Implications of using 348 battery disconnect switch

Newer Ferraris have issues with using the battery disconenct, but our 348's can handle it just fine.

However, you are correct about the ECUs needing to go through the relearn process (which just means starting up the car and letting it idle until the cooling fans kick on - without touching *anything* on the car in the meantime).

 

I've never understood this "relearn process" that is sometimes mentioned. What exactly is there to relearn? The idle speed should start out a bit higher than normal and then reduce to the normal warm idle... around 1000rpm or whatever. This is handled by the coolant temp. sensor sending the temp. info. to the computer which adjusts the idle bypass valve slowly closing it until normal idle is reached. This is common to any modern car.

Idle mixture probably starts off a bit richer on a cold engine and then comes down to about 14.7 to 1 for normal idle. Again, coolant temp. sensor, air temp. sensor and O2 sensor feedback loop feeding info. to the computer. Standard on any modern car. So what is there to learn?

I have heard of cars that "learn" your driving habits, particularly gearbox shift points, how much throttle you use... things like that. None of that comes into play when you are just letting the engine idle until it gets to operating temperature.

Any detailed explanation on what must be learned?

Erich

 

Good questions, Erich!

All mechanical devices wear with age. Likewise, so do electronic and electrical components, especially those devices that are in high heat areas (e.g. O2 sensors in your exhaust flow).

This wear impacts performance. A dirty air filter, for instance, flows less air. A dirty fuel filter flows less fuel. Aged fuel injectors flow at different rates, etc.

Motronic 2.7 learns your car as you drive. M2.7 makes adjustments to your air/fuel mixture based on your car's individual wear/condition. Install a new muffler? M2.7 will learn this and make adjustments.

Each M2.7 engine ECU stores two different types of things:
1. error codes
2. individual vehicle fuel air maps

When you disconnect the battery on a 348, you erase both of the above. When the battery is reconnected, each M2.7 ECU shows no error codes and uses only stock factory air fuel maps.

As you drive, the above will be modified as M2.7 learns your car through feedback from its sensors (e.g. MAF, O2, coolant temp, etc.).

This is not an instantaneous process.

Until M2.7 completely relearns your car, however, your initial ignition timing and your initial fuel air maps will be giving your car fuel and spark as if you had a factory new, stock 348.

Of course, once your car is warm your O2 sensors and MAFs will enable M2.7 to gradually adjust your air fuel mixture as appropriate to your car, but again, this is not an instantaneous process. There will be time periods where your car isn't running at its peak capacity/refinement. It's also worth mentioning that your O2 sensors are always *ignored* by M2.7 when you are driving at full throttle.

Moreover, until your indiviual fuel air maps are completely relearned, M2.7 will have a more difficult time coping with a failed sensor (e.g. a bad O2 or MAF).

Of course, when you force M2.7 to go through the relearning process, all of the above is sped up to a small window of 10 minutes or less.

So the difference between driving after going through the relearning process versus driving without going through the forced startup relearning process amounts to:
1. better idle
2. better running
3. better revving
4. better full throttle performance
5. better cold engine and cold weather performance
6. better gas mileage...
...until M2.7 relearns your car's condition through enough driving. Of course, M2.7 does less learning but more acting when you are driving versus when it is going through the forced relearn process on a cold engine at idle, so expect it to take considerably more driving to complete the relearn process than the ~~ 10 minutes it would take to go through the forced relearn process.

 

Thanks for the reply.

Maybe it is a question of semantics here. I would use the word compensates for MOST cases where the word "learns" is being used.

When your air filter gets dirty any MAF sensor puts out a different signal because there is less air flowing. The engine management then compensates by reducing the amount of fuel injected to a mixture level that is needed for current operating conditions.

Fuel supply is always "overrated" (more supplied than needed) so until the fuel filter is so clogged that the fuel pressure regulator(s) can no longer supply the minimum fuel needed the engine management system doesn't know one way or the other the condition of the fuel filter.

If you install new components such as a K&N filter or more open exhaust you increase air flow through the engine to some extent. Again, this is compensated for (within limits) by the MAF sensor. It's not what I would call a learning process.

It's common to most any modernish car with EFI for the engine management system to substitute a signal of 0.45 volts for an O2 sensor (0 to 1 volt type) when it does not see a signal that is not in the range expected or not varying as expected. This is a substitution or compensation process.

As far as I know all O2 sensors are "ignored" by engine management system at full throttle and a fixed, slightly rich, fuel map is substituted for O2 feedback. This helps avoid a lean condition at full throttle which could have catastrophic consequences. This is not specific to M2.7 or the 348.

There is not much to learn about start up to idle from cold to full temperature. If coolant or air temp. sensors do not supply a value within expected limits, again, an engine management system substitutes values or maps and generates an error code.

I guess it just seems to me that it is being made to sound much more mysterious or complicated and critical than it really is.

 

Erich, what you are describing is the old style fuel injection management that was fully analog (a control system).

The MAF input and O2 feedback *can* be used for a fully analog control system. The problem with a fully analog system is that it always lags true conditions. It takes time, for instance, for a charge to explode inside the cylinder, then for those waste gasses to pass through the exhaust valves, travel out the headers, and pass by an O2 sensor. Thus, a fully analog control system is modifying the fuel management for conditions that were over some time ago.

As computers became smaller and less costly, manufacturers added digital capacity on top of the old analog control systems.

The early computers stored fuel air maps that were mapped from extensive dyno testing. These maps anticipated future conditions in the engine, such as what to expect as the rpms revved up when throttle was fully depressed or when at partial throttle. In this manner some of the old analog delays were avoided.

By the time M2.7 came out, not only were the factory dyno'd fuel air maps stored, but the system itself was capable of learning your car as it aged. Which is to say, new fuel air maps were written on the fly by Motronic 2.7.

This avoided/compensated-for the problem of the factory maps not being ideal for every car, on every day, at every altitude/temperature, for the current wear in each individual car.

Also, keep in mind that M2.7 only allows the O2 sensors and MAFs to modify 20% of the fuel flow. Those analog components can add or subtract up to 10% of your fuel (20% range in sum).

The stored air fuel maps, however, can set the initial fuel flow to anywhere from 0 to 100% of capacity.

How much fuel to set initially is determined by those maps, which are then fine-tuned as you drive by the O2 and MAF inputs (lambda controls).

M2.7 compares the factory maps to the O2/MAF feedback to create even more accurate (for your car) stored maps. This allows the system to better anticipate the current/future needs of your engine, which is important because you want fuel flowing in advance of when the engine actually needs it (otherwise you are back to the old analog systems that always lagged the true conditions of the motor).

So M2.7 really does "learn." It learns your car as your car ages. You can speed up this learning process by doing the ECU reset.

Likewise, M2.7 compensates on the fly based on the feedback from the O2's and MAF's (though this compensation lags true conditions by some amount of time).

 

I understand now what your saying about the ability to write new fuel maps. Certainly that is something the older EFI systems did not have. However that ability does not make a system "digital" and certainly not "fully digital." The various sensors that make up the engine management system (along with the computer etc.) on even the newest cars still provide analog inputs which are converted to 1's and 0's by an analog to digital converter. There are no coolant or air temp. sensors or O2 sensors that put out 1's or 0's or other discrete values.

The K-Jetronic along with the points igntion system on 308's could be considered fully analog (if one were even to consider this as engine management) in the sense that the fuel system "voltage" (in reality, fuel pressure) and air flow determine the fueling. Both the fuel pressure and air flow can be at any one of an infinite number of points just as the timing could be at any infinite number of points based on rpm and distributor spring tension etc. This would be fully analog (as in no discrete levels).

Certainly, as you mentioned, an O2 feedback loop can never "anticipate" fueling requirements by its very nature. The O2 sensors on any car are downstream of the combustion process so there is always some delay in the feedback they provide to the computer. Their job is to make adjustments, via the computer, "after the fact" although in reality very quickly indeed. So the O2 sensor circuitry does not learn anything. ALL engine management systems that employ O2 sensors and MAF sensors compensate on the fly for changing conditions. That is the reason they are there. It's not something peculiar to the M2.7

Anyway, to go back to the original point, there is nothing the system can learn about the driver or the driver's habits when the battery is reconnected and the car is started and left to idle, without touching anything, until reaching full temperature. It can perhaps gather some basic baseline data and that's about it. I suppose you could call that learning or you could call it a calibration procedure.

Erich

 

No, that's not entirely accurate. M2.7 writes fuel maps on the fly that are unique to your 348. If you have one or more fuel injectors that are not linear such that they are outputting an unusual amount of fuel inside a certain rpm range or at a certain throttle position, then your dynamic fuel maps are going to look considerably different from a 348 that has fuel injectors operating linearly (or, as expected per factory specs).

Better still, M2.7 uses your unique fuel air maps to proactively schedule fuel flow and spark advance so that you receive peak performance.

These dynamic maps are created by the M2.7 system paying attention to the factory fuel air maps compared to your throttle position (e.g. wide open throttle or partial throttle or idle) versus feedback from your O2's and MAF's.

So M2.7 is *specifically* learning your unique 348, and M2.7 is changing as your components wear.

Those dynamic fuel maps are lost, however, when you disconnect the battery (but that can be a good thing when you want the system to relearn your car after a component change).

At the very least, you can view the relearning process as a recalibration, but it is that much and more.