Spark and Fuel Diagnostics
I’ve observed that many 928 owners are trying to diagnose problems
with their cars. I’m pleased to share some of the facts I’ve learned
since I began providing LH module rebuild service.
I must advise the reader that extreme care must be
taken when working near high voltage spark sources.
IF YOU DO NOT KNOW WHAT YOU ARE
DOING, DON’T DO IT. You can hurt yourself and damage your car.
Have a qualified Porsche technician do the work, in the
long run you’ll save money (no self-damaged parts) and you won’t get
hurt. Odds are your Porsche shop may not know this detail of information,
so feel free to take this information to them, it could save you money by
reducing the time required and improve the quality of the diagnosis.
There were two different generations of LH fuel
injection controllers used during the life of the 928.
The model year 1984 – 1986 Euro/ROW and 1985 –
1986 US/Japan 928s used an 8039 microprocessor-based design and have a
25-pin connector to the module.
For model years 1987 to 1995, the LH module was based on
an 8031 microprocessor and the increased functionality required a 35-pin
Spark control also was supported by two different
generations of modules.
The 1984 – 1986 Euro/ROW and 1985 – 1986 US/Japan
928s use a 25-pin EZF controller and an inductive RPM pick-up and had a
vacuum coupling provide engine load information.
From 1987 onwards, a 35-pin EZK module was used. The EZK
had a three-dimensional RPM, engine load and spark advance map stored
within the ROM code, so no vacuum connection was needed.
The EZK system used an inductive pick-up for RPM signals
plus a Hall Effect sensor to recognize which ignition coil gets the next
spark signal. Also, knock sensors were added to the spark control system.
On-going improvements and fine-tuning were achieved by changing ROM codes,
so modules were often specified to specific year applications.
When diagnosing no-start
situations, ALWAYS clean and tighten connectors, check ground
connections and inspect for broken wires. Only after those potential
problems are checked-out should a module failure be investigated. The
928 workshop manuals are a necessity for diagnosis, the system is
complex and the interconnections are not intuitive.
Here are some hints to follow if your LH-equipped 928
isn’t starting or running well.
It is not commonly known that the LH fuel injection
module is slave to the EZx spark control. So if you have a no-start
condition, always check for spark first. If there is no spark, fix that
problem before testing for LH ECU functionality.
The EZx spark unit looks for rotation pulses from the
engine RPM sensor as a condition to start the spark system. The EZx then
turns on the LH ECU, so a bad EZx ECU may cause the fuel injection not to
work. Therefore, always check for RPM pulses from the sensor located at
the top of the bell housing for a no-start & no-spark conditions.
Both systems use the inductive sensor (master-sensor).
Inductive RPM sensors generate voltage so a VOM can be used for testing.
For the EZF, it is connected at pin 19 and 7, where pin 7 is connected
internally in the ECU to pin 12 (master-ground). Pin 20 is the shield of
the sensor cable and is connected through internally to master-ground pin
12. For the EZK, pins 23 and 6 are used where pin 6 is connected to
master-ground pin 18. Pin 24 is the shielding of the sensor cable and is
connected internally to pin 18.
Hall Effect sensors can be tested with an LED tester.
The EZK generates the Power supply for the Hall Sensor at pin number 5
(12V). The EZK generates a 5V Level at pin number 22, which the Hall
sensor can pull down to ground. Pin 22 is the "hot" signal cable
and therefore is shielded. The cable shielding is connected to pin 4, and
pin 4 is connected internally to master-ground pin 18.
The EZx spark and LH ECUs are under relay switch power
control. First verify that voltage is present at both LH and EZx relay pin
numbers 30. There should always be voltage at pin 30, regardless of the
ignition key position because a direct circuit is provided from the
battery. If voltage is present, next jumper pins 30 and 87 and attempt to
start the engine. If it starts, there is a relay or control signal to the
relay problem. If it doesn't start, you may have an ECU problem.
The LH ECU controls power to the fuel pump. If there is
a "no fuel pressure" situation, first verify that the fuel pump
fuse is OK. If the fuse is OK, next check for voltage at fuel pump relay
pin 30. If voltage is present, jumper the fuel pump relay (pins 30 and 87)
and check for a running fuel pump. If the fuel pump runs, there is a relay
or control signal problem to the fuel pump relay. If the fuel pump doesn't
run when the relay is jumpered, there may be a fuel pump or wiring problem
to the fuel pump. One of the symptoms of a failed LH module is no
"turn-on signal" to the fuel pump relay.
For 1984 – 86 model year 928s, the fuel pump relay is
the source for voltage to the fuel injectors and the LH module provides
the fuel injection current path to ground. Therefore, a bad fuel pump
relay can also result in no voltage to the fuel injectors. The fuel
injector voltage source was changed to the LH relay for model year 1987
and newer 928s. In 1989 an ignition circuit control was added as a safety
feature to open the fuel injection circuit (stop injection) if no heat is
found in the exhaust stream. Look for this module in the passenger
compartment near the EZK and LH modules.
A simple way to determine if the
“hot wire” Mass Air Flow sensor is working is to disconnect it and
start the motor. If it runs better when the MAF sensor disconnected (in
limp-home mode), then the MAF sensor is likely failing.
As crazy as it may seem, cars are different. A friend of
mine had an LH module that would work in other cars, but not in his, so he
spent many more weeks looking for the problem in other areas. The “suspect”
LH module ultimately failed, even in other “test cars,” when only a
few weeks prior it was running fine.
If you have some specific questions diagnostic
questions, please feel free to send me a note at Email: info AT
electronikrepair DOT com.
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