Sensor Plate Potentiometer - Checking, Adjusting

9229audi01

Group: 25

Number: 92-03

Date: 03/10/92


Subject:
Sensor Plate Potentiometer Checking, Adjusting


Model(s):
100 with NF engines
80/90 with NG engines

M.Y. 1989-1991
1988-1991



THIS TECHNICAL BULLETIN SUPERSEDES ALL PUBLISHED INFORMATION CONCERNING CHECKING AND ADJUSTING SENSOR PLATE POTENTIOMETERS.


Symptoms

A bad sensor plate potentiometer as well as other malfunctioning engine components may cause:

^ Engine idle normal, but drops with load (i.e.: A/C)
^ Engine idle too low or too high
^ Engine dies coming to a stop
^ Engine idle does not increase when cold


Requirement

^ Engine at operating temperature
^ Idle stabilizer valve OK
^ All possible vacuum leaks eliminated





^ Check Engine Basics:
- Idle speed, CO mA, ignition timing, adjust if necessary
- Perform vehicle self diagnosis
- See repair group 25 of repair microfiche for additional information


Note
A proper check and adjustment of the sensor plate potentiometer can only be obtained if the static, sweep and dynamic tests are carried out as described below.





Static Test/Adjusting

- disconnect potentiometer harness connector and connect VW 1501 test adaptor between potentiometer and harness connector
- switch multimeter US 1119 to 20 volt range and connect to terminals 1 and 3 (U1 voltage) of VW 1501 adaptor
- switch ON ignition
- must be 4.35 to 5.35 volts

Note
U1 : voltage supplied from fuel injection ECU
U2 : sensorplate potentiometer output voltage


If NO
- check power supply fuse of fuel inj. ECU
- check wiring between fuel injection control unit and potentiometer using wiring diagram, repair as necessary If wiring OK
- replace fuel injection ECU
- switch multimeter US 1119 to 2 volt range and connect to terminals 2 and 3 (U2 voltage) of VW 1501 adaptor





- switch ON ignition (but do not start engine)
- U2 must fall within the center third of the shaded area of the graph

If NO
- remove sealing compound covering head of potentiometer trim screw

Caution
Do not damage the trim screw when scraping away the sealing compound





- adjust U2 voltage via trim screw (arrow) while observing multimeter


Note
in order to obtain proper U2 reading during the dynamic test (engine at idle), U2 voltage at rest must fall within the center third of the shaded area of the graph


- if U2 rest voltage can not be adjusted, replace potentiometer/airflow sensorplate assembly.

Note
After replacing the potentiometer/airflow sensor plate assembly check and if necesary adjust the new potentiometer.

After adjusting:
- Secure the adjusting screw with sealing compound
- Erase the fault memory


Sweep Test





- Remove air filter cover and element
- To check U2 voltage "full sweep", push sensor plate from below slowly from rest position to full throttle
- U2 voltage must increase smoothly and continuously from U2 voltage at rest to approx. 3 - 5 volts

If not
- Replace potentiometer/airflow sensorplate assembly


Note
After replacing the potentiometer/airflow sensorplate assembly, check and if necesary adjust the new potentiometer.

After adjusting:
- Secure the adjusting screw with sealing compound
- Erase the fault memory


Dynamic Test

- Insert spare fuse into top of fuel pump relay (do not remove until test is completed)
- Start engine and let it idle
- Remove harness connector from idle air control valve (engine speed must increase)
- Remove idle air control valve from mount





- Clamp upper hose with pliers until an engine speed of 800 +/- 20 RPM along with an ignition angle of 13~-17~ BTDC is displayed on the VW 1367





- Observe U2 voltage.
Must fall within shaded area of graph


Note
To make accurate U2 voltage readings during the dynamic test, engine must be at operating temperature (radiator fan must cycle at least once)


If not
- The U2 reading is being influenced by another fault condition:
If too low: check for vacuum leaks - slightly low readings are usually caused by small vacuum leaks i.e.: injector O-rings, inserts

- correct vaccum leaks and recheck

If too high: replace potentiometer/airflow sensorplate assembly

Note
After replacing the potentiometer/airflow sensorplate assembly, check and if necesary adjust the new potentiometer.

After adjusting:
- Secure the adjusting screw with sealing compound
- Erase the fault memory
- Turn off ignition
- Remove hose clamp pliers
- Reconnect harness connector to idle air control valve
- Remove fuse from feel pump relay
- Restart engine (disregard U2 reading)