ACURA NSX 1991 Service Repair Manual

4. A/C Compressor Clutch Relay

When the ECU receives a demand for cooling from the air conditioning system (compressor control unit), it delays

the compressor from being energized, and enriches the mixture to assure smooth transition to the A/C mode.

5. Purge Cut-off Solenoid Valve

When the coolant temperature is below 70 °C (158 °F), the ECU supplies a ground to the purge cut-off solenoid valve

which cuts vacuum to the purge control valve.

6. Chamber Volume Control Solenoid Valve (CVCSV)

When the engine rpm is below 4,800 rpm the CVCSV is activated by a signal from the ECU, intake air flows through

a smaller chamber, then high torque is delivered. At speeds higher than 4,800 rpm, both solenoid valves are deac-

tivated by the ECU, and intake air flows through the a larger chamber in order to increase airflow.

7. EGR Control Solenoid Valve (EGR CSV)

When the EGR is required for control of oxides of nitrogen (NOx) emissions, the ECU supplies ground to the EGR CSV

which supplies regulated vacuum to the EGR valve.

ECU Back-up Functions

1. Fail-Safe Function

When an abnormality occurs in a signal from a sensor, the ECU ignores that signal and assumes a pre-programmed

value that allows the engine to continue to run.

2. Back-up Function

When an abnormality occurs in the ECU itself, the injectors are controlled by a back-up circuit independent of the

system in order to permit minimal diving.

3. Self-diagnosis Function (Check Engine light)

When an abnormality occurs in a signal from a sensor, the ECU lights the Check Engine light and stores the failure

code in erasable memory. When the ignition is initially turned on, the ECU supplies ground for the Check Engine light

for two seconds.ProCarManuals.com

PGM-FI Control System
Troubleshooting Flowchart
ECU

Check Engine light never comes

on (even for two seconds) after

ignition is turned on.

Is the oil pressure light on?
Turn the ignition switch OFF.
Inspect No. 5 fuse.

Is No. 5 fuse OK? Replace fuse.
Connect the ECU test harness
between the ECU and connector

(page 11-21).

Connect A13 terminal to body
ground.
Repair open in YEL between

No. 5 fuse and gauge assembly.

A13

Turn the ignition switch ON. Replace the light

bulb.
Repair open in BLU

wire between ECU

(A13) and gauge

assembly.

A23

A24 A26 B2
G101

Less than 1 V

Is Check Engine light on?
Measure voltage between body
ground and the following ter-
minals individually: A23, A24,

A26,
B2.

Is there less than 1V?
Repair open in wire

between ECU and in-
take manifold (G101)
that had more than 1V.
Substitute a known-good ECU and recheck. If symp-
tom/ indication goes away,
replace the original ECU.ProCarManuals.com

Idle Control System
NOTE: Across each row in the chart, the sub systems that could be sources of a symptom are ranked in the order they should

be inspected, starting with . Find the symptom in the left column, read across to the most likely source, then refer to

the page listed at the top of that column. If inspection shows the system is OK, try the next system , etc.

If the idle speed is out of specification and the Check Engine light does not blink CODE 14, go to inspection described

on page 11-70.

PAGE

86

70

72

74


76

78


80

82

83 *

System Troubleshooting GuideProCarManuals.com

System Description
The idle speed of the engine is controlled by the Electronic Air Control Valve (EACV).
The valve changes the amount of air bypassing into the intake manifold in response to electric current sent from the ECU.
When the EACV is activated, the valve opens to maintain the proper idle speed.

BLU/REDEACV

ECU

YEL/BLK

TO MAIN

RELAY

IDLE

ADJUSTING

SCREW
FAST

IDLE

VALVE
(cont'd)
ProCarManuals.com

Idle Control System

System Description (cont'd)

1. After the engine starts, the EACV opens for a certain time. The amount of air is increased to raise the idle speed about 150

-300 rpm.

2. When the coolant temperature is low, the EACV is opened to obtain the proper fast idle speed. The amount of bypassed air

is thus controlled in relation to the coolant temperature.

ENGINE SPEED

(rpm)

COOLANT TEMPERATURE

(°C)ProCarManuals.com

1. When the idle speed is out of specification and the Check Engine light does not blink CODE 14, check the following
items:Adjust the idle speed (page 11- 84)
Air conditioning signal (page 11-72)
Alternator FR signal (page 11-74)
A/T shift position signal (page 11-76) M/T neutral switch signal (page 11-78)
M/T clutch switch signal (page 11- 80)
Starter switch signal (page 11- 82)
Fast idle valve (page 11- 83)
Hoses and connections
EACV and its mounting O-rings
2. If the above items are normal, substitute a known-good EACV and readjust the idle speed (page 11- 84).
If the
idle
speed
still
cannot
be
adjusted
to
specification (and
the
Check Engine
light
does
not
blink
CODE
14)
after
EACV replacement, substitute a known-good ECU and recheck. If symptom goes away, replace the original ECU.ProCarManuals.com

Idle Control System

Troubleshooting Flowchart
Air Conditioning Signal
This signals the PGM-FI ECU when there is a demand for cooling from the air conditioning system.

Inspection of Air Condition-

ing Signal.

Connect the ECU test harness

between the ECU and connector.

Disconnect "C" connector from
the engine wire harness only, not

the ECU (page 11-21).

Turn the ignition switch ON.
Measure voltage between C3 (+)

terminal and A26 (-) terminal.

Is there approx. 10 V?
Substitute a known-

good ECU and re-

check. If prescribed

voltage is now avail- able, replace the ori-

ginal ECU.
C3

A26

10 V

Turn the ignition switch OFF. Reconnect "C" connector to the
engine wire harness.
Turn the ignition switch ON.
Momentarily connect A15 ter-
minal to A26 terminal several
times.
Is there a clicking noise from the
A/C compressor clutch? Connect the RED/BLU terminal of

the 4P connector on the A/C

clutch relay to body ground. RED/BLU

A 15

Short
A26

View from UNDER-HOODFUSE/RELAY BOX side

Start the engine.
Is there a clicking noise from the
A/C compressor clutch?
See air conditioner in-

spection (section 22).

Repair open in RED/BLU wire be-

tween ECU (A15) and A/C clutch
relay.

Blower switch ON.
(To page 11-73)ProCarManuals.com

(From page 11-72)
Less than 1 V

C3

A26

A/C switch ON.
Does A/C operate ?

Air conditioning signal is OK.
Is voltage less than 1V?
Measure voltage between C3

( + ) terminal and A26 (-) ter-

minal.

Repair open in BLU/
BLK wire between ECU
(B5) and A/C switch.
Substitute a known-good ECU
and recheck. If symptom/indica-
tion goes away, replace the orig-
inal ECU.ProCarManuals.com

Idle Control System
Troubleshooting Flowchart Alternator FR Signal

This signals the PGM-FI ECU when the alternator is charging.

Inspection of Alternator FR

signal.

Connect the ECU test harness
between the ECU and connector.
Disconnect "D" connector from
the engine wire harness only, not

the ECU (page 11-21).

Turn the ignition switch ON.
Measure voltage between D9 ( + ) terminal and A26 (-) ter-

minal.

Is there approx. 5 V?
5V

D9

A26

Substitute a known-good ECU and recheck. If pre-
scribed voltage is now avail-
able, replace the original ECU.
Turn the ignition switch OFF.
Reconnect "D" connector to the
engine wire harness.
Warm up engine to normal oper-
ating temperature (cooling fan

comes on).
Voltage

D9

A26

Measure voltage between D9
( + ) terminal and A26 (-) ter-
minal.
Stop the engine.

Does the voltage decrease when

headlights and rear defogger are

turned on ?

Alternator FR signal is OK.

(To page 11-75)ProCarManuals.com

(From page 11-74)
Disconnect "D" connector from
ECU only, not the engine wire
harness.
Disconnect the negative battery
cable from the battery.

Check for continuity between D9

terminal and body ground.
Continuity ?

Disconnect GRN connector from
the alternator.
Disconnect GRN connector from
the alternator. GRN CONNECTOR
Connect WHT/RED wire to body
ground.
Check for continuity between D9

terminal and body ground.

Check for continuity between D9

terminal and body ground.
Does continuity exist ?
Does continuity exist ? See Alternator Inspec-
tion.
See Alternator Inspec-

tion.

Repair open in WHT/RED wire
between ECU (D9) and alter-
nator. Does continuity exist ?
Repair short in WHT/RED wire
between ECU (D9) and alter-
nator.ProCarManuals.com