Distributor Advance Unit: Testing and Inspection

VACUUM ADVANCE CONTROL SYSTEM
The distributor vacuum advance unit is connected by a hose to a fitting on the throttle valve. The vacuum signal is controlled by an electromagnetic valve mounted on the firewall. A relay controls the ground for the valve which is powered closed.

The relay is wired to a 113°F temperature switch (which is combined with the temperature gauge sensor), an intake air temperature sensor (with two sensing points: 50°F and 100°F) and the fuel injection control unit (for load sensing).

The relay logic circuits control vacuum advance as follows:
1. Intake air temperature < 50°F - vacuum advance to aid starting
2. Intake air temperature > 50°F - no advance for faster
Coolant temperature < 113°F - warm up
3. Intake air temperature > 50°F - vacuum advance
Coolant temperature > 113°F
4. Intake air temperature high engine load > 100°F - no advance to avoid pinging

In order to properly test the vacuum advance control relay the various temperature and load signals must be simulated. For this purpose a special tool has been developed and should be used while performing Engine Test 06 on the BMW Service Tester.

Checking Operation of Vacuum Advance Control System
- Pull off relay 3 and plug it on VACUUM ADVANCE CONTROL SIMULATOR.
- Connect plug 4 - see picture.
- Connect BMW SERVICE TEST Unit.
- Start engine and run at idle speed.
- The ignition timing displayed will already be "retarded" about 20 degrees on crankshaft by the simulator when switch 1 is positioned at "a".
- Lamp MV comes on to indicate that the solenoid is activated; the hose to the vacuum box is vented to atmosphere through the solenoid. This indicates no vacuum advance.
- Lamp tp (load signal) will flash during the entire test.
- Check the following points-see instructions on simulator.
1. Switch 1 set to "a"-lamp MV on (no advance). Switch 2 switched over to other position -ignition timing should not change.
2. Switch 1 set to "b"-lamp MV goes out. Ignition timing "advanced" about 20 degrees on crankshaft. Switch 2 switched over to other position - no ignition timing change.
3. Switch 1 set to "c"-same reading as "b" above.
4. Switch 1 set to "d" and switch 2 to 0 -lamp MV goes out, vacuum on advance box- ignition timing "advanced" about 20 degrees, switch 2 position 1 - lamp MV on (no advance)

Troubleshooting -VACUUM ADVANCE CONTROL SIMULATOR connected-
Lamp "tp" (Load signal) not flashing
- No Load signal
- No power supply on simulator
- LJetronic control unit, connections or wire harness defective
- Engine running erratically or not at all

Lamp "MV" not on
- No power Supply on simulator
- Wire harness for simulator defective
- Vacuum advance control unit detective

No advance control
- Vacuum advance control unit defective
- Connections or wire harness for solenoid defective

Ignition timing not reached
- Solenoid defective
- Vacuum hose leaks
- Distributor defective
- Vacuum advance control unit defective

Checking Coolant and intake Air Temperature Sensors
In this test the function of temperature switches and corresponding wires is checked. The values can be checked in the plug jacks with an ohmmeter.





The following values should be reached.

Sensor for
- air temperature > 10°C ... < 38°C approx. 950 ohms
- coolant temperature opens at < 45°C infinite ohms (see NOTE Below)
closes at > 45°C 0 ohms

NOTE: If the COOLANT temperature switch is checked with coolant temperature < 45°C the plug on the idle speed control unit (in glove box above L-Jetronic control unit) must be disconnected.


ENGINE CHANGES
1. The cooling capacity and flow rate have been increased and an 8 blade visco fan provides temperature controlled ventilation.
2. The compression ratio is 9.3:1 to improve fuel economy efficiency.
3. The intake and exhaust manifolds are engineered for increased engine torque and good emission control.