morgan

 

Dave Morgan,
Phew....
Now I know EXACTLY why I had a spare, complete, known good
Dizzy, Ignition amp and coil when I worked for Vx!...
It's great that you have the time, Paitience and
(considerable) knowlege to achieve the results that you have posted.
But, Imagine your car the dealer's I worked at, and being presented
with a bill for the time you have spent working on your vehicle at (at
time of leaving) £52.88 per hour + VAT
A simple substitution of parts would give the same result
in a fraction of the time, thus saving time and money for the
customer. As a technician, I applaud your logic and explanation of a
simple problem, solved in a complex way.
As a customer, I would not want to pay your bill, Even
though your explanation of the fault would be, in no doubt, exact.
On reflection, this posting reads derogratory. It's not
meant to be. If there were no people who could explain these faults,
no-one would understand.
Ever considered teaching?
Regards.
Bilko130

 

Maybe the post is a bit long winded but the fault drove us all nuts so
needs explaining.

I agree with you that component substitution is a quicker and a
preferred method of locating a problem like this but this could have
been a costly mistake. Even two mechanics I consulted concluded this
problem was with the ECU when it fact it wasn't.

Anyhow you are probably right with hindsight, replace the distributor
first by substitution if you get an intermittent engine cutout fault
and see if it cures the problem. However, at £100 odd pound for a new
unit, you have to be sure its faulty if you haven't got one on the
shelf to substitute.

regards
Dave

 

So maybe some dealers / stockists could take the initiative here, and offer
a small range of such "hard to diagnose" components for hire (at say 5% or
10% of full price + returnable 100% deposit). They could attract quite a
bit of business, I imagine.