This morning the torque tube on my automatic 1990 S4 (70,000 miles) snapped, and really ruined my day.

I have just been reading Porsche Bulletin 9206 (repeated below for reference - Thanks to both Dave Roberts for posting it and Wally Plumley for previously bringing this bulletin to my attention) and am interested in views on whether the failure today was due to past problems or whether it would have been caused by a more recent problem.

The background is as follows:

1.I bought the car 2nd hand at 60,000 miles 2.Within the first few weeks, I found it would sometimes go in to 4-cylinder mode.
3. The first time that happened I didn't know what to do, so drove home 10 miles on 4-cylinders.
4.Subsequently, and whilst trying to diagnose the problem, any time it happened I'd shut it down, restart, and away I'd go on all 8 cylinders 'till it next occurred, maybe 500 miles later.
5.In due course, and with Wally's assistance, the problem was tracked down to the cylinder head temp sensors.
6. These were replaced with new ones P/N 928 606 155 02 about 5,000 miles ago, along with a new left hand coil wire, and the problem has never recurred.
7. Today the torque tube snapped. Pulling out from an intersection accelerating briskly from 20 mph to 40mph, and suddenly no more power to the rear wheels. Truck it to workshop, remove flex plate cover and sure enough torque tube has snapped just behind the pinch bolt.

Porsche Bulletin 9206 below clearly shows a strong link between the faulty cylinder head temp. sensors, faulty left hand coil wire, and snapped torque tubes on 1989-1991 automatic S4s.

So here's the crux of my question: - Does anyone have any views on whether the snapped torque tube today is likely to be a carry-over from the previous problems that were fixed 5,000 miles ago, or whether the snapped torque tube today is due to a new and as yet unidentified cause?

Broken Central Tube Shaft Vehicles With A/T

July 7, 1992
Model 928 S4
Group 3
Part Identifier 3903
Number 9206

Subject: Broken Central Tube Shaft Vehicles with Automatic Transmission

ATTENTION: Service Manager/Service Technician

Models Affected: 928 S4 Model Year 1989 to 1991

Concern: Activation of the ignition monitoring system (injection circuit switched off fault codes 1131 or 1231) can lead to breakage of the central tube drive shaft.

General Information: Oscillations in the central tube system that occur when the ignition monitoring system has switched (flywheel effect) can lead to breakage of the central tube drive shaft. This can occur only in vehicles with automatic transmission where the vehicle is operated at an engine speed of approximately 1000 RPM. Possible causes are:

- 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: A new version temperature sensor, central tube and coil wire are installed in production.
Temperature sensor, Part Number 928 606 155 02 Central tube with changed material for drive shaft, Part Number 960 421 012 07
New coil wire with hose covering (for left side), Part Number 928 602 040 01

Repair Information:
1. If the central tube drive shaft is broken, replace the central tube with the new version part (see parts information). Refer to Technical Bulletin Group 3, Number 9203, dated May 5, 1992 for hints on central tube installation.

2. Replace the left side ignition coil wire with the part number listed in this bulletin. Be certain the coil wire is routed freely and not under tension.

3. 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

4. Replace both temperature sensors located in the exhaust ports:
Model '89-'90, cyl. 4 and 8
Model '91 cyl. 3 and 7
Use the new version temperature sensors listed in this bulletin.

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.

5. After the temperature sensors are installed, the voltage difference of the 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

Kind regards,

Dave Overington
1990 S4 Auto

PS Also posted to Landsharkoz for views from any persons on that site who are not Rennlist subscribers

Excess vibration is a problem for automatic torque tubes. When the engine is out of balance due to poor valve timing or worse running in four cylinder mode, vibrations are set up that can loosen the bolts on the torque tube.

Think of the drive shaft as suspended between two fixed points with bearings set in rubber. As such it is very easy to set up harmonics in the drive shaft.

When I was an aerospace engineer, we tested airframe components on a vibration table. At certain frequencies - different for every material - a harmonic will be set up. Harmonics will very quickly destroy materials.
The challenge for an airframe is to insure that airframe components do not have common harmonic frequencies. It is amazing to see how violent harmonic frequencies are.

Any component subjected to harmonic frequencies will experience metal fatigue. Removing the harmonic will not cause the metal to heal. Micro cracks will remain and over time the material will fail.

Your torque is probably a victim of both running in four cylinder mode for short periods and a long term out of balance from the valve timing. When the valve timing is out on the 928 32 valve engine the engine is running in opposition and both looses power and sets up dangerous vibration.

Dan the Pod Guy
Portia's Parts