Failure diagnosis controller of pressure sensor

A failure diagnosis controller of a pressure sensor whereby abnormalities of the pressure sensor can precisely be diagnosed, wrong abnormality diagnoses can be avoided, users' perplexity and confusion on maintenance avoided, users' distrust removed, and unnecessary maintenance eliminated. The invention provides a control means which diagnoses the pressure sensor to be in a failure mode when the intake air temperature is in a state of being measured and the integrated load of the internal combustion engine from its starting time exceeds the criterion of the integrated load.

FIELD OF THE INVENTION 
The present invention relates to a failure diagnosis controller of a 
pressure sensor and, more particularly, to a failure diagnosis controller 
of a pressure sensor capable of precisely diagnosing abnormalities of a 
relative pressure sensor mounted on an intake system of an internal 
combustion engine. 
BACKGROUND OF THE INVENTION 
In an internal combustion engine, sensors such as a pressure sensor for 
detecting intake manifold pressure and a temperature sensor for detecting 
intake air temperature are mounted on the intake system so as to diagnose 
an operational state of the internal combustion engine. The foregoing 
pressure sensor includes a relative pressure sensor used for detecting a 
variation of an intake manifold pressure of an exhaust gas recirculation 
system (EGR system) at its on-off operation, in order to diagnose whether 
the EGR system is or is not in a normal operational state. As shown in 
FIGS. 17 and 18, the relative pressure sensor has a characteristic such 
that it gives a specific output voltage (V) against an input pressure 
(mmHg), and there is a specific dispersion or pattern to the 
characteristics, as shown in FIG. 19. 
As shown in FIG. 19, the relative pressure sensor represents the 
characteristics that the input pressure is high when the engine load is 
high, and the input pressure is low when the engine load is low. 
Accordingly, in the relative pressure sensor the operational state is 
diagnosed to be abnormal when the output voltage is 0.5 volts or less 
where the engine load is "a" or more, and when the output voltage is 4.5 
volts or more where the engine load is "b" or less (areas shown with cross 
hatching in FIG. 19). 
A method of diagnosing a failure of such a pressure sensor is disclosed, 
for example, in Japanese patent publication JP-A-6-58210. The method 
disclosed in this patent publication is to prevent a misdiagnosis due to 
an abnormal state or mode, for example a freezing of the pressure sensor, 
by providing a judgment step to judge whether or not at least one of a 
coolant water temperature, intake air temperature, and oil temperature at 
starting of the engine is lower than a predetermined value corresponding 
to the lower limit temperature in a normal operational state of the 
pressure sensor system, and thus invalidating a failure diagnosis 
execution step when at least one of these three temperatures is lower than 
the predetermined value. 
However, an abnormality of the pressure sensor can not be precisely 
diagnosed by the aforesaid conventional diagnosis method, and increasing 
the aforementioned judgment voltages, 0.5 volts and 4.5 volts, raise the 
probability of a misdiagnosis, which is disadvantageous. The failure of 
the pressure sensor includes some deviations in the characteristics, 
although the voltage is produced from the pressure sensor. This situation 
makes it impossible to diagnose an abnormality of the pressure sensor, 
which is also disadvantageous. 
Furthermore, when moisture penetrates into a hose communicating with the 
pressure sensor and intake system due to accidents in an extremely cold 
region and the moisture freezes, the pressure sensor is likely to be 
diagnosed as abnormal and the EGR system can be misdiagnosed as abnormal. 
Still further, if the pressure sensor is diagnosed as normal in spite of 
the characteristics of the pressure sensor being abnormal, the EGR system 
for diagnosing abnormalities using this pressure sensor will diagnose 
itself as abnormal. Accordingly, some parts can be replaced from the 
normal EGR system, and unnecessary maintenance can be done with the true 
cause being unknown, thereby increasing the maintenance cost, thus 
amplifying distrust of users, which is another disadvantage. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of solving the foregoing 
problems, and it is characterized in that the invention provides a control 
means for a failure diagnosis of a pressure sensor to measure an intake 
manifold pressure of an internal combustion engine. The control means 
diagnoses the pressure sensor to be in a failure mode when intake air 
temperature is in a state of being measured and an integrated load of the 
internal combustion engine from the engine starting time exceeds a 
criterion of the integrated load. 
The failure diagnosis controller of this invention diagnoses that the 
pressure sensor has gone into a failure mode when the intake air 
temperature is in a state of being measured and the integrated load of the 
internal combustion engine from the starting time exceeds a criterion of 
the integrated load; and therefore, abnormalities of the pressure sensor 
can be diagnosed with high precision, misdiagnosis of abnormalities can be 
avoided, confusions regarding maintenance and distrust of users can be 
removed, and still unnecessary maintenance can be eliminated.

DETAILED DESCRIPTION 
The embodiment of this invention will hereafter be described with reference 
to the accompanying drawings. FIG. 20 shows an internal combustion engine 
2 installed in a vehicle (not illustrated), which engine includes a 
cylinder block 4, a cylinder head 6, an oil pan 8, a crankshaft 10, an air 
cleaner 12, an intake pipe 14, a throttle body 16, a throttle valve 18, a 
surge tank 20, an intake manifold 22, an exhaust manifold 24, a front 
catalytic converter 26, an exhaust pipe 28, a rear catalytic converter 30, 
and a fuel tank 32. The fuel tank 32 is provided with a level gauge 34. 
An evaporative fuel controller 36 is provided between the surge tank 20 and 
the fuel tank 32. In the evaporative fuel controller 36, a canister 42 is 
provided between an evaporative fuel passage 38 communicating with the 
fuel tank 32 and a purge passage 40 communicating with the surge tank 20. 
In sequence from the fuel tank 32, a tank inner pressure sensor 44, a 
separator 46, and a pressure control valve 48 are provided in the 
evaporative fuel passage 38. The pressure control valve 48 communicates 
with the surge tank 20 through a pressure passage 50. The pressure passage 
50 is provided with a negative pressure control valve 52. The purge 
passage 40 is provided with a purge valve 54. The canister 42 is provided 
with an atmospheric control valve 56. 
In the intake system of the internal combustion engine 2, an exhaust gas 
recirculation (EGR) unit 58 is provided which supplies the intake system 
with a portion of an exhaust gas. The EGR unit 58 includes an EGR control 
valve 60, a back pressure regulating valve 62, and an EGR judgment valve 
64. 
The surge tank 20 is provided through a filter 66 with a relative pressure 
sensor 68 for detecting an intake manifold pressure. The relative pressure 
sensor 68 outputs a characteristic, specific output voltage (V) against an 
input pressure (mmHg), as shown in FIGS. 17-19. 
The internal combustion engine 2 is provided with a crank angle sensor 70. 
The crank angle sensor 70 also functions as an engine speed sensor and 
comprises a crank angle plate 74 having a plurality of teeth 72 on the 
periphery thereof, fitted on the crankshaft 10, and an electromagnetic 
pickup 76 mounted on the cylinder block 4. The crank angle sensor 70 
communicates with an electronic control unit or means (ECU) 78. 
The control means 78 communicates with a coolant temperature sensor 80 
mounted on the cylinder head 6, an intake air temperature sensor 82 
mounted on the intake pipe 14, a throttle travel sensor 84 mounted on the 
throttle body 16, an ignition unit 86, the level gauge 34, the relative 
pressure sensor 68, the tank inner pressure sensor 44, the negative 
pressure control valve 52, the atmospheric control valve 56, the purge 
valve 54, the EGR control valve 60, the EGR judgment valve 64, a front 
oxygen sensor 88 mounted on the exhaust manifold 24, a rear oxygen sensor 
90 mounted on the exhaust pipe 28 on the downstream of the rear catalytic 
converter 30, an atmospheric pressure sensor 92 for detecting the 
atmospheric pressure, an ignition key 94, and a starter 96. The control 
means 78 is provided with a failure diagnosis unit 78a therein. 
The control means 78 diagnoses the relative pressure sensor 68 to be in a 
failure mode when intake air temperature is in a state of being measured 
and an integrated load from the starting of the internal combustion engine 
2 exceeds a criterion of the integrated load. 
The control means 78 further diagnoses that the relative pressure sensor 68 
is in a disconnection or short-circuit mode when an intake manifold 
pressure is not within a range between a higher limit and a lower limit of 
the criterion range, and that the relative pressure sensor 68 is in a 
functional abnormality mode when a pressure variation against an engine 
load variation is outside the criterion range. 
Furthermore, the control means 78 measures the intake manifold pressure 
when the ignition key is ON, measures an atmospheric air pressure, 
measures a first pressure when the starter 96 turns ON from OFF, and 
measures a second pressure at a complete explosion within the cylinder 
when the starter 96 turns OFF from ON. The control means diagnoses that 
the relative pressure sensor 68 is disconnected when the intake manifold 
pressure is lower than the reference criterion. The control means 
diagnoses that the relative pressure sensor 68 is short-circuited when the 
first pressure is lower than a first criterion value or the second 
pressure is lower than a second criterion value. The control means 
diagnoses that the relative pressure sensor 68 is functionally abnormal 
when the first pressure is lower than a third criterion value or the 
second pressure is lower than a fourth criterion value. The control means 
sets a correction factor on the basis of the intake manifold pressure and 
atmospheric air pressure, and sets a corrected input pressure on the basis 
of an input pressure of the relative pressure sensor 68 and the foregoing 
correction factor. The control means also diagnoses that the relative 
pressure sensor 68 is short-circuited when an output voltage is higher 
than a lower limit for judgment when fuel is not being supplied, and 
diagnoses that the relative pressure sensor 68 is disconnected when an 
engine load is higher than a set value or the output voltage is lower than 
a higher limit for judgment. 
The control means 78 also measures the intake air temperature when a 
vehicle speed continues for a specific length of time or more with a set 
vehicle speed, measures the intake air temperature each time when the 
vehicle speed satisfies the foregoing specific length of time, and 
performs a statistical processing of the intake air temperature. 
Furthermore, the control means 78 delays the diagnosis of the relative 
pressure sensor 68 from the starting of the internal combustion engine 2 
depending on the integrated load, and varies the criterion range of the 
integrated load according to the intake air temperature. 
The operation of this embodiment will now be described with reference to 
the flow chart shown in FIG. 1 
When the internal combustion engine 2 is started (step 102), first, whether 
or not the intake air temperature sensor 82 can measure the intake air 
temperature is judged (step 104). If step 104 gives YES, a measurement 
process of the intake air temperature is executed (step 106). 
The measurement process of the intake air temperature in step 106 is 
executed according to the flow chart shown in FIG. 2. When, the internal 
combustion engine 2 starts running and starts the program (step 202), 
first, whether or not the vehicle speed (SPD) (hereinafter simply referred 
to as the speed) is higher than the speed criterion (SPD1) being the set 
speed, that is, SPD.gtoreq.SPD1 is judged (step 204). If step 204 gives 
NO, this judgment process is continuously repeated. 
If step 204 gives YES, a judgment process on the condition whether or not 
the state of SPD.gtoreq.SPD1continues more than a specific length of time 
(ts) is prepared (step 206), and the next step 208 concludes whether the 
condition is satisfied. This is to prevent that a correct measurement of 
the intake air temperature becomes impossible when the ambient temperature 
around the intake air temperature sensor 82 increases owing to an idling 
engine running left alone (i.e., a non-moving vehicle). If step 208 gives 
NO, the process returns to step 204. 
If step 208 gives YES, the intake air temperature is measured and the 
measured intake air temperature (THA) is obtained (step 210). 
After the internal combustion engine 2 starts running, the judgment whether 
or not the first intake air temperature measurement is done is prepared 
(step 212), and the next step 214 judges whether or not the measured data 
is the first measurement of the intake air temperature. If step 214 gives 
YES, the measured intake air temperature (THA) is specified as an intake 
air temperature (THAn) that is employed for correction (step 216). On the 
other hand, if step 214 gives NO, the intake air temperature used for 
correction (THAn) is specified as (THA.smallcircle.+THA)/2.fwdarw.THAn; 
here, THA.smallcircle. is the intake air temperature used for the previous 
correction and THA is the currently measured intake air temperature (step 
218). 
After steps 216 and 218 are processed, the intake air temperature 
measurement process is repeated (step 220). 
Therefore, in the intake air temperature measurement process in FIG. 2, the 
intake air temperature is measured when the speed (SPD) is more than the 
speed criterion (SPD1) and continues for more than the specified length of 
time (ts), as shown in FIG. 3, and the intake air temperature measurement 
is done each time when the foregoing condition is satisfied and the 
statistical processing thereof is performed. 
Next, in FIG. 1, after the intake air temperature measurement (step 106), 
an integrated load measurement is done (step 108). The integrated load 
measurement of step 108 is performed according to a flow chart as shown in 
FIG. 4. 
When the internal combustion engine 2 starts running and starts the program 
(step 302), first, whether the coolant temperature (THW) is higher than a 
criterion of the coolant temperature (TWH1) which is a set temperature, 
that is the equation THW.gtoreq.THW1, is judged (step 304). If step 304 
gives NO, this judgment process is continuously repeated. 
If step 304 gives YES, the engine load of the internal combustion engine 2 
is integrated by the control means, and thus the quantity of the 
integrated load (value of the integrated load) (KLOAD) is acquired (step 
306). The integrated load (KLOAD) is obtained, for example, from an intake 
air flow as shown in FIG. 6. The integrated load (KLOAD) thus obtained is 
judged whether it is more than a criterion (KLOAD1) of the integrated load 
(step 308). The criterion (KLOAD1) of the integrated load is determined 
depending on the state of the intake air temperature used for correction 
(THAn) as described above, and as shown in FIG. 5. If step 308 gives NO, 
the step returns to step 304. 
If step 308 gives YES, the integrated load (KLOAD) is maintained until the 
internal combustion engine 2 stops (step 310), and step 312 ends the 
program. 
Therefore, in the integrated load measurement process in FIG. 4, the 
diagnosis of the relative pressure sensor 68 is delayed from the starting 
of the internal combustion engine 2 by means of the integrated load 
(KLOAD), and in the meantime the diagnosis is not executed. This is to 
avoid misjudgment even if moisture in a hose (not illustrated) 
communicating the relative pressure sensor 68 with the intake system 
freezes in extremely cold weather. The criterion of the integrated load 
(KLOAD1) by the intake air temperature is set with a sufficient time delay 
to melt the frozen moisture. 
Next, after the integrated load measurement (step 108), the integrated load 
(KLOAD) is compared with the criterion of the integrated load (KLOAD1), 
and whether the equation KLOAD.gtoreq.KLOAD1 is satisfied is judged (step 
110). If step 110 gives NO, the step returns the process to step 104. 
If step 110 gives YES, the relative pressure sensor 68 is diagnosed (step 
112). For example, the flow chart shown in FIG. 7 as Example 1, or the 
flow chart shown in FIG. 12 as Example 2, can be applied to the diagnosis 
of the relative pressure sensor 68. 
The diagnosis of the relative pressure sensor 68 in Example 1 is done 
according to the flow chart shown in FIG. 7. When the diagnosis of the 
relative pressure sensor 68 starts (step 402), an intake manifold pressure 
(PEG) is measured (step 404). And, whether or not the intake manifold 
pressure (PEG) is within the criteria is judged (step 406). 
As shown in FIG. 8, the criterion of the intake manifold pressure (PEG) is 
dependent on the load of the internal combustion engine 2 and is 
determined by the higher criterion (PEGH) and the lower criterion (PEGL), 
and the area (i.e., the criterion range) between the higher and the lower 
criterion is judged as normal. The data in FIG. 8 is acquired at a 
constant engine speed of 1500 rpm. In the middle of the area between the 
higher criterion (PEGH) and the lower criterion (PEGL), a standard value 
is determined. That is, when the engine speed is constant, the correlation 
between the engine load and the intake manifold pressure (PEG) comes in a 
region between the higher criterion (PEGH) and the lower criterion (PEGL), 
deviated from the standard value due to the characteristic dispersion of 
the relative pressure sensor 68. 
Although the higher criterion (PEGH) and the lower criterion (PEGL) are set 
in a judgment map by means of the intake manifold pressure and the engine 
load in FIG. 8, an abnormality judgment map for the higher criterion 
(PEGH) is expressed by means of the engine speed (Ne) and the engine load 
(FIG. 9), and an abnormality judgment map for the lower criterion (PEGL) 
is expressed by means of the engine speed (Ne) and the engine load (FIG. 
10). 
If PEGL.ltoreq.PEG.ltoreq.PEGH is not satisfied at step 406, this step 
gives a NO result and diagnoses the relative pressure sensor 68 to be 
disconnected and/or short-circuited, and step 408 judges it as abnormal. 
If PEGL.ltoreq.PEG.ltoreq.PEGH is satisfied at step 406 and this step gives 
a YES result, a variation of the intake manifold pressure (DPEG) and a 
variation of the engine load (DLOAD) are measured (step 410). Based on a 
correlation map as shown in FIG. 11 relating to the variation of the 
intake manifold pressure (DPEG) against the variation of the engine load 
(DLOAD), the diagnosis of normality and/or abnormality of the functional 
relative pressure sensor 68 is executed (step 412). In FIG. 11, the 
failure of the relative pressure sensor 68 is diagnosed depending on 
whether the variation of the intake manifold pressure (DPEG) against the 
variation of the engine load (DLOAD) is within the normal judgment range. 
The normal judgment range is set between the higher criterion (DPEGH) and 
the lower criterion (DPEGL). A standard value is set in the middle of the 
higher criterion (DPEGH) and lower criterion (DPEGL) judgment range. 
The step 414 judges whether the variation of the intake manifold pressure 
(DPEG) against the variation of the engine load (DLOAD) is satisfied with 
DPEGL.ltoreq.DPEG.ltoreq.DPEGH. If step 414 gives NO, then step 408 judges 
the function of the relative pressure sensor 68 as abnormal. If step 414 
gives YES, the next step 416 judges the relative pressure sensor 68 as 
normal. 
After steps 408 and 416 judge the abnormality and normality of the relative 
pressure sensor 68, this program is repeatedly executed until the internal 
combustion engine 2 stops (step 418), and step 420 ends the program. 
In Example 1 of the diagnosis of the relative pressure sensor 68, although 
the output voltage of the relative pressure sensor 68 is normal, the 
pressure characteristics of the relative pressure sensor 68 are not normal 
is judged on the basis of the variation of the intake manifold pressure 
(DPEG). The relation between the variation of the engine load (DLOAD) and 
the variation of the intake manifold pressure (DPEG) is shown in FIG. 11. 
The values of the higher criterion (DPEGH) and the lower criterion (DPEGL) 
are obtained by respectively adding and subtracting a variation factor to 
the intake manifold pressure, for example 0.8 (.+-.0.8), the range 
therebetween being the dispersion or variation of the characteristics of 
the relative pressure sensor 68 relative to the standard value. If the 
variation of the intake manifold pressure (DPEG) against the variation of 
the engine load (DLOAD) is out of the range DPEGL.ltoreq.DPEG 
.ltoreq.DPEGH, the function of the relative pressure sensor 68 judged as 
abnormal. 
Furthermore, the diagnosis of the relative pressure sensor 68 in Example 2 
is performed according to the flow chart in FIG. 12. In FIG. 12 "PE" is 
the input pressure of the pressure sensor and "TPE" is the corrected input 
pressure. If the program of diagnosing the relative pressure sensor 68 
starts (step 502), first, as shown in FIG. 13 and 14, the intake manifold 
pressure (PEG) is measured when the ignition switch 94 is turned ON (step 
504) as indicated at "1" in FIG. 13; and the atmospheric air pressure (PA) 
is measured (step 506). The intake manifold pressure when the ignition 
switch 94 is turned ON is equivalent to the atmospheric air pressure 
because the internal combustion engine 2 is not yet started. 
A first pressure (DPEG1) when the starter 96 is turned from OFF to ON is 
measured, and a second pressure (DPEG2) is measured at a complete 
explosion when the starter 96 is turned from ON to OFF (step 508) as 
indicated at "2" in FIG. 13. 
The step 510 judges whether PEG .gtoreq.KPEG is satisfied; where KPEG is a 
criterion reference for the intake manifold pressure (PEG). If step 510 
gives a NO result, it shows that the intake manifold pressure (PEG) is 
lower than the criterion reference (KPEG); and step 510 diagnoses the 
relative pressure sensor 68 as disconnected, and step 512 indicates it as 
abnormal. 
If step 510 gives a YES result, the first pressure (DPEG1) is compared with 
the first criterion reference (KDEG1) (FIG. 13), and whether 
DPEG1.gtoreq.KDPEG1 is met is diagnosed (step 514). 
If step 514 gives a NO result, it shows that the first pressure (DPEG1) is 
lower than the first criterion reference (KDPEG1); and step 514 diagnoses 
the relative pressure sensor 68 as short-circuited, and step 512 indicates 
it as abnormal. 
If step 514 gives a YES result, the second pressure (DPEG2) is compared 
with the second criterion reference (KDPEG2) (FIG. 13), and whether 
DPEG2.gtoreq.KDPEG2 is met is diagnosed (step 516). If step 516 gives NO, 
it shows that the second pressure (DPEG2) is lower than the second 
criterion reference (KDPEG2); and step 516 diagnoses the relative pressure 
sensor 68 as short-circuited, and step 512 indicates it as abnormal. 
If step 516 gives a YES result, the first pressure (DPEG1) is compared with 
a third criterion reference (KDPEG3) (FIG. 14); and whether 
DPEG1.gtoreq.KDPEG3 is met is diagnosed (step 518). If step 518 gives NO, 
it shows that the first pressure (DPEG1) is lower than the third criterion 
reference (KDPEG3); and step 512 indicates the relative pressure sensor 68 
as functionally abnormal. 
If step 518 gives YES, the second pressure (DPEG2) is compared with a 
fourth criterion reference (KDPEG4) (FIG. 14); and whether 
DPEG2.gtoreq.KDPEG4 is met is diagnosed (step 520). If step 520 gives NO, 
it shows that the second pressure (DPEG2) is lower than the fourth 
criterion reference (KDPEG4); and step 512 indicates the relative pressure 
sensor 68 as functionally abnormal. 
If step 520 gives YES, a correction factor (TPEG) is acquired from the 
intake manifold pressure (PEG) divided by the atmospheric air pressure 
(PA) (step 522). 
The corrected input pressure (TPE) is acquired from input pressure (PE) of 
the relative pressure sensor 68 correction factor (TPEG) (step 524). The 
correction factor (TPEG) is stored in a backup memory when the ignition 
switch 94 is turned ON, and is statistically processed in the form of 
TPEG=TPEG old.multidot.coefficient (for example, 0.9)+TPEG 
new.multidot.coefficient (for example, 0.1). Here, "TPEG old" is a 
previous correction factor, "TPEG new" is a present correction factor, and 
the coefficients 01.9 and 0.1 are a type of annealing coefficient. By 
means of the correction factor (TPEG) obtained by an atmospheric air 
pressure (PA) and the relative pressure (PE) after a complete explosion of 
the internal combustion engine 2, the relative pressure (PE) is converted 
into an absolute pressure (TPE); as shown in FIG. 15, the pressure 
characteristics shown in FIG. 17, 18 are corrected into the "design 
center" rating. 
The output voltage (PV) of the relative pressure sensor 68 during the time 
that the internal combustion engine 2 is not supplied with fuel within the 
combustion chamber is compared with the higher criterion (PVH) and the 
lower criterion (PVL) of the judgment range (step 526) while the engine is 
running. In this judgment range, as shown in FIG. 15, the output voltage 
(PV) against the input pressure (TPE) is illustrated, the higher criteria 
(PVH) and the lower criterion (PVL) are set, and the "design center" 
rating is determined in the middle of the range between the maximum value 
(MAX) and the minimum value (MIN), and thereby the dispersion of the 
characteristics of the relative pressure sensor 68 can be corrected into 
the design denter (PVmean) by the correction factor (TPEG). The step 526 
is to diagnose abnormalities of the relative pressure sensor 68 during 
running in a short period, during the time that the engine is not supplied 
with fuel if PV.ltoreq.PVL is met, the relative pressure sensor 68 is 
diagnosed as short-circuited. Furthermore, an operational condition of the 
engines are, when the engine integrated load (KLOAD).gtoreq.the criterion 
of the integrated load (KLOAD1) is met, and the output voltage 
(PV).ltoreq.the higher criterion (PVH) is met, the relative pressure 
sensor 68 is diagnosed as disconnected. 
And, the step 528 diagnoses whether the relative pressure sensor 68 is 
normal or not. If step 528 gives NO, the step 512 judges the relative 
pressure sensor 68 as abnormal. If step 528 gives YES, the relative 
pressure sensor 68 is normal, and step 530 ends the program. 
Still, if step 512 judges the relative pressure sensor 68 as abnormal, step 
530 ends the program. 
Furthermore, in the diagnosis of the relative pressure sensor 68, as shown 
in FIG. 16, by means of the engine speed (Ne) and engine load (KLOAD), the 
design center of the corrected input pressure (TPE) can be set by the map 
in FIG. 16; and it is possible to diagnose that the relative pressure 
sensor 68 is functionally abnormal, when the corrected input pressure 
(TPE) corresponding to the engine speed (Ne) and the engine load (KLOAD) 
is higher than TPE.+-.a specific error value (X), for example .+-.100 
mmHg. 
Therefore, according to Example 2 of the diagnosis of the relative pressure 
sensor 68, the relative pressure sensor 68 can be diagnosed as 
disconnected or short-circuited depending on variation of the intake 
manifold pressure when the ignition switch 94 is turned ON, the functional 
abnormality can be diagnosed, and the relative pressure sensor 68 can be 
diagnosed even when an abnormality occurs when the engine is running after 
the internal combustion engine starts. 
Next, in FIG. 1, step 114 judges whether the relative pressure sensor 68 is 
normal or not. If step 114 gives a NO result, an abnormality display unit 
such as a lamp (not illustrated) is illuminated to inform the abnormality 
of the relative pressure sensor 68 (step 116). 
On the other hand, if step 114 gives YES, the next step 518 diagnoses the 
EGR system. If step 120 gives NO, then step 116 displays an abnormality. 
If step 120 gives YES and step 116 finishes the process, then step 122 
ends the program. 
According to this embodiment, the relative pressure sensor 68 can precisely 
be diagnosed from the states of the intake air temperature and the engine 
load to determine the abnormality such as disconnection or short-circuit, 
or the functional abnormality thereof. 
Furthermore, the system according to this embodiment does not involve an 
incorrect abnormality judgment of the relative pressure sensor 68 due to 
frozen moisture in a hose communicating the relative pressure sensor 68 
with the intake system, which can happen in an extremely cold region, or a 
wrong abnormality diagnosis of the EGR system and the like in which the 
relative pressure sensor 68 is used. Therefore, confusion regarding 
maintenance and users' perplexity can be avoided. 
Still further, the system facilitates a precise diagnosis, and it can 
remove users' distrust due to a wrong diagnosis and eliminate performance 
of unnecessary maintenance work. 
The above method is performed in a suitable electronic device adapted to 
receive the appropriate input signals, perform the comparisons and output 
a corresponding result signal. Examples of the suitable electronic device 
are integrated circuits or electronic circuitry. 
As clearly understood from the detailed description hereinabove, it is 
possible to precisely diagnose abnormalities of the pressure sensor, avoid 
a wrong abnormality diagnosis, eliminate users' perplexity and confusions 
on maintenance, remove users' distrust, and eliminate unnecessary 
maintenance, by installing the control means diagnosing the pressure 
sensor to be in a failure when the intake air temperature is in a state of 
being measured and the integrated load of the internal combustion engine 
from its starting exceeds the criterion of the integrated load. 
Although a particular preferred embodiment of the invention has been 
disclosed in detail for illustrative purposes, it will be recognized that 
variations or modifications of the disclosed apparatus, including the 
rearrangement of parts, lie within the scope of the present invention.