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Copyright by Dr.-lng. h.c. F. Porsche


Test Step Description Page

Precautions for Working on Electronic Ignition Systems 0 - 02

Conditions Caused by Faults 2 -01

Relay and Plug Connections on CEL 2 - 02

Plug Connections - Control Unit LH 2 - 03

Plug Connections - Control Unit EZK 2 -04

Wiring Diagram for 928 S 4 Engine Electrical System 2 05

1. Ground Points/Plug Connections - EZK and LH 2 -07

2. Power Supply for EZK Control Unit 2 12

3. Speed and Reference Mark Senders 2 -14

5. Throttle Valve Switch for Idle Spped and Full Load - EZK 2 -16

6. Ignition (Ignition Timing, Ignition Pattern, Test Values) 2-17

7. Hall Sender Signal 2-21

8. Power Supply for LH Control Unit 2 22

9. Power Supply for Fuel Pump / Speed Signal from EZK Control Unit to LH Control Unit 2 -25

10. Air Flow Sensor Test 2 26

12. Throttle Valve Switch for Idle Speed and Full Load - LH 2-29

13. Fuel Injectors and Injection Timing 2 - 30

15. Leaks in Intake System 2 - 34

16. Idle Speed Control Test 2 - 34

17. Resonance Flap Function Test 2 - 35

18. Tank Vent Solenoid Valve Test 2 - 37

19. Speed Drop in Cars with Automatic Transmission 2 - 39

20. Oxygen Sensor Control Test in Cars with Catalytic Converter 2 - 40

21. Generator and Regulator 2 41

Engine Adjusting/Testing Values 2 - 44

Codes Depending on Country Equipment 2 - 45

The requirements of modern engines on ignition systems and necessity for maintenance-free systems had led to the introduction of electronic ignition systems in standard production some time ago. Normally the ignition power of electronic systems (almost all makes) will be higher than that of conventional systems and even greater power can be seen in the near future.

Consequently electronic ignition systems are coming into a power range, where even touching power-carrying parts or terminals could be very dangerous (both primary and secondary sides).

In conjunction with this matter we must refer you to VDE regulations, especially VDE 0104/7.67, for Germany and/or regulations/legislation of a pertinent country when repairing or testing an ignition system.

Always turn off the ignition (switch off ignition or source of power) when working on the ignition system. This work would include:

9 Connecting engine testers

(ignition test lamps, dwell angle/speed testers, ignition oscilloscopes, etc.).

• Replacing parts of the ignition system

(spark plugs, ignition coil, distributor, ignition leads, etc.).

The mentioned dangerous voltage will be in the entire system when it is necessary to turn on the ignition for ignition system tests or engine adjustments.

Consequently there is not only dangerous power in or on parts of the ignition system (for example: distributor, ignition coil, control unit, ignition tackle), but also on the wire harness (for example: tachometer connections, diagnosis plug connections), on plug connections and on testers.


928 S 4

Important Car Instructions

Only measure resist?"*"^® after turning off the ignition or disconnecting the battery (tester could be destroyed if not).

Pull off both control unit plugs when testing the compression pressure (otherwise there could be dangerous high tension, insulation damage on the ignition coil, high tension distributor and ignition leads).

Specified ignition coil (see Order Number) must never be replaced with a different ignition coil.

A shielded capacitor must never be connected on ignition coil terminals 1 and 15.

Ignition coil terminal 1 must never be connected on ground for a burglar alarm system (ignition coil and control units could be destroyed).

Battery positive (+) or any type of test lamp must never be connected on ignition coil terminal 1 (control unit would be destroyed).

Never disconnect the ignition lead from ignition coil terminal 4 to high tension distributor terminal 4 while operating the engine.

There must never be voltage flashover from ignition coil terminal 4 to ignition coil terminals 1 and 15 (control unit could be destroyed).

Secondary end of the ignition must be shielded with at least 4 k-ohms to avoid destruction of the control unit, whereby the original distributor rotor with a 1 k-ohm shielded resistor must be installed (do not use a 5 k-ohm distributor rotor even with radio or intercom shielding).

Only disconnect EZK control unit plugs with the ignition turned off.

Flashover or disruptive discharge in the area of the high tension distributor cap (poor insulation) could destroy the control unit.

Never disconnect the battery while the engine is running.

Mixing up battery pole connections could destroy the control units, ignition coil or EZK control unit.

Starting the engine with outside help of more than 16 V or a fast battery charger is forbidden.

Working with Oscilloscope

Sun Engine Motor Tester 1010

SUN Engine Tester 1010

Bosch Mot300

Bosch Engine Tester Mot 300/400

Old Bosch Engine Tester

Working with Oscilloscope

Bosch Engine Tester Mot 300/400

All sender and ignition timing signals of Porsche models can be tested with SUN 1010/1019/ Master 3 or Bosch Mot 300/400 engine testers recommended by Porsche. Since instructions for connecting a tester on a car vary depending on the make of tester, you should study the documents supplied with a tester thoroughly before making connections.

SUN Engine Tester 1010

Sender Signals Which Can Be Tested: 1 — Reference mark sender

4 - Idle speed control activation

5 — Hall sender signal

6 — Ignition triggering signal to final stages

7 - Speed pulse from EZK to LH control unit

8 — Activating signal for resonance flap

9 - Activating signal for tank vent also:

TO - ti consumption signai (consumption display)

11 — Ignition triggering signal from breakerless distributors

12 — Reed switch signal (speedometer)

13 - ABS wheel sensor signals

Hall Signal 928 1501

Test leads must always be used for tests!


928 S 4

Equipment Required for LH and EZK Tests:

Plug Terminal Designations:

Multiple-pin plugs of control units, looking at plug terminals! LH and EZK

1 oscilloscope (for example SUN or Bosch)

1 Bosch L-Jetronic test lead Bosch No. 1684 463 093 (Check leads for correct polarity in plugs.)

1 digital displaying multimeter (for example Fluke 23)

2 control unit plug test leads

(made locally) with 2 flat male plugs, N 17.457.2, in order to avoid damaging plug pins on control unit plug during tests.

1 adapter test lead, consisting of: 4 plug connectors N 017.483.1 with two approx. 150 mm long leads, soldered.

1 test lead 3-pin (for example VW VAG 1501)

Engine in perfect running condition.

Battery charged.

Starter used to crank engine.



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LH-Jetronic c o


CL 3







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eu 3





































c ta

CO 3 ff

Test Step

10 11 12

Engine cannot be started

Engine hard to start

Erratic idling

Poor engine pick-up


High fuel consumption

Poor engine power

Engine hesitation

ro o

Poor hot starting of engine


L Ira

^ IfFTFfl









LH-Jetronic relay XXV, which in the past only had a burn-off function, is now additionally responsible for power supply to fuel injectors, idle speed control, tank venting valve, coding element and switching valve for the resonance flap in contradiction to LH cars up to and including 1986 models, of which the fuel injectors, oxygen sensor and idle speed control were activated by fuel pump relay XX. This was necessary, in order to permit self-diagnosis in the future.

Power Supply For:

Fuel pump Oxygen sensor

2 - EZK relay XVI

LH control unit EZK control unit

3 - LH-Jetronic relay XXV

Burn-off function

Fuel injectors

Idle speed control

Tank venting valve

Coding element

Resonance flap switching valve

Central electric interface

Power supply for EZK and LH control units

Data code Pump relay XX 85

LH-Jetronic relay XXV 85

Display (check)

CO poti (without CAT) Family curve corr. (with CAT)

Oxygen sensor Load signal to EZK 9

Tank venting valve

Data code Code for oxygen family curve

Automatic idle speed drop

Consumption output

Idle speed control

Intake pipe resonance flap

Power supply terminal 15 EZK relay XVI 87

O cni

cni cni co

cni lO cnj cd


co lo co cn co vj

o cnj co in cd co

Speed from EZK 13

Throttle valve switch Idle speed

Throttle valve switch Full load

Electronic ground MP IX

Ground from LH 2

Voltage from LH 3

HLM burn-off

Voltage from XXV 87

Diagnosis (K-lead) from diagn. conn. 10


AC compressor

Diagnosis (L-lead)

Ground MP VIII Fuel injectors ti

Temperature sensor NTC II Knock sensor 1

Knock sensor 2 Hall sender 0

Speed sender +

Speed sender shield

Transmission protection switch

Throttle valve switch Full load

Coding input

Coding input

Power supply Term. 15 from XVI 87

Diagnosis plug term. 11

Ignition pulse to control unit 1 (left)

Permanent positive 30

O cm

cm cxi co cm cm

CD cm

O co

cm CO


Diagnosis (K-lead)

Shielding Knock sensor 1

Shielding Knock sensor 2

Shielding Hall sender

Hall sender +

Diagnosis inciting lead (L-lead)

Throttle valve switch Idle speed

Load signal from LH 25 Coding input

Speed pulse to LH 1

Diagnosis plug 4

Ignition pulse to control unit 2 (right)

Ground MP IX

Wiring Diagram Type 928 S Model 87


Hand Cranked Generator


Sender Unit Fuel Level Porsche 928



Fan Motor Wiring Capacitor Yellow


Mak chec

Sym AIwj


Engi bodv engir

2 - Fuel filter

3 — Fuel pipe t) 4 — Distributor injection pipes

5 — Fuel injectors

A — Throttle valve housing

B — Idle speed control

C - Ejector

D — Pressure dampen

E — Pressure regulator

F - Vacuum valve

G — Electric tank venting valve

H - Oil filler neck

K — Vacuum switching valve

L — Vacuum box a — From fuel tank b — To fuel tank c — From active carbon tank d — From vacuum reservoir e — To automatic transmission

Note: Test steps should be carried out in the given order!

Test Step 1 is especially important!

Make sure that you are testing with the correct tester, with correct tester settings and on correct checkpoint in conformance with specified requirements.

Symbols in Test Plan: V = voltmeter, Q = ohmmeter,^= oscilloscope, <= smaller (less) than,

> = greater (more) than, °KW = crankshaft degrees

Always read the text precisely and without stress, in order to avoid wrong test results.


Engine ground between engine and body on right-hand side underneath engine.

Porsche 928s Ezk Realy Diagram

Ground of ignition control units above right ignition coif.

Ground Points Porsche 928


928 S 4


Power ground underneath fuel pressure regulator and fuel pressure damper.

Is the contact of connections good and tight?

Plug Connections

Fuel pump (1), EZK (2) and LH (3) relays as well as plug W (4) on central electric.

Shown here on removed engine for better recognition.



Ezk Porsche

One each 35-pin plug on control units EZK (1) and LH (2).

Plug connection for activation of ignition final stages (green/white lead). Above central electric.


Six-pin plug on air flow sensor (only 5 plug contacts are used in the plug receptacle).

Two-pin plug on temperature sensor It.

Three-pin plug on speed/reference mark sender.

Two-pin plug on each of eight fuel injectors (first remove cover for ring pipes).


Plug connections of knock sensors. Front sensor — plug connection underneath fuel ring pipe at left front.

Rear sensor — plug connection above fuel ring pipe at right rear.

Porsche 928 Central Lock Drive

Hall sender plug connection. Behind housing for camshaft drive gear — cylinder bank 1 - 4.

Porsche 928 Belt Driven Fan Replacement

Plug connection of solenoid valve for resonance flap operation (at toothed belt cover, cylinder bank 5 — 8),

Resnance Flap Porsche


Plug connection of solenoid valve for tank venting (at toothed belt cover, cylinder bank 1 — 4, underneath water hose).

Tranings Hosen Porsche

Plug connections of final stages (on hood lock cross wall at left-hand side).

Resonance Flap Valve

Are these plugs connected correctly, not loose and not corroded?

Disconnect plugs; plugs must not be bent on the pin or receptacle ends and also not corroded!

Since 928 S 4 cars are only fitted with one standard control unit (EZK as well as LH) worldwide since 1987 models and these control units are coded to pertinent country equipment requirements (see page 2 - 47 of Test Plan), this code must be checked in case of complaint.

Coding element is located on back of the control unit carrier. Control units must be removed for checking.


Power is supplied to EZK and LH control units of 928 S 4 cars via ignition lock terminal 15 as well as additionally from battery + (permanent positive terminal 30) since 1987 models.

a) Permanent Positive

Connect voltmeter on terminal 18 and terminal 35 of EZK control unit plug. Display: battery voltage.

No Display:

Connect voltmeter on W 22 of connected CEL plug and ground (body). Display: battery voltage.

No Display:

Bridge U 12 and W 22 on CEL plug with a test lead. Connect voltmeter on terminals 18 and 35 of control unit plug.

Display: battery voltage.

If the control unit plug has voltage in this test, the condition is located within Central Electrics. If the control unit plug does not have voltage, check wiring to CEL plug U 12 according to the Wiring Diagram.

Con Turi

No I Con Dlsf

If hi EZK

Ifth Now rem;

If bi

If bi

Central Electric Plug W

^18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 f^ f b> Power Supply via EZK Relay (XVI)

Connect voltmeter on terminal 18 and terminal 29 of disconnected EZK control unit plug. Turn on ignition. Display: battery voltage.

No Display:

Connect voltmeter on W 14 of CEL plug and ground (body). Turn on ignition. Display: battery voltage.

If battery voltage is displayed, check power flow from CEL plug W 14 to terminal 29 on EZK control unit plug.

If there is no display, remove EZK relay (XVI) and bridge terminal 87 and 30 on relay socket. Now connect voltmeter on terminals 18 and 29 of control unit plug (however, ignition remains off for this test). Display: battery voltage.

If battery voltage is displayed, check EZK relay and/or activation.

No Display:

Check voltage and ground supply on EZK relay socket to wiring diagram.

Important: Burglar alarm control unit or a plug bridge must be connected between B 23 and F 25 of the CEL plug.

35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 ¡^


This test step can only be carried out with an oscilloscope.

Connect and adjust a workshop oscilloscope to instructions supplied with the tester for this purpose (set display screen range to 20 V for Bosch MOT 300/400 testers).

Connect positive test lead via auxiliary lead on control unit terminal 23 and negative tester lead via auxiliary lead on control unit terminal 6 {terminal 24 via auxiliary lead on ground*).

Crank engine with the starter motor. Sine oscillation of at least 3 V (speed pulses) and an inserted higher amplitude (reference mark) must be seen on the screen.

Speed signal

2 — Reference mark signal

Sender distance cannot be adjusted. If there is insufficient or no deflection «3 V speed pulse) displayed on the screen, check connections or measure lead connections; replace sender.

* Note: If there is a signal with tester connections on terminals 23 and 6 of the EZK control unit and terminal 24 is connected with ground during the test, the signal must be visible in the same form. !f not, there is a short circuit in the sensor lead.


Pull off EZK control unit plug.

Connect ohmmeter on terminal 19 and terminal 18 of EZK control unit plug,

15-30° C = 1.4- 3.6 kQ 40° C = 1 - 1.3 k Q 80° C = 250 - 390 Q 100° C= 160 — 210 Q

If values are not reached, check sensor itself. In so doing don't connect ohmmeter on both contact lugs, but instead check each plug contact on temperature sensor against ground (two independent temperature sensors).

Note: Temperature sensor II advances the ignition timing of a cold engine for better engine pickup.


a) Idle Speed Contact

Pull off LH control unit plug.

Connect ohmmeter on terminal 8 and terminal 18 of disconnected EZK control unit plug with help of auxiliary leads.

With throttle valve closed display: < 10 Q

With throttle value opened display: °° Q

Idle speed contact must open (break) immediately « 1° throttle valve 4).

Note: A breaking idle speed contact places the ignition timing in partial load family curve (advanced approx. 10°).

Break: Idle speed ignition timing approx. 20° crankshaft.

No coasting shutoff, higher idle speed.

Short Circuit: Coasting shutoff while racing engine, one time after each engine start. Ignition timing retarded approx. 10°.

b) Full Load Contact

Connect ohmmeter on terminal 26 and terminal 18 of disconnected EZK control unit plug with help of auxiliary leads. LH control unit plug must also be disconnected.

With throttle valve closed display: » Q

After approx. 2/3 throttle valve travel display: < 10 Q

Note: A closing (making) full load contact retards the ignition timing into the full load family curve!

Break: Ignition timing advanced approx. 15 -25° in full load range.

No full load enrichment.

Short Circuit: Ignition timing retarded approx. 15 - 25° in partial load range.

Check whether throttle valve moves easily throughout entire operating range.

c) Adjusting Throttle Valve Switch

Throttle valve in idle speed position.

Loosen screws of throttle valve switch and turn throttle valve switch far enough, until the idle speed contact switches.

Idle speed contact must switch while opening the throttle valve after a gap of approx. 1°. Full load contact is adjusted automatically with the idle speed contact.

Note: Intake branch must be disassembled to adjust a throttle valve switch.

Controlling Ignition Timing

At control value of 3000 rpm 32 + 4° before TDC

EZK control unit does not have a vacuum connection (load sensor), but instead now receives the "engine load" information as an electronic signal from the LH control unit. This signal cannot be checked with common workshop equipment.

If this load signal is missing on an EZK control unit, ignition timing will be retarded up to 25° on crankshaft in partial and full load ranges (full load family curve, as protective circuit)!

If a secondary display appears on the oscilloscope, engine tester must be set to 4 cylinders since the ignition system is divided into two separate ignition circuits. Firing order: 1-3-7-2-6-5-4-8.

Ignition Circuit I:

High tension distributor on exhaust camshaft, cylinders 1-4:

Ignition Circuit II:

High tension distributor on exhaust camshaft, cylinders 5—8:

An ignition coil installed at right front as seen looking forward in car is responsible for high tension power supply of ignition circuit I, while the ignition coil installed at left front is responsible for high tension power supply of ignition circuit II.

a) Connection of Engine Tester

Connect engine tester to instructions supplied with tester and adjust (note 4 cylinder test range}.

If ignition pattern of ignition circuit I (right-hand side} is visible, connect tester on cylinder 1 ignition lead and right front ignition coil.



Prin Sect Vol


Dist as Vt


Shie Visi oxic c) A

This Con

If ignition pattern of ignition circuit 11 (left-hand side) is visible, connect tester on cylinder 5 ignition lead and left front ignition coil.




Note: If a fault is displayed for all cylinders, it is in the primary or secondary circuit between the ignition coil and distributor rotor.

If a fault is displayed for only one cylinder, it is after the distributor rotor.

b) Resistance on Ignition Coil, High Tension Distributor and Spark Plug Connectors Ignition Coil ( Q / V):

Primary resistance Terminals 1 + 15 0.4 - 0.7 Q

Secondary resistance Terminals 1+4 5.0 - 8,7 kQ

Voltage on terminal 15 with engine running > 10 V

High Tension Distributor (Q):

Distributor finger shielded resistance 1 kQ

Visual inspection for damage and traces of burning as well as cracks in cover.

Spark Plug Connectors (Q):

Shielded resistance 3 kQ

Visual inspection for damage, traces of burning and oxidation.

c) Activation of High Tension Side

Ignition triggering signal can be simulated for fast ignition end troubleshooting.

This is accomplished by disconnecting a two-pin plug (arrow) above CEL (green/white lead).

Connect plug pins with an auxiliary lead.

Connect oscilloscope tester lead terminal 4 on ignition lead terminal 4.

Start Ignition Porsche 964 Sch

Turn on ignition. Now if positive is applied on one of both plug pins in intervals with help of the auxiliary lead, a high tension needle {ignition triggering) must appear on the oscilloscope each time.

If no signals appear, check

- power supply of final stage and ignition coil,

-- ground point for final stages (underneath right ignition coil),

- plug connections on final stages, ignition coils and high tension distributor,

- components (e.g. ignition coils, ignition leads, spark plug connectors, etc.) 'responsible for hiqh tension and

- power flow in leads from control unit plug to final stage (green lead for ignition circuit white lead for ignition circuit II).

and d) Checking Control Signal for Final Stage (ea)

Carry out test on both disconnected final stage plugs.

Connect positive tester lead on terminal 5 and negative tester lead on terminal 2 of disconnected final stage plug. Operate starter motor. Oscilloscope should display a control signal on each final stage plug.

Dis bef a 3 plu sigi Co on ne< tes

Fo ha

If na ta1 ut

Important Note: Baseplate must be coated with a heat conducting paste prior to installation of final stages. Never apply heat conducting paste on painted parts. Heat conducting paste: Bosch No. 5 942 860 003.


Disconnect plug on cylinder head 1 — 4 behind camshaft drive gear and connect a 3-pin adapter lead (VW 1501) between plug and cylinder head to test the hall signal.

Connect positive and negative tester leads on terminals 1 and 2 of adapter lead, connect tester to instructions supplied with tester and start engine.

Following signal will appear on screen, if hall sender system is okay.

If hall signal or one of knock sensor signals is missing, ignition timing will be retarded by about 6° for all cylinders in upper partial load and full load ranges by the control unit (protective circuit).

1 - Final stage

2 - Baseplate

Voltage signal approx. 5 V


a) Permanent Positive

Connect voltmeter on terminal 17 and terminal 4 of LH control unit plug with help of auxiliary lead Display: battery voltage.

No Display:

Connect voltmeter on CEL plug W 22 and ground with help of auxiliary lead. Display: battery voltage.

If battery voltage is displayed, check power flow in lead from CEL plug W 22 to control unit plug ; terminal 4.

No Display:

Connect U 12 and W 22 of CEL plug with help of auxiliary lead. Connect voltmeter on terminal 17 and terminal 4 of LH control unit plug with help of auxiliary lead. Display: battery voltage.

Battery voltage displayed = condition within CEL. No display = check power supply to wiring diagram.

^ 35 34 33 32 31 30 2928 27 26 25 24 23 22 21 20 19 ^ 18 17 16 IS \4 13 12 ?l 10 9 8 7 6 5 4 3 *"

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