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LH Module Failure

The main cause of failure in the S4 and newer 928 35-pin 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.

35 Pin LH Hybrid Circuit
35 Pin LH Hybrid Circuit

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% to 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 please contact me.

Rich Andrade
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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