Ignition Monitoring System (1989 and newer)

Ignition Monitoring System (1989 and newer)

1989 and newer 928's employ a system to detect a malfunction in the ignition system. Due to the dual distributor design it is possible for the car to run on four cylinders, passing unburned fuel from the other four into the hot catalyltic converter. This quickly results in a cat fire which has resulted in the untimely demise of more that a few of our beloved cars.

So here I am (well not right now, but today) doing one of my least favorite things, trying to troubleshoot the
electrics in my 90 GT.
I finally get to the right section of the right book that mentions what I think is wrong. It tells me that a
glowing green relay means that Injection circuit II is faulting, causing my car to stop running on half it's
cylinders. If the red one was glowing, it would be the other half that was not working. O.K. fine.. Well, I can NOT find anywhere where it says which side is Circuit I, and which side is Circuit II. All those books, but nothing that I can find to tell me which half of the motor is stopping. Mind, this only happens when I am at the track, or a hillclimb, or a highway race, so I can't just get out slap the headers to see which side is cold...
Can anyone help out an electrical dunce here? I think this is a GT and better (or should I say newer) thing, as I understand the older cars do not have these particular relays (which, BTW, are mounted on top of all the other controlers down in the passengers foot well)
Which is Injection Circuit I and which is Circuit II?
Please...I know it is in the manuals somewhere, but I sure can't find it.
Don Hanson

Diagnosing Ignition Monitoring System

July 7, 1992

Model: 928 S4/GT/GTS from 1989 Models

Checking of components after activation of the ignition monitoring system
(injection circuit switched off fault codes 1131 of 1231).

Activation may be caused by the following:

- Damaged or defective exhaust gas temperature sensors.

- Possible damage to the ignition coil wire left side (in driving direction) between the ignition coil and distributor cap.

- Poor grounding of the mounting plate for the ignition final stages.

- Poor physical connection of the electrical plugs on the ignition final stages.

Parts Information:
New version temperature sensor Part Number 928 606 155 02 New coil wire with hose covering Part Number 928 602 040 01 (for left side)

Repair Information: 1. Check the left side ignition coil wire. Be certain the coil wire is routed freely and not under tension. If damaged, replace with new version (see parts information).

2. Check all ignition components and connectors for corrosion, tightness, correct connection and damage. Repair or replace as necessary. If an ignition circuit has malfunctioned, an LED indication will be given by the ignition monitor relay located on the L-H control unit mounting plate.

Ignition circuit I (cyl. 1-7-6-4) Red diode
Ignition circuit II (cyl. 3-2-5-8) Green diode

3. Before replacing temperature sensors, check the voltage difference of both temperature sensors as per step 4 of this bulletin. Sensors are located in the exhaust ports:
Model '89-'90, cyl. 4 and 8
Model '91 cyl. 3 and 7

The function of the ignition circuits and light diodes located in the ignition monitor relay remains unchanged. It is not possible to determine from the LED display of the ignition monitor relay if one or both temperature sensors are defective or which temperature sensor has failed.

When installing temperature sensors, coat the sensor adapter threads with molykote paste HTP (white) and torque sensors to 10 N-m.

4. The voltage difference of the temperature sensors must be checked in order to ensure proper operation of the ignition monitor system.

Checking sensor voltage difference:

- Start engine and bring to operating temperature.

- Loosen the mounting bolt for the ignition monitor relay and pivot the relay up to gain access to the plug terminals Do not disconnect the relay plug.

- Set volt meter to the millivolt range and connect leads between E1 and E2 of ignition monitor relay (Figure 3, white wires). A digital volt meter must be used. Polarity is not important.

- Measure voltage with the engine idling and again at approximately 2000 RPM. A maximum difference of +2.5 mV or -2.5 mV (depending on polarity) is permitted. If the voltage difference is above 2.5 mV, stop the engine, loosen and rotate one temperature sensor. Retighten sensor and check voltage difference. If the difference is above 2.5 mV, stop the engine and rotate the other sensor. If after rotating the sensors to different positions, the voltage difference is too high (above 2.5 mV) the temperature sensors are defective and must be replaced.


David Roberts

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