The ECM supplies a 5 volt signal to the IAT through a resistor in the ECM and measures the voltage. THE VOLTAGE WILL BE HIGH WHEN THE INTAKE AIR IS COLD, AND LOW WHEN THE INTAKE MANIFORLD AIR IS HOT. By measuring the voltage, the ECM knows the intake air temperature. QUESTION: Does anyone know what the ECM does with this information, and how it affects the fuel supply?
:rolleyes:

 

Though the chip burner guys say this data is not used in the CalPac tables, I'm not convinced that it isn't used by programming that's hard coded into the ECM's PROM.

 

Thanks for the response, 65Z01. If it is hard coded data, do you know what the ECM is doing with the data. Why does the ECM care about the temperature of the intake air, unless it is metering fuel accordingly? The reason I ask is I just returned from a 500 mile trip with the IAT unplugged. A code 23 reminded me to reconnect the plug. The car ran super, and there was no change in mileage.

 

So far it's just speculation on my part, but it's reasonable that the General put the MAT sensor there to be used by the ECM.

When in closed loop mode the O2 sensor is king and it is used by the ECM to assure a stoch A/F ratio. I'm thinking that the MAT data may be used for open loop operation such as during a WOT burst.

 

Thanks again, Jim. Your speculation may be quite accurate. :yesnod:

 

MAT (or IAT) is used in the calculation of intake air density, per GM's version of Clapeyron's equations. In the 727 code that I am familiar with, it starts with a couple of MAT look-up tables - discussed here by TunerCat:

"These two tables are used in the calculation of the inverse MAT term used in
the of grams of air per cylinder calculation.

Depending on the setting of a switch, the 'Inv. MAT Term Lookup Delta
Multiplier Vs. Airflow' table may or may not be used.

If this table is disabled then the MAT Compensation Counts Vs. MAT table is
indexed with the normal value of MAT. If this table is enabled, then the
table index for the MAT Compensation Counts table is a corrected value of
MAT. The corrected value is calculated as follows:

Corrected MAT = MAT + Table Value * (Coolant Temp - MAT)

where 'Table Value' is the value looked up from the Inv. MAT Term Lookup
Delta Multiplier table.


The Inverse MAT term is then calculated as:

Inverse MAT Term = Cylinder Volume / (((Table Value ** 0.5) + 233) * 128)

where 'Table Value' is the value looked up from the MAT Compensation Counts
table (indexed by either MAT or the corrected MAT depending on the option
switch).

This value is then used in the calculation of the grams of air per cylinder
as follows:

grams air per cylinder = Volumetric Efficiency * (MAP - EGR Partial
Pressure) * Inv. MAT Term

This value and engine RPM are then used to calculate air flow which is then
obviously used to calculate the base pulse width."

This is much more entertaining when you tr to follow it in the original code.

HTH
DrJ

 

Thanks for the input, Doctor J. Based on the equations you presented, what do you think was happening to the calculation process of air density with the IAT disconnected? The engine seemed very contented with, or without the IAT connected.

 

DOCTOR J, are you discussing caclulations used with a SD or a MAF system?

I understood that the MAT temp. data was used along with MAP pressure data only with the SD system. However the MAT sensor was used since the first TPI which used a MAF to directly measure air flow and the MAP sensor wasn't used till seven years later.

Is the 727 code used with MAF based systems?

 

oldman

Um, actually they're GM's equations, from the $8D ECM mask & code. I just presented them because you asked a technical question that seemed to be looking for a factual answer. Besides, I had it on file. I was looking into that topic on some tuning I was doing last winter .

I don't have a copy of the $DA code, so I could only guess on what your ECM did (GM code seems to be similar from one ECM generation to the next):

IIRC, in the $8D code an open IAT would be interpreted as -40degF. The computer would look at that a few times. If the value were unchanging the ECM would set a code and ignore the value. I don't remember off the top of my head whether it then uses some default T , or just skips that step and falls back on the BPW. Seems to me it uses BPW, but I'd have to look it up. YMMV.

FWIW, GM uses "adaptive tuning" for most of the engine control loops. Several input variables that are not critical to safe operation are handled by so-called 'soft-fail' mode, in which the engine continues to run (under less than ideal conditions) with a code set, so that you can get home to repair the problem. The GM Tech Manual for your car has more complete descriptions of each sensor's function.

In any case, the MAT calc is most important for things like summer/winter start-ups in northern climates. Daytime cruising in sunny CA wouldn't put much of a strain on the ECM or fuel train, IMHO. Once you were in closed-loop the O2 PID would control fuel without worrying too much about the missing MAT value.

If you are interested, some annotated source codes have been copied at the ECM Guy's site, or in the DIY EFI library. Looking at what GM implemented is the best way to see what the ECM is really trying to do, and how GM configured it.


65Z01

A 93 has an SD system. I have no first-hand info for you on MAFs, or their history at GM.

Vehicles that use a hot-wire anemometer (MAF) have temp & baro corrections built-into the load tables AFAIK. Other systems used other means (or none at all) for correcting air density. The source codes above would be your best reference on how each system works.

As far as which ECMs were in which system, Ludis' site has a pretty complete list of that info across most GM product lines. That is the best reference I'm aware of.

DrJ