ACURA NSX 1991 Service Repair Manual

Idle Control System

System Troubleshooting Guide

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 Malfunction Indicator Lamp (MIL) does not blink code 14, go to inspec-

tion described on page 11-88.

PAGE
SUB-SYSTEM

SYMPTOM
DIFFICULT TO START
ENGINE WHEN COLD
WHEN COLD FAST IDLE OUT OF
SPEC
(1,000-2,000
rpm)
ROUGH IDLE
WHEN WARM RPM TOO HIGH
Idle speed is

below specified
rpm (no load)
Idle speed does
not increase after
initial start up.

WHEN WARM
RPM

TOO LOW
On models with

A/T, the idle

speed drops in gear

Idle speeds drops

when air

conditioner in ON or

OFF

Idle speed

fluctuates with

electrical load
WHILE
WARMING UP

FREQUENT
STALLING

AFTER
WARMING UP
FAILS EMISSION TEST
IDLE

ADJUST-

ING

SCREW
IDLE AIR

CONTROL
VALVE
AIR

CONDI-

TIONING
SIGNAL
ALTER-

NATOR
FR

SIGNAL
NEUTRAL

SWITCH

SIGNAL
(M/T)
STARTER

SWITCH
SIGNAL
FAST

IDLE

THERMO
VALVE
HOSES

AND

CONNEC-
TIONS

CLUTCH

SWITCH

SIGNAL
(M/T)

AUTOMATIC

TRANSAXLE
GEAR

POSITION
SIGNAL

102

88

90

92


94

96

98


100


101ProCarManuals.com

System Description

The idle speed of the engine is controlled by the Idle Air Control (IAC) valve.

The valve changes the amount of air bypassing into the intake manifold in response to electric current controlled by the

ECM.

When the IAC valve is activated, the valve opens to maintain the proper idle speed.

(cont'd)
IDLE

ADJUSTING

SCREW

FAST

IDLE

THERMO

VALVE
IAC VALVE

TO PGM-FI MAIN

RELAY
YEL/BLK
BLU/REDProCarManuals.com

1. When the idle speed is out of specification and the Malfunction Indicator Lamp (MIL) does not blink code 14, check

the following items:
Adjust the idle speed (see page 11-102)
Air conditioning signal (see page 11 -90)
ALT FR signal (see page 11-92)
A/T gear position switch signal (see page 11-94)Neutral switch signal (M/T) (see page 11-96)
Clutch switch signal (M/T) (see page 11-98)
Starter switch signal (see page 11-100)Fast idle thermo valve (see page 11-101)
Hoses and connections
IAC valve and its mounting O-rings
2. If the above items are normal, substitute a known-good IAC valve and readjust the idle speed (see page 11-102). If the idle speed still cannot be adjusted to specification (and the MIL does not blink code 14) after IAC valve
replacement, substitute a known-good ECM and recheck. If symptom goes away, replace the original ECM.ProCarManuals.com

Idle Control System

Troubleshooting Flowchart
Air Conditioning Signal

This signals the ECM when there is a demand for cooling from the air conditioning system.

Inspection of Air Condition-

ing Signal.
Connect the test harness be-
tween the ECM and connector. Disconnect "C" connector from
the engine wire harness only, not
the ECM (see page 11-37).
Turn the ignition switch ON.
Measure voltage between C3 (+)

terminal and A26 (-) terminal.

Is there approx. 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?

Start the engine.

Blower switch ON.
(To page 11-91) Repair open in RED/BLU wire be-
tween ECM (A15) and A/C
clutch relay. 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. View from relay side
(sub relay box A)

RED/BLU

See air conditioner in-
spection (see section
22).
Substitute a known-
good ECM and re-
check. If prescribed
voltage is now avail-
able, replace the ori-
ginal ECM.ProCarManuals.com

(From page 11-90)
A/C switch ON.
Does A/C operate ?

Air conditioning signal is OK.
Is voltage less than 1.0 V?
Measure voltage between C3

( + ) terminal and A26 (-) ter-

minal.

Substitute a known-good ECM
and recheck. If symptom/indica-
tion goes away, replace the orig- inal ECM. Repair open in BLU/
BLK wire between ECM
(C3) and A/C switch.ProCarManuals.com

Idle Control System
Troubleshooting Flowchart

Alternator (ALT) FR Signal
This signals the ECM when the alternator is charging.
Inspection of ALT FR signal.
Connect the test harness be-
tween the ECM and connector.
Disconnect "D" connector from
the engine wire harness only, not
the ECM (see page 11-37).

Turn the ignition switch ON.

Measure voltage between D9 ( + ) terminal and A26 (-) ter-
minal.
Is there approx. 5 V?
Turn the ignition switch OFF.
Reconnect "D" connector to the
engine wire harness.
Warm up engine to normal oper-ating temperature (the cooling
fan comes on).
Measure voltage between D9( + ) terminal and A26 (-) ter-
minal.

Does the voltage decrease when

headlights and rear defogger are

turned on ?

Do the ECM Reset Procedure
(see page 11-35).
ALT FR signal is OK.(To page 11-93) Substitute a known-good ECM

and recheck. If prescribed

voltage is now available, replace
the original ECM.
Turn the ignition switch OFF.ProCarManuals.com

(From page 11-92)

Disconnect "D" connector from

ECM only, not the engine wire

harness.
Disconnect the negative battery
cable from the battery.

Check for continuity between D9

terminal and body ground.

Does continuity exist ?
Remove the harness covers.
Disconnect GRN connector from

the
ALT.
Connect WHT/RED wire to body

ground.
Check for continuity between D9

terminal and body ground.

Does continuity exist ?
Repair open in WHT/RED wire between ECM (D9) and ALT.

See ALT inspection
(see section 23). Repair shor in WHT/RED wire
between ECM (D9) and ALT.
Does continuity exist ?

Check for continuity between D9

terminal and body ground.
Disconnect GRN connector from

the
ALT. Remove the harness covers.
See ALT inspection(see section 23).
GRN CONNECTORProCarManuals.com

Idle Control System
Troubleshooting Flowchart Automatic Transaxle (A/T) Gear Position
Signal
This signals the ECM when the transmission is in Neutral or Park.
Inspection of A/T gear position

signal.

Turn the ignition switch ON.
Observe the A/T gear position in-
dicator and select each position
separately.

Does the indicator light properly?
Turn the ignition switch OFF.
Connect the test harness be-
tween the ECM and connector.
Disconnect "B" connector from

the engine wire harness only, not

the ECM (see page 11-37).

Turn the ignition switch ON.

Measure voltage between B7

( + ) terminal and A26 (-) ter-

minal.
Is there approx. 10V?
Turn the ignition switch OFF.
Reconnect "B" connector to the
engine wire harness.
Turn the ignition switch ON.
(To page 11-95) See A/T gear position switch in-
dicator inspection (see section
14).
Substitute a known-good ECM
and recheck. If prescribed voltage is now available, replace
the original ECM.ProCarManuals.com

(From page 11-94)
Measure voltage between B7 (+)

terminal and A26 (-) terminal
with the transmission in and

range individually.

Is there less than 1.0 V?

Measure voltage between B7 ( + ) terminal and A26 (-) ter-

minal with the transmission in

gear.

Is there approx. 10 V?
A/T gear position signal is OK. Repair short in LT GRN wire be-
tween ECM (B7) and gauge

assembly.

Repair open in LT GRN wire be-
tween ECM (B7) and gauge

assembly.ProCarManuals.com