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The MABC (Maserati Automatic Boost Controller).
The MABC is also known under the name APC, but that is when it is mounted in a SAAB.
This system for adaptive boost control were introduced by SAAB, and made by Luxor in Motala, Sweden.
The MABC is nearly the same as the first generation APC. It is in fact built in Sweden too. The differences
is in the Maserati scripts embossed in the unit and some component values.
It's purpose is to try to keep the boost pressure up while still protecting the engine from the
disastrous knock that will occur if the boost is too high for the conditions the engine is running
under at any moment. These conditions include fuel octane rating and ambient temperature, as well
as a range of other variables.
In theory the MABC would allow you to run any octane fuel and still keep the performance as high
as possible while not hurting the engine. I say "in theory" because this is only partially successful.
The same concept has been incorporated in the microprocessor equipped electronic ignition control unit
made by Weber Marelli for the later models with fuel injection. In these units the calculations used
and the method of avoiding knock is much more refined.
Sensors and actuators.
- Manifold Pressure Sensor. (Input)
This sensor measures the pressure in the intake manifold.
If the throttle is wide open, this pressure can become positive, or in other words greater than ambient pressure.
This is different from a normally aspirated car, where it will always be lower than ambient. Note that this is
the pressure after the carburettor butterflies, and therefore can be a lot lower than current boost pressure.
When the throttle is closed, the manifold pressure will be lower than atmospheric pressure, regardless
of how high the boost may be at the moment.
- Engine speed "sensor". (Input)
The engine speed is not measured by a sensor as such. It is calculated from the frequency of the ignition pulses
which are picked off from the ignition amplifier in the same way as an electronic revolution counter.
- Knock Sensor. (Input)
This is a piezoelectric microphone built into a metal stud that is bolted to the block in the bottom of the "V".
It is mounted directly over one of the crank bearings, which will be an area that is greatly affected by the
shockwaves that are results of engine knock. As these shock waves are the same as a sound, a microphone is a
good sensor to pick it up. Remember that it will also pick up all other sounds present in the engine block.
We'll come to that later.
After reading the general description, you may not be surprised to learn that a SAAB knock sensor will
fit the BiTurbo, and work as it is supposed to.
- Boost Control Solenoid. (Output)
This is a solenoid air valve that will bleed off the boost pressure to the air filter or direct it to
the wastegates, depending on whether it gets energized or not. Since it has only two possible states,
it is pulsed by the MABC control unit. This exploits the slow mechanical reaction of the solenoid to
position it in states between ON and OFF, and thus obtaining a continously variable valve from this
simple solenoid. There are around 16 pulses/second, and the length of time it is energized during
each 1/16sec interval will determine the position of this valve. This is what you hear as a "clattering"
from the engine room at 2000RPM, and although some owners think something may be wrong, it is quite normal.
- Overspeed Relay. (Output)
This relay is used by the control box to disable the ignition if it determines the RPM is above a set limit.
- Diagnostic port. (Input/Output)
The MABC control unit has an input that can be used to disable parts of it for diagnostic purposes.
It also has a couple of diagnostic outputs. One of these can be used to give a visual signal when the
unit determines that the engine knock level is higher than it should be.
MABC Control Unit.
This electronic device is an analog computer. In our time of digital computers this might sound a bit strange,
but such devices are not as uncommon as you might believe. One thing you should know is that there is no EPROM
chips in this unit, so if anyone tries to sell you a "chipstuning" for this device you should ask him to
go sell some snake oil instead.
One of the main units is the addition unit that takes several variables as inputs, adds them up
and give an output that is used for comparison against a set value. This comparison will at any moment
be a representation of the "stress level" imposed upon the engine compared to the factory set acceptable
level. If it is exceeded, the output circuitry will be given directions to lower the stress by lowering
the boost pressure.
The knock amplifier multiplies the input from the microphone by 10, filters out high and low frequencies,
takes the absolute value of the result and sends it off as one input to the addition unit. The effect is that more engine
knock will add to the calculated stress level. The filtering will shave off low and high frequency vibrations
that during the development of the control unit was determined not to be results of engine knock but other
One part of the speed sensing circuitry takes the instantaneous value of the engine speed, inverts
it and send this off as another input to the addition unit. The effect is that higher RPM will
lower the calculated stress level.
Another part of the speed sensing circuitry will find the change in engine speed, invert this and
send it off as input to the addition unit. The effect is that rapid acceleration will lower the calculated stress level.
This makes sense, as the situation is likely to change very soon as the RPM gets higher. The quicker the
acceleration, the more tolerant it gets to knock.
The boost level is scaled and sent off as input to the addition unit.
The effect is that low intake manifold pressure will lower the calculated stress level.
There is no reason to lower boost if the manifold pressure is already low.
The output circuitry will use the instantaneous perceived stress level to preset value ratio to decide if it
should lower boost or not. And if it should, then by how much. It also has a timing device that will be started
if the stress level ratio is high. This timer will prohibit raising the boost for a short time after it has
been determined that the engine stress was much too high. This avoids "swinging" the boost level up and down,
which would make the car very unnerving to drive.
The control unit also has some failure detection. It will multiply the knock microphone input by 100,
and take the absolute value of this. This signal is not filtered like the knock signal.
If the result is below a set limit, it will set an internal signal
telling the output unit that something is wrong, and it better lower the boost to be on the safe side.
The reasoning here is that if there is no background noise to be heard in the engine block, the microphone
or the connections to it must be faulty, and it cannot be relied upon.
One output from the engine speed sensing circuit is compared to a factory preset limit. If found too
high, it will simply shut down the ignition to protect the engine from overrevving. This circuit has a
bit of lag built into it. This allows a somewhat higher engine speed during rapid acceleration than
it does continously. This lag is a linear function, which means that if the engine has no resistance
as when you miss a gear, the RPM will be allowed to rise substancially higher than the preset level
before the engine is cut off. This means you could do damage to the engine if hoofing it under no load.
So don't let the rev limiter take care of overrev protection, it is actually your own responsibility.
When something goes wrong.
If you experience that the boost level is never rising up to the start of the yellow area on your
boost meter, the first thing to suspect is the knock microphone or the connection between this and the
control box. Check the plug/receptacle in the engine bay first.
If you replace the knock sensor microphone, take care not to apply too much force when unscrewing it.
It seems very sturdy, but there is in fact only a 8mm bolt in the center, and with a recessed neck too.
It breaks easily, and getting the remaining stub out of the block is no easy task. If it's not coming
out easily, you should have it checked first to see if it really is damaged.
Boost coming and going.
If your valves are not correctly adjusted, the noise from them will not be discernible from knock for
the control unit, and it will lower boost as a consequence. In this case it may happen only some of
the time, as opposed to the case of a broken knock microphone.
Also see previous paragraph.
If the wastegate actuators are not correctly adjusted, this may cause confusion to the MABC, and
strange results can come from this. If attempts at raising boost by tampering with the wastegates
are made, the MABC should be recalibrated to get a properly working system again. Certainly not a
task for the uninitiated.
Chattering sound from engine room.
When you have a chattering from the engine area especially at 2000RPM, just keep your calm.
This is not a sign of anything being wrong, it is just the boost solenoid at work.
Questions or comments are welcome. Click here
hits to this page since 1998/02/22.
Revision 1.1, 1998/02/22