The diagram shows the basic ignition timing curve verses engine speed, without the ignition timing changes which result from input from the pressure sensor and intake air temperature sensor.
To make starting easier, the ignition timing is relatively retarded during engine cranking. As the engine starts, the timing is advanced quickly and then retards as the engine approaches idle speed. If the idle speed drops (e. g. when switching on electrical equipment or the air conditioner), the timing is advanced in order to stabilize the idle speed.
As engine speed increases above idle the timing is at first advanced quickly and then more gradually so that engine response, power output and fuel economy are optimized. •
Other input data, such as intake vacuum and intake air temperature, wil up or down depending on operating conditions.
change the timing curve
The "reference mark tooth" eliminates the necessity for adjustment of the ignition timing.
Since the engine speed and ignition timing will change when adjusting the idle air volume, it is now necessary to make idle adjustments dependent on the ignition timing (see idle adjustments).
The DITC (digital ignition timing control) includes idle speed regulation.
The timing control unit stabilizes the idle speed by varing the ignition timing at idle. As a result, basic idle speed adjustment must be made using an ignition timing light.
To eliminate the influence of the intake air temperature sensor, it must be removed during the idle adjustment procedure.
— Unscrew temperature sensor from intake manifold and place it in the fresh air tray behind the engine fire wall, leaving the wire connected (sensor temperature must be below 50 °C — 120 °F).
A suitable plug (M 14 x 1.5) must be screwed in the opening of the intake manifold during adjustment procedures.
Tip: Use stop screw from manual transmission 924, Part No. 088.311.721.
— Run engine to operating temperature (about 90 °C oil temperature).
— Connect CO tester according to instructions supplied with tester.
— Connect stroboscope timing light on ignition wire for cylinder no. 1.
— Turn idle control screw (bypass air screw) until ignition timing mark (dot) is fully visible at reference adge and jumps below reference mark partially.
— The speed should then be below 900 rpm (timing will vary slightly because the ignition timing is being regulated).
If necessary, the CO level must be corrected parallel to the idle speed adjustment' (as previously). No electric equipment should be switched on during adjustments.
Notes on Removal and Installation of Distributor
— Set cylinder no. 1 to TDC (Z 1 mark on flywheel must align with reference edge on clutch bell housing and mark on camshaft sprocket must be opposite valve cover.
— The distributor must be installed so that tab for distributor cap faces flywheel and mounting clips face in direction of car's longitudinal axis.
The distributor rotor must be mounted so that it faces mark for cylinder no. 1 on the distributor housing.
— Make sure ignition wires are connected correctly.
DITC Wiring Diagram
3 — Integral pressure sensor
4 — Temperature sensor
5 — Throttle switch
6 — Flywheel sensor
9 — Ignition control unit 10 — Ignition coil
A — Ignition switch term. 15 B — Tachometer term. 1
• The ignition timing will only change depending on the speed with a closed throttle and intake air temperatures below 50 - 60 °C (120 - 140 °F).
• A wire plug connection can be seen in the engine compartment on the left wheel housing. If it is disconnected, the entire ignition timing will be retarded by about 7°.
This plug can be disconnected, if necessary, in countries with poor quality gasoline in order to eliminate detonation.
• If the temperature sensor fails or the wire is pulled off, the ignition timing will be retarded. Also when making idle adjustments, make sure sensor temperature is below 60 °C (120 °F) (see idle adjustments).
• I f the pressure sensor of the control unit fails or the pressure hose is disconnected or leaks, the ignition timing will be advanced. Consequently always be sure of tight connections and correct routing of pressure hose!
• If the throttle switch fails or a wire is pulled off, there will be no ignition retard during starting or coasting, and the idle speed will be too high.
• If the flywheel sensor fails, the entire ignition system will not operate.
The engines for USA and Canada have the ignition system "DITC" in conjunction with the oxygen sensor system. The circuitry of the oxygen sensor control unit has been changed because of this modification and the revised oxygen sensor system operation.
The former speed relay has been omitted. This relay is replaced by an acceleration enrichment control unit (18). This control unit performs the function of the former speed relay and also has a circuit for cold engine acceleration enrichment.
The water temperature switch (12) installed in the coolant return hose near cylinder no. 4 is new. It activates control unit (18) below 60 °C / 35 °C (140 °F / 95 °F).
The former throttle micro-switches replaced by a new switch housing with three built-in contacts. The switching points are
1° throttle angle = activation of DITC and control unit for acceleration enrichment,
7° throttle angle = idle switch for oxygen sensor system and activation of control unit for acceleration enrichment
66° throttle angle or > 3500 rpm = full throttle enrichment
Throttle switch with 3 integral contacts
There will only be a fuel/air mixture enrichment during acceleration when the coolant temperature is below 60 °C / 35 °C (140 °F / 95 °F) and the sensor temperature is below about 250 °C (480 °F) (during warm-up). If the temperature value is exceeded in the one or the other case there will be no acceleration enrichment.
If the throttle is opened, control unit (18) will be activated at throttle angle of 1° or 7°. Control unit (18) puts out a voltage signal to oxygen sensor control unit (19), which increases the duty cycle to 75 % for about 2.5 seconds. The fuel/air mixture will be enriched during this time.
Was this article helpful?
The History of Porsche Cars through its generational changes. A must have ebook for any Porsche fanatic, this ebook is a reference book for Porsche enthusiasts. Discover all there is to know about the greatest Porsches ever made. All models are fully described and illustrated providing a definitive production history plus an accurate guide to original specs and equipment.