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 1984 – 1986 Euro/ROW and 1985 – 1986 US/Japan model 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 module connector. 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 seam, 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 info@electronikrepair.com .

Rich Andrade

'93 GTS
'79 Euro 5-liter hybrid track beast


First, check all connections and earth points. Corrosion is the number one cause of electrical problems.

Have you checked the engine position sensors? If you look to your schematic there are two, one is called the "RPM" sensor and it is inductive and the other is called the Hall sensor and it is a Hall Effect device. Focus on the RPM sensor for now. If there is no signal generated by the RPM sensor, the EZK will not turn on. The EZK is the system master, so with no signal from the EZK, the LH will not function either!
The RPM sensor is connects to pins 6 and 23 of the EZK and is shielded (connected to earth) at pin 24. The best way to look at the RPM sensor is with an oscilloscope. The Hall sensor provides information to the EZK so can calculate the cylinder position for the knock sensor reading. If there is no Hall signal, spark timing is retarded.

Next, look to the ignition trigger units (black finned boxes at the front of the engine compartment). I've heard of corroded connections to these units causing a no-start condition. Clean the connectors and check the connections to the EZK unit and the ignition coils.

Good luck with your efforts.

'93 GTS
'87 S4
'79 Euro 5-liter hybrid track beast


 If I understand correctly the EZK unit looks for the RPM sensor before firing the spark plugs, and that it also tells the LH unit to start operation which in turn trips the fuel pump relay to flow fuel, is this correct?

>>> Correct.

In searching the archives I have read some about the Hall sensor but I don't quite understand what it does or where it's located on my car?

>>> The engine speed is provided by an inductive sensor. A volt meter can be used to check the output of the sensor.

The Hall sensor is used to provide info to the EZK for spark knock detection, it tells the EZK which cylinders the spark knock detectors are listening to. If the Hall sensor isn't functioning the EZK still works, it just retards the timing by 6 degrees for safety sake.

a. Testing the engine RPM sensor. The EZK spark control is the system master, if engine rotation signals are not provided to the EZK, spark will not be generated and EZK will not provide a turn-on signal to the LH Jetronic fuel injection system. The RPM sensor input to the EZK is from an inductive RPM sensor. Inductive RPM sensors generate voltage so a VOM can be used for testing the sensor. EZK pins 23 and 6 are connected to the RPM sensor; pin 6 is ground and is connected to master-ground pin 18.
EZK pin 24 is the shielding of the sensor cable and is connected internally to pin 18.

b. The Hall Effect engine position sensor can be tested with an LED tester. The EZK provides the power supply for the Hall Sensor at pin number 5 (12V). The EZK provides a 5V level at pin number 22, which the Hall sensor can pull down to ground. Pin EZK pin 22 is the "hot" signal cable and therefore is shielded. The cable shielding is connected to EZK pin 4, and pin 4 is connected internally to master-ground EZK pin 18.

c. The EZK spark module provides the signal to turn on the LH fuel injection module; it is the circuit from EZK pin 13 to LH control Pin 1.
Check for continuity, if the wire is broken or the connection isn't clean, the car will not start.

Also, the temp II sensor, MAF sensor etc., but I don't see any of them causing a no spark problem?

>>> you are correct. A likely problem is a corroded connection or cut wire. Check the connections to the controllers up front of the radiator.

Good luck,



1987 928s4 of a friend: car almost starts, makes a few turns and then stops.  Mostly it doesn't fire up at all. Sparkplugs are wet and spark is there. Timing is ok, ecu's are ok. Fuel pump delivers enough fuel in volume.

cause: The fuel pressure was way low. Maybe 2-2,5 bar when relay bridged.  That drops even further when the car starts. This causes a bad fuel mixture to enter the engine and prevents it from starting. Using quick start spray in the intake (careful with the MAF) may show that it does pick up on evaporated fuel.

Replacing the fuel pump did it.