1997 AUDI A8 check engine

24-166

Engine speed signal, checking


Note:
The engine speed signal is output at terminal 37
of the Engine Control Module (ECM). The signal
is used, among other things, for RPM display in
the instrument cluster.

- Connect VAG1598/31 test box at wiring harness
to ECM, do not connect ECM Page 24
-17
.

- Check the following wire connection for open
circuit and short circuit to Ground (GND) and B+:

Test box
VAG1598/31
Socket
Instrument cluster

Terminal
37 Electrical Wiring Diagrams,
Troubleshooting & Component
Locations binder

- Repair open circuit or short circuit if
necessary.
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24-171

Wire for fuel level signal, checking


Notes:

The Engine Control Module (ECM) receives the
fuel level signal from the instrument cluster.
The processed signal enters at terminal 30 of
the ECM and is used to determine the cause of
certain recognized malfunctions (e.g.
combustion misfires).

- Connect VAG1598/31 test box at wiring harness
to ECM, do not connect ECM Page 24
-17
.

- Check the following wire connection for open
circuit and short circuit to Ground (GND) and B+:

Test box
VAG1598/31
Socket
Instrument cluster

Terminal
30 Electrical Wiring Diagrams,
Troubleshooting & Component
Locations binder

- Repair open circuit or short circuit if
necessary.
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24-172

A/C compressor shut-off, checking


Notes:

The A/C compressor signal communicates to
the ECM that the compressor will be switched
on in 140 ms.
The ECM can switch off the A/C compressor via
the same wire connection.
Engine Control Module (ECM) switches off A/C
compressor:

-- during hard acceleration (Wide Open
Throttle 1st gear)

-- in emergency mode (emergency
operation)

-- after initiating basic setting (function 04)

Test requirements:
A/C system OK
No DTCs in DTC memory of Engine Control
Module (ECM).
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24-175

A/C system signal, checking

The A/C system signal notifies the Engine
Control Module (ECM), that the A/C system
requires a higher idle speed to achieve the
desired interior temperature (regardless of
whether heating or cooling is required).
The increase in idle speed based on presence of
the A/C system signal is control module
dependent and occurs only in vehicles with All
Wheel Drive (AWD) and manual transmission.

Test sequence

- Connect VAS5051 tester or VAG1551 scan tool
and select the control module for engine
electronics using "address word" 01 Page 01
-
9 .

Engine must run at idle for this.Rapid data transfer
HELP
Select function XX

When indicated on display:
- Press buttons -0- and -8- to select "Read Measuring Value Block" and
press -Q- button to confirm input.
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24-176
If specified values are not obtained:
Read Measuring Value Block
Q
Input displa
y group number XXX

When indicated on display:
- Press buttons -0-, -5- and -6- to select "display group number 056" and press -Q-
button to confirm input.
- Switch on A/C system, press "Auto" button and select the lowest temperature.
- Check indications in display field 3 and 4.

Display fields

1 2 3 4
Display group 056: Idle stabilization with engine at idle and operating temperature
Display xxx /min xxx /min
---
---
Indicated Engine speed (actual) Engine speed (specified)
A/C readiness
A/C compressor
Functional range

A/C - LOW
A/C - HIGH Compr. OFF
Compr. ON
Specified value
720 to 820 RPM
1)
630 to 730 RPM 2) 760 RPM
1)
680 RPM 2) A/C - HIGH
Compr. ON
1) Front Wheel Drive Vehicles 2) All-Wheel Drive Vehicles
- Connect VAG1598/31 test box at wiring harness to ECM, do not connect ECM Page 24
-17
.
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24-178

Brake light switch and brake pedal
switch, checking


Note:
For safety reasons, the instruction from the
accelerator pedal sensor (potentiometer) to the
ECM to open the throttle valve is suppressed
when brake is being operated. This requires the
brake light switch signal and also the brake pedal
switch signal in the control module.
If brake pedal is operated with accelerator pedal
constantly depressed, engine is immediately
regulated down to idle speed. A faulty switch can
lead to undesired regulation.

Test sequence

- Connect VAS5051 tester or VAG1551 scan tool
and select the control module for engine
electronics using "address word" 01 Page 01
-
9 .

Ignition must remain switched on for this.Rapid data transfer
HELP
Select function XX

When indicated on display:
- Press buttons -0- and -8- to select "Read Measuring Value Block" and
press -Q- button to confirm input.
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24-179

Read Measurin
g Value Block
Q
Input display group number XXX

When indicated on display:
- Press buttons -0-, -6- and -6- to select "display group number 066" and
press -Q- button to confirm input.
Read Measuring Value Block 66
1
2
3
4

Indicated on display:
- Check brake light/brake pedal switch in display field 2.

Display fields

1 2 3 4
Display group 066: Signals to Engine Control Module (ECM)
Display xxx km/h
XXXX xxx km/h XXXX
Indicated ACTUAL speed
Switch positions SPECIFIED speed Switch positions
Functional range
XX00
XX11
Specified value
XX00
(with brake pedal
not operated)

XX11
(with brake pedal
operated)
Notes
Value:
X = no significance.
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24-183

Clutch vacuum vent valve switch -F36-,
checking


Note:
This signal helps prevent excessive RPM
oscillations and tip-in shocks when clutch is
disengaged and is required for the cruise control
system.

Test sequence

- Connect VAS5051 tester or VAG1551 scan tool
and select the control module for engine
electronics using "address word" 01 Page 01
-
9 .

Ignition must remain switched on for this.Rapid data transfer
HELP
Select function XX

When indicated on display:
- Press buttons -0- and -8- to select "Read Measuring Value Block" and
press -Q- button to confirm input.
Read Measurin
g Value Block
Q
Input displa
y group number XXX

When indicated on display:
- Press buttons -0-, -6- and -6- to select "display group number 066" and
press -Q- button to confirm input.
Read Measuring Value Block 66

Indicated on display:
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