928 LH Repair


928-S4, -CS, -SE, -GT and -GTS models have all a LH Jetronic system (the L stands for "Luftmengenmessung" in German, meaning "air flow management" and the H stands for "Hot Wire" referring the Mass Air Flow Sensor that is used). The L and LH systems are very similar, where the LH system outperforms the L type by more accurate control and improved flow capacity. It picks up the information of various sensors, and adjusts fuel delivery and timing to the engine.


The LH fuel injection computer is frequently the cause of failures. Especially after 10 years they are prone to fail as recent polls showed. Bosch is not producing these LH's anymore, so new ones are virtually impossible to get and used ones are just a problem waiting to happen. 928-ecu-repair can effectively repair the defective controller for you and restore it to specs !

Common symptoms of a failing LH injection module are:

  • Fuel injection valves click when the ignition turned is on and engine is not running.

  • Engine will not run at idle.

  • Air-fuel mixture is too rich (this can damage catalytic converters).

  • No fuel pump operation, but the fuel pump operates when its relay is jumpered.

  • The engine will not rev higher than 3000 RPM.

  • Automatic Transmission: no RPM regulation (idle speed) when switching from "D" to "N" or "P".

  • Engine will not start, spark plugs are dry and the ignition system produces spark.

  • Removing the LH from the 928

    The fuel injection system in the 928 S4/CS/SE/GT and GTS is operated by the Bosch LH injection computer LH2.3 (also called the brain or ECU). First disconnect the car battery by removing the Earth strap which is below the toolkit in the boot of the car. Ensure the battery earth cannot be accidentally reconnected until you are ready to do so.

    On the left the LH & EZK in the assembly (England).

    The Ignition (EZK) and Fuel Injection (LH) control units are held in a special carrier located on the outer bulkhead to the left (England) or right (other countries) of the passenger footwell. The carrier is held by two 5mm Allen bolts or 10mm bolts depending on year / model. First raise the steel latches on the cable plugs. This releases the plugs from their sockets in the units. Then carefully remove the cables from their slots. Move the cables to one side if possible. Remove the auxiliary relay (where fitted) as shown in the picture. Now remove the two bolts and move the entire assembly outwards.

    Access is much improved by first removing the passenger parcel shelf, as in the picture above, but this is not essential. The Porsche part numbers for the LH and corresponding Bosch part numbers are listed up in the table below.

    There are small differences between the controllers relating to different 928 models and manual/auto gearbox. Most differences are handled by firmware (program) stored in the LH but there are also differences in the electronics PCB. Generally speaking all LH's work for testing purposes.

    Porsche and Bosch partnumbers on the LH:

    928 model Porsche part number Bosch part number
    87 928 618 123 10 0280.002.504
    88 928 618 123 11 0280.002.504
    88 CS 928 618 123 12 0280.002.506
    89/90 928 618 123 13 0280.002.507
    89/90 CS/GTS 928 618 123 14 0280.002.506
    91 S4 928 618 123 25 0280.002.508
    91 GT 928 618 123 26 0280.002.509
    GTS 928 618 123 30 0280.002.514

    The problem

    The most common cause of LH-Jetronic failure is the hybride circuit. This thick film component is constructed with the internal components exposed and then the integrated circuit is filled with a "rosin" to protect the internal components.

    LH without cover.

    Unfortunately the rosin that they selected is shrinking and/or expanding with changes in temperature over the years and finally cracks which also cracks and damages the internal components of the IC in the process. 928-ecu-repair offers you a repair of the LH-unit for a reasonable price. Due to the large number of variants of the basic unit, it is often not possible to offer an exchange unit from stock. Your faulty unit can usually be turned around by our workshop in 2 working days.

    The 928-repair-service

    Once you disconnect the faulty LH from your car, send the LH-unit to 928-ecu-repair. When we receive the LH, the first thing we do is connect the LH on a 928-testbench. This PC controlled testbench acts like a 928 car and with a logic analyser we measure the signals of your faulty LH. Most of the time the hybride circuit is the defective, but sometimes other circuits can be the problem as well. If so, the 928-ecu-repair service will contact you and agree on how to continue the repair service. 928-ecu-repair have all circuits on stock.

    Desolder hybride and mount replacement.

    If the hybride circuit is the defective, 928-ecu-repair remove the hybride circuit from the LH and mounts a new replacement printed circuit board on the same position as the hybride circuit. When the solder process is finalized, the repaired LH will be tested again on the 928-testbench.

    928-ecu-repair has re-engineered the replacement of the hybride with a version that is well protected to suit the difficult environment found in a the car. This replacement is electronically 100% compatible and functions within the specs of the original hybride, but lacks the typical ageing problems of the hybride circuit. We verify the operation of each repaired LH, and test it on specs on our workbench and test-drive in a 928 prior to shipping to our clients.

    Warrant and prices

    We offer a 2 year warranty on replaced parts and labor. Any obvious abuse of the unit will render the warranty invalid. Modifications to the car's systems are beyond our control, and are at owners' risk. Please email us for a quotation for your LH repair. Our preferred payment method is bank transfer (or Paypal for our international clients).

    LH with soldered replacement.

    Collection of hybrides.

    Collection of hybrides.

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