VW Passat & Audi A4 1990-2000

Engine Oil Pressure Sender

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TESTING

WARNING
If the oil pressure light in the instrument cluster stays on while the engine is running, it should be assumed that an oil pressure problem exists. Turn off the engine as soon as possible. Before performing any electrical testing, the oil pressure should be checked with a mechanical gauge. If the mechanical oil pressure gauge indicates little or no oil pressure, the oil pump and pressure relief valve should be inspected and repaired/replaced as necessary. If the oil pressure is correct, proceed with electrical testing.

Low Pressure Oil Pressure Switch

NOTE
The switch pressure is usually inscribed on the switch itself.

The low oil pressure warning lamp should illuminate anytime the ignition switch is turned to the ON position without the engine running. When the engine is running, the light also illuminates if the engine oil pressure drops below 3-7 psi (0.2-0.5 bar).

The oil pressure switch is a pressure sensitive on-off switch. The switch is calibrated such that if the oil pressure supplied to the switch falls below the minimum safe oil pressure for the engine to operate, the pressure switch completes (closes) the circuit between the electrical lead and ground.

If the engine's oil pressure falls below the pressure rating of the pressure switch, the pressure switch completes the ground circuit for the oil pressure warning light, and the low oil pressure warning light is activated.

The wire leading to the oil pressure switch is typically a blue wire with a black tracer on early model Passats, and a white wire with a gray tracer on later Passat and A4 models.

If the oil pressure light does not work, perform the following:

  1. Turn the ignition switch to the ON position.
  2.  
  3. If the lamp does not light, check for a broken or disconnected wire around the engine and oil pressure sending unit switch.
  4.  
  5. If the wire at the connector checks out OK, carefully remove the connector from the switch.
  6.  
  7. On models with a single wire electrical terminal:
  8.  
    1. Attach a jumper wire to a known good chassis ground and to the electrical connector.
    2.  
    3. If the lamp lights, replace the pressure sensor.
    4.  

  9. On models with a two wire electrical terminal:
  10.  
    1. Attach a jumper wire to a known good chassis ground and terminal 1 of the electrical connector.
    2.  
    3. If the lamp lights, place a jumper between terminals 1 and 2 of the electrical connector.
    4.  
    5. If the lamp lights, replace the oil pressure sensor.
    6.  
    7. If the lamp fails to light, check for an open circuit in the terminal 2 wiring.
    8.  

  11. If the light failed to operate during the above checks:
  12.  
  13. Check the wiring from the ignition switch to the bulb socket in the instrument cluster.
  14.  
  15. Check for a burned out lamp or disconnected socket in the instrument cluster.
  16.  
  17. Check for an open circuit in wiring between the bulb and the sensor.
  18.  

To diagnose an oil pressure light that stays on, perform the following:

  1. Check the engine oil level and top off as necessary. If the engine was 2 or more quarts low on oil, test start the engine after topping off the oil level and recheck the oil pressure light operation. If the light stays on, stop the engine immediately and go to the next step.
  2.  
  3. Detach the electrical connector at the sending unit, then turn the ignition switch to the ON position. If the light remains on, check the wire from the sending unit electrical connector (terminal 1 on a 2-wire system) to the warning light for being shorted to ground and repair as necessary.
  4.  
  5. If the light stops working, turn the ignition switch OFF and go to the next step.
  6.  
  7. Replace the oil pressure sending switch with a known good switch.
  8.  
  9. Start the engine and check the oil pressure light operation. If the light stays on with the engine running, stop the engine immediately to avoid severe engine damage until the cause of the problem can be determined.
  10.  


WARNING
If the oil pressure light remains on while the engine is started, stop the engine immediately. Do NOT operate the engine until the cause of the problem can be determined. Operating an engine with low oil pressure will cause severe internal damage.

Possible causes for an oil pressure light to stay on are:



Pinched or grounded sensor wire
 
Defective oil pressure sending unit
 
Restricted or severely leaking oil filter
 
Excessive internal engine bearing clearance.
 
Insufficient or severely contaminated engine oil
 
Blocked oil pump pickup screen or severely worn oil pump
 

High-Pressure Switch

Some models are also equipped with a high pressure oil pressure switch. The high-pressure sensor (normally open) operates in conjunction with electronic control unit. The electronic control unit receives reference signal from the tachometer when it reaches 2000-2500 rpm. The high-pressure switch should attain the closed position when the engine oil pressure exceeds 26 psi (1.8 bar) If the switch is not closed before the engine reaches approximately 2500 rpm, the electronic control unit will illuminate the oil pressure light.

  1. With the engine idling, disconnect the harness connector from the high pressure switch.
  2.  
  3. Raise the engine speed to 2500 rpm; the light should flash.
  4.  
  5. While holding the idle speed at 2500 rpm, ground the high-pressure switch harness connector.
  6.  
  7. With the high-pressure switch harness connector grounded, and the idle speed above 2500 rpm, the light should cease to flash. This indicates that the circuitry in the instrument panel is operating properly, and the switch is faulty.
  8.  

REMOVAL & INSTALLATION

Removal of the switch is simply a matter of unscrewing it from the cylinder head, engine block or oil filter housing. When installing the switch, apply a light coating of thread sealer to prevent leakage. Tighten the switch to 18 ft. lbs. (25 Nm).

 
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