Page 21
32> S576
wo
wd
Technical description 200-19
Modulating valve
The modulating valve is a solenoid valve which
opens and closes at a constant frequency
(70 Hz) as soon as the engine has started.
How long the valve remains open and closed is
determined by the pulse ratio,
A long opening time (e.g. pulse ratio of 70/30)
results in a rich mixture, and a short opening
time (e.g. pulse ratio of 30/70) a lean mixture.
A pulse ratio. of 70/30 is known as a 70% ratio
and means that the modulating valve is open for
70 per cent and closed for 30 per cent of the
time.
Stepless adjustment of the pulse ratio of the
modulating valve is made continuously by the
ECU, thereby providing the fine-tuning of the mix- 7
ture required in cars equipped with catalytic con- 70/30 .
verters. |
60/50 q
Frequency of 70 Hz
Saab 900
Page 22
ML SEES
200-20 Technical description
Fuel distributor
When the pulse ratio of the modulating valve is
high, a larger quantity of fuel will flow from the
chamber below the steel diaphragm in the pres-
sure-regulating valves, thereby momentarily re-
ducing the pressure below the diaphragm by ap-
prox. 0.2 bar (2.9psi). This pressure drop in-
creases the flow of fuel through the pressure-
regulating valves and, because the opening of
the metering slots remains constant, a richer
mixture is delivered to the engine.
If the oxygen sensor signals a low oxygen level
(rich mixture), the valve-open time will be re-
duced and a smaller quantity of fuel will flow
from the chamber below the diaphragm in the
pressure-regulating valves. This results in a re- (
duction in the pressure difference and fuel flow,
and hence a weakening of the mixture.
Fuel distributor
Gm Line pressure
Gay «SCS «Upper chamber pressure a
Magy = Lower chamber pressure 1 Injection valve
Ga «= njection pressure 2 Control pressure regulator
3 Inlet
Control pressure .
| P 4 Return line
i ~CCOReturn, no pressure 5 Modulating valve
Saab 900
Page 23
Nr
Technical description 200-21
Summary
The pressure difference across the diaphragm in
the pressure-regulating valves is controlled by
variation of the pulse ratio. Variations in the
pressure difference give rise to a richer or
weaker mixture. The magnitude of the rise or fall
in pressure is governed by the signal voltage
from the oxygen sensor.
Because the oxygen sensor first becomes opera-
tive at a temperature of 300°C (572°F), while
the starter motor is running and for a period of
three seconds after starting, the pulse ratio will
be raised to at least 92% (via ECU terminal 11).
If the engine temperature is below 19°C (66°F),
the pulse ratio will be 60% for the first three
seconds after starting, after which the mixture
will be enriched via the cold-start valve (temper-
atures below 45°C/113°F) or via relay P11
(temperatures 45 °C/113 °F or above).
A ratio of 50/50 is that obtaining under normal
conditions of engine load. If the oxygen sensor
should fail to emit a signal for any reason, the
system will revert automatically to this ratio.
Fuel boosting
There are four types of fuel boosting:
© Cold-start boosting (via the cold-start valve or
relay P11 depending on temperature)
* Boosting during acceleration (via fuel booster
pressure switch for 140 20s after starting)
¢ Rpm-governed boosting (via the speed relay
(138) at engine speeds above 3800 rpm (M88
onwards: 4600 rpm). Provides a fixed pulse
ratio of 85%. Applicable to Turbo cars only).
* Full-load boosting (via the throttle-position
sensor)
Turbo cars M87 and earlier: minimum pulse
ratio of 92% when throttle butterfly angle
>72°
or
Fixed pulse ratio of 85% minimum at speeds
greater than 3800rpm (M88 _ onwards:
<4600 rpm).
* Pressure-governed full-load boosting (M88
onwards).
At an inlet manifold pressure of 0.25 - 0.30
bar (3.6 - 4.4psi) a pressure switch is ac-
tuated to effect full-load boosting (fixed pulse
ratio of 85%).
Full-load boosting is not fitted to normally as-
pirated engines in M87 cars onwards.
Fuel boosting is provided to improve drivability
during the warm-up period and on acceleration.
On turbo engines, it is also designed to ensure
sufficient internal cooling of the engine at
maximum load and at speeds above 3800 rpm
(M88 onwards: 4600 rpm).
Saab 900
Page 24
006 Gees
Oxygen-sensor blocking Operation of cold-start and boosting systems
Cold-start valve
|
Cold-start
valve + P14. |
|
|
| |
|
|
Cold-start valve
| P14 for max. 1205
fue! shut-off
Cold-start
valve + P14
Cold-start valve
P11 for max. 120s
fuel shut-off
—— ee el
Cold-start
valve + Pit
y
P11 for max, 120s
fuel shut-off
e
P11
Pii for max. 120s
fuel shut-off
- oa
P11 for max. 120s
pa fuel shut-off
Enrichment functions triggered
by fuel booster pressure switch
Closing temp oo _ ;Prening temp Closing temp, | ta =I
t +
1942
© Regardless of temperature, Pi1 boosting
(pulse ratio 92% minimum) always takes
place while the starter motor is running and
for three seconds after starting.
* Pulsed signal (0.10s open and 0,.3s closed)
to cold-start valve occurs after starter motor
|
|
|
|
|
|
|
|
|
|
|
Zz
Opening tem
Sis. Oe
has been running for 1.4s. This function is
operative when the 45 °C thermostatic switch
and/or TTK is/are open,
© Oxygen-sensor blocking (P12) results in a
fixed pulse ratio of 60% (also that obtaining
during fuel shut-off).
—ge- Coolant temperature, °C
Ud!}dOSap jeI1UYy9a| ZZ-00T
Page 25
. . a= |
% (also that obtaining
~~
fixed pulse ratio of 60
during fuel shut-off).
sed)
motor
- Ss Spr pw ae > UpC anu U.SS CIO
to cold-start valve occurs after starter
s-é
Technical description 200-23
Fuel-boosting system
M81-83 incl.
A Cold-start function
The cold-start valve can open only when the star-
ter motor is running. The time the valve remains
open, injecting fuel, is governed by the thermo-
static time switch, which senses engine temper-
ature. The maximum injection time of approx.
9.5.s occurs at temperatures below -20°C (-4 °F),
At higher engine temperatures the injection time
becomes progressively shorter and the cold-
start valve ceases to operate at temperatures
above 45°C (113°F).
B Hot-start function
If, when hot, the engine is slow to start because
of vapour in the fuel lines, the cold-start valve
will be energized by current supplied by a hot-
start relay. Thus, the fuel required by the engine
for starting is provided by the cold-start valve and
more-rapid purging of the vapour in the fuel lines
is obtained.
If the engine fails to start after 1.4, injection
pulses lasting 0.4 s will occur at 0.3-s intervals.
1 Thermostatic time switch
2 Cold-start valve
3 Starter motor
4 Cold-start/hot-start relay
1 Starter motor
2 Cold-start valve
3 Hot-start relay
Saab 900
Page 26
200-24 Technical description
C Fuel boosting on acceleration
This function provides better drivability after
starting. During acceleration, the fuel booster
pressure switch responds to the pressure differ-
ence caused by the increased load on the engine
and closes, making the circuit to the cold-start
valve,
Engine cold
Fuel boosting takes place until the engine temp-
erature rises above 45°C (113°F).
Engine hot
Fuel boosting takes place for about 100s if the
engine has been standing for more than half an
hour.
Fuel booster pressure switch
When the car is travelling at a constant speed,
the depression is the same on either side of the
diaphragm. On acceleration, the depression
above the diaphragm falls, but because of the re-
striction there is a delay before the depression
below the diaphragm falls. At the moment of
delay, the pressure above the diaphragm is rela-
tively higher and the diaphragm is forced down,
closing the switch contacts.
1 Diaphragm
2 Spring
3 Restriction
ooh
Spring contact arm
Vacuum connection
Electrical terminals
st
Saab 900
Page 27
CE _L Ee
Technical description 200-25
Electrical functions: engine cold
The cold-start valve is energized when the circuit
is made by the fuel booster pressure switch. The
n circuit is earthed via terminals 30 and 87a of the
cold-start/hot-start relay and the thermostatic
time switch.
1
Sit
1 Thermostatic time switch
2 Cold-start/hot-start relay
3 Cold-start valve
4 Fuel booster pressure switch
Electrical functions: engine hot
If the engine has been switched off for more than
about 30 minutes ( 10 minutes) but the engine
wd temperature is above 45°C (113°F), and the
thermostatic switch on the throttle housing is
therefore closed, fuel boosting will take place for
100s after starting. Current flows from the fuel
| pump relay through the coil and terminal 85 in
we the cold-start/hot-start relay, through diode 85
and terminal 31 in the fuel booster time relay
and to earth via the thermostatic switch, thereby
operating the cold-start/hot-start relay.
If the time relay has not been energized for 30
minutes (10 min.), the contacts in the relay will
be closed. The cold-start valve will then be
earthed via terminals 30 and 87 in the cold-start/
hot-start relay, terminals 87 and 31 in the time
relay and the thermostatic switch.
Saab 900
Page 28
200-26 Technical description
In 1981-model Turbo cars and 1982-model Tur-
bos with automatic transmission, this system
would deliver an excessive quantity of fuel, for
which reason an additional relay is fitted to re-
strict the flow of fuel by modulation of the cold-
start valve. The relay is operated by the fuel
booster pressure switch and is wired into the cir-
cuit between the fuel booster time relay and the
cold-start/hot-start relay.
Thermostatic time switch
Cold-start valve
Fuel booster pressure switch
Time relay
Cold-start/hot-start relay
Thermostatic switch
Fuel pump relay
Pulse operation relay
OAANAOAEWNH
Saab 900
Page 29
200-28 Technical description
ee ee
b When the starter motor is running, relay
P11 also receives a signal and connects
pin 11 in the Lambda control unit to
earth. The control unit sets the modulat-
ing valve to a fixed pulse ratio greater
than 92%, causing the valve to remain
open and the injection valves to provide a
boost of fuel. This function continues to
operate for three seconds after the en-
gine has started.
ei etet vi
i a7 TIP TAK} OX?
S 2/419
1 Thermostatic switch
2 Modulat
3 Lambda
ing valve
control unit
4 P11 relay
2 Hot starting, i.e. engine temperature above
25°C (77°F).
a Asis the case when the engine is started
from cold, the P11 relay receives a signal
when the starter motor is running. The
modulating valve opens wide and the in-
jection valves deliver a boost of fuel. This
function continues to operate for three
seconds after the engine has started.
b If the engine has failed to start after
1.4s, the hot-start relay, which is a
pulse-operation relay, operates and the
cold-start valve boosts the supply of fuel
in injection pulses of 0.1s.
1
P<]
1 Cold-start valve
2 Pulse-operation relay (hot starting)
S 2/420
Saab 900
Page 30
6
Technical description 200-29
B Acceleration system
This system improves the drivability of the car
after starting. It is controlled by the fuel booster
pressure switch which senses changes in the de-
pression in the inlet manifold.
Driving with the engine cold, i.e. engine
temperature below 18°C (65°F).
Fue! boosting is provided by the cold-start valve.
When the fue! booster pressure switch closes,
current flows to the cold-start valve, which is
earthed via the thermostatic switch and the ther-
mostatic time switch. When the engine tempera-
ture reaches 25°C (77°F) the thermostatic
switch breaks the circuit, and fuel boosting
ceases.
if the engine temperature reaches 25°C (77 °F)
within two minutes of starting, the P11 relay op-
erates, the modulating valve opens fully and ad-
ditional fuel is injected by the injection valves.
Driving with the engine hot, i.e. engine tempera-
ture above 25°C (77°F)
Each time the fuel booster pressure switch
closes, a signal is sent to the P11 relay, which
senses that the thermostatic switch is open, and
no fuel boosting can be provided by the cold-
start valve. The P11 relay then earths pin 11 in
the Lambda control unit, the modulating valve
opens wide and a boost of fuel is delivered by the
———» |
50 TKK
11
0 TRK
ri 87 TIP X23!
injection valves. This cycle is repeated every
time the pressure switch makes, within a
maximum period of two minutes from starting.
1
ae
~<
i
1 Fuel booster pressure switch
2 Cold-start valve
3 Thermostatic switch
S$ 2/421
S 2422
|
+
1 Thermostatic switch
2 Starter motor
3 Lambda control unit
4 Modulating valve
5 Pressure switch
6 Pit-relay
Saab 906