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
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
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.
If you have some specific questions diagnostic
questions, please feel free to send me a note at Email: info AT
electronikrepair DOT com.