Diagnosing Problems with LH-equipped Engines

Diagnosing Problems with LH-equipped Engines

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


Cold start problems with LH-based 928s

When troubleshooting, always start with the "easy" and inexpensive items first and work your way to eliminate problems until you end up to the difficult and expensive items. The majority of electrical problems are connection related.

If the ECU doesn't know that the engine is cold, it does not provide cold start enrichment and it will not start. Eventually after cranking enough, there may be enough "raw" fuel sprayed into the intake to start the engine. You would see smoke after starting because the fuel wasn't metered properly.

The Temperature Switch II is located in the front center of the engine just to the right of the fuel pressure damper.

1. Check the plug to the sensor, is it on tight? Are the contacts clean? If not, clean the connectors and clip it on tightly. Check engine starting now. If it does, you're done. If not got to step 2.

2. To check the temperature II sensor, you'll need an ohm meter. Remove the connector from the LH control unit. Connect the ohm meter to pins 2 and 5 of the LH connector (not to the pins on the module itself). The correct value ranges are:

0 C / 32 F: 4.4 - 6.0 k-ohm
15 - 30 C / 59 - 86 F: 1.4 to 3.6 k-ohm
40 C / 104 F: 0.9 - 1.3 k-ohm
60 C / 140 F: 480 - 720 ohm
80 C / 176 F: 250 - 390 ohm

Since your problem is cold start, you could just look for the proper resistance range for the ambient temperature of the cold engine. If the sensor is shorted (zero ohms resistance) the mixture will be too lean and the engine will not start when cold. For problems at other engine temperatures, I'm including all of the resistance ranges for higher engine temperatures. If the sensor has an open circuit (infinite resistance), the fuel mixture will be too rich, the engine will not run when warm, and will be difficult to start when warm.

If the values aren't correct, measure them directly at the Temperature Sensor II. You'll see that there are two prongs on the sensor. There are two separate temperature sensors housed in the one sensor body, one for the LH and the other for the EZK module. Note the orientation of the protruding alignment notch on the outside of the sensor. If you are viewing the alignment notch orientated to the left side, the prong closest to you is the prong for the LH controller and the prong further away is for the spark control module. Clip one of the ohm meter leads to the prong closest to you (LH) and clip the other lead to a ground point [Do NOT connect the ohm meter leads between the two prongs of the sensor]. Repeat the resistance measurements.

If the resistance readings are in the correct range, there is a problem with wiring to the LH controller or the connector itself could be corroded. If the resistance readings are not correct range, the temperature sensor must be replaced.

If replacing the sensor doesn't fix the cold start problem, check the fuel filter, fuel line pressure, injectors etc. Fuel injectors can be removed and tested for flow and flow pattern. There are several shops that provide this service. The problem could also be attributed to bad ground connections. Also, don't overlook potential weak spark conditions. If you've eliminated all other areas, a failing LH controller could be the problem.

Rich Andrade

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