At 11:26 PM 12/26/01,  wrote:
>Hello ALL,
>I'm having a problem with my 89 S4 AT 116K. It seems that it has been going into a limp mode after about every 10 miles of driving. It's done this in the past, but placing in neutral and restarting seemed to get it running again and maybe 5000mi would pass before the symptom was repeated.

Most likely cause is that one of the exhaust temp sensors of the ignition monitoring system is going bad. You can sometimes fix it by rotating the sensors in the exhaust.

You can tell which ignition system is dead by seeing which LED is illuminated. Pull the cover off of the ECUs, and watch for an LED in the ignition monitoring relay, attached to the back of the ECU. Red LED - Right coil, cylinders 1-7-6-4 Green LED - Left coil, cylinders 5-8-3-2

>It seems to be getting much worse now. Maybe or maybe not related is some pinging around 2K to 3K seems worse now.

May not be related, but is a problem. The knock sensors should be retarding the ignition timing slightly if pinging is present. If you are getting audible pinging, then either one or both of the knock sensors are not functional, or the cause for the pinging is so bad that retarding the
timing isn't curing it.

Wally Plumley
928 Specialists


Hello all,

Here are some more tech hints that may come in handy for 928 owners. If your 928 is older than a 1989 model, be sure to turn off the engine immediately if you suspect that you've lost 1/2 of your ignition system. It could save you from a car fire. If your 928 is a 1989 or newer, Porsche will shut things down for you .... this explains how

Best regards,
Electronik Repair, Inc.

The Ignition Circuit Monitoring Relay

The 928 S4 and newer engines have two separate ignition circuits. Ignition circuit 1 is comprised of cylinders 1,4,6,7, the right side coil and distributor. Ignition circuit 2 is comprised of cylinders 2,3,5,8, the left side coil and distributor. If there is a failure in one of the ignition circuits, raw fuel is pumped out of the cylinders and into the exhaust system. When the raw fuel hits the hot catalytic converters a fire may occur - OUCH!

From model year 1989 onwards, Porsche added the Ignition Circuit Monitoring Relay as a safety feature. This circuit is completely independent of the LH injection system. It is identified in the workshop manual wiring diagram as the Ignition Control Circuit. The function of the system is to turn off the fuel injection to the failing ignition circuit. The inputs to the relay are two exhaust temperature sensors that provide input to the relay. If one of the ignition circuits fails, the "Ignition Circuit Monitoring Relay" shuts off the pulse signal to all of the fuel injectors of the affected circuit.

As you might imagine, the symptoms of a shut down ignition circuit are a significant reduction in power and a rough running engine. For trouble shooting, you need to determine if the shut-down is due to a failed ignition circuit or a failure of the Ignition Circuit Monitoring Relay itself.

The first step is to quickly identify which circuit is affected. Look at the clear relay module that is fitted next to the EZK spark control unit in the passenger compartment.

When ignition circuit 1 (cylinders 1,4,6,7) is shut-down, a red LED is illuminated.

When ignition circuit 2 (cylinders 2,3,5,8) is shut-down, a green LED is illuminated.

Check the ignition circuits and repair as necessary. If no problem is found with the ignition circuit, there may be a failure of the Ignition Circuit Monitoring Relay circuit, the signals to test are:

1. Terminal 31: ground
2. Ground must be present at terminal AL when the ignition switch is in the off position.
3. Battery voltage must be present at terminals A1, A2, 15 and 87 when the ignition is in the on position.
4. Battery voltage must be present at terminals AL and 61 when the engine is running.
5. A Voltage value of approximately 2.7 V must be present at both terminals E1 and E2 when the ignition is in the on position.
6. The resistance between E1 and E1 is approximately 5 - 10 Ohms (measured at the disconnected relay socket).

The temperature sensors that fit in the exhaust system should also be inspected.

A more detailed explanation is provided in the factory workshop manuals.

Above true, mostly, but be aware however, resistance is just an indication, the thermo element creates a voltage.  Even is resistance is ok, it can still be broken. Both sensors produce a few mV, but are connected +- to -+ polarity. Thus equal voltages result in a zero value. That is what the relay uses to determine any faults and which side is faulty (result negative or positive)

As noted it does NOT shut down one bank but 2 cylinders on each bank those which are on the same coil, same distributor cap. Also running on 4 cylinders is NOT LIMP HOME MODE !! Driving on 4 cylinders is said by Porsche to be very hard on the drive shaft inside the torque tube. Porsche felt the need to add that system in 1989 because the dual ignition cars ...(two coils ) were sometimes experiencing a failure in one circuit. The car would run poorly have little power BUT BIG BUT people would still try to "limp home" so with four cylinders not firing and pumping unburned fuel and air into the exhaust the catalytic converter would glow red hot and the undercoating would catch on fire and the flames would burn up everything on top of the engine. So to avoid that stupidity Porsche added the protection circuit to shut of the injection to those 4 cylinders 2 on each side) which were not firing. However like any other device which can shut off it sometimes will fail and do it at the wrong is after all a RELAY getting information from temp sensors which passes through wires ! The real limp home is a stored program in the LH computer which under certain failures of the mass air sensor allows the engine to run at low engine speed without the information from a mass air sensor.....which is why most cars will run with a disconnected MAF just not rev up well . Just run well enough to pull off the freeway.

So is it safe to summarize that:
87-95 928s have a limp home mode (MAF related, but stored in LH computer)
87 & 88 928s do *not* have an intentional 4 cylinder mode
89-95 do have an intentional 4 cylinder mode, which was only intended to drive to a safe parking space and not drive home.