Dear 928 Porsche friends,

I’ve read much about the topic of injection controllers and their repair in this forum. Unfortunately, I must state that some information provided is incorrect due to a misunderstanding of how the LH controller works or perhaps is simply wrong by intention. You will be very surprised at the many “secret tips” that follow.

Circuit diagrams of the LH are posted in the internet, however, they contain elementary errors so please use them with caution. Someone once suggested adding a 12 Volt Zener diode to protect the LH module. Given that the nominal electrical system voltage of a German automobile is 13.8 Volts and all the electronic elements of the system are designed to function at that voltage, there is no need for such a diode. Furthermore, a protection diode is already present in the controller!

How might I know these facts? We repair controllers. For 13 years we created for our internal use innumerable circuit diagrams, test software and two controller diagnostic stations. The controller which we best understand is the 928 LH controller.

The main cause of failure in the LH is the hybrid circuit. To understand the function of the hybrid, please envision it as a type of interface that converts the 5 Volt signals of the microprocessor to 12 Volts, or 12 Volt signals of the controller to the level lower 5 Volts for the microprocessor. Additional functions such as Lambda regulation, resets and reference voltages are also integrated. The hybrid circuit contains two integrated circuits (ICs) and several discrete components which provide these functions. The ICs are protected by a silicone covering.
Over the course of the time, oxygen and water molecules diffuse through the silicone covering and cause oxidation at the chip which eventually leads to a loss in functionality. Unfortunately, the loss of the hybrid circuit functions can lead to the complete loss of controller functionality and thus the engine stops running.

When asked how the failure occurred and how it affects the car, nearly 95 % of the time the answer is: "it worked properly when turned off, afterwards the engine would no longer start."

Beyond "oxidation" and “turning off your engine" you need to understand these two facts.

1. The age of the controller is the main enemy here, not the actual hours of operation

2. The controller will fail mostly after turning the off the engine. As Murphy’s law dictates, it will fail most often at the place where it will cause you the highest possible inconvenience.

From these facts it follows:

a) It is it your "Karma" that will get you!

b) If your controller fails, never buy a used or repaired controller that contains an old hybrid chip because then “it will get you again.” The record that one customer holds is three successive failures when purchasing used controllers, each failure came approximately 6 months apart.

c) as a precaution, have the controller tested – or - better yet have it rebuilt – or – buy a new one (that hasn’t been sitting on a shelf for 10 years!). Some functions of the hybrid can fail without becoming noticeable to the driver. The loss of some functions is, however, already a clear indication for that the "remainder" of failures will soon follow.

d) Don’t give in to your Karma, but rather you should be like a customer of ours who always carried a used replacement controller in the glove compartment. So, when his module actually went out on him, he simply swapped them. But he was lucky, since his replacement controller could have also died of “oxidation” in the glove compartment during this time.

If you should decide to have your controller a repaired or rebuild, then you should contract with someone who COMPLETELY replaces the hybrid circuit. Our hybrid circuit replacement, in contrast to the original, possesses substantially improved characteristics such as lower power consumption (therefore it does not become so hot) and most importantly incorporates fail-safe functions thus making it practically impossible for the complete failure of the controller. This becomes possible by a coprocessor function incorporated into our hybrid replacement. However, do not fear. There is a hardware function that takes over this failsafe too. But that has never happened before. Moreover, as part of our rebuild process, where it is necessarily other elements of the controller are brought up to date with the newest state-of-the-art components.

The good news is that other than the hybrid circuit, the LH controller is a durable and very reliable controller.

If you desire further information about us, our repairs, or the LH module please contact our North American business partner, Richard Andrade at

Achim Müller
ACM Elektronik
74747 Ravenstein/Oberwittstadt
Tel +49 (0)6297/929462 (europe) (intl)