September 2020

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SEPTEMBER 2020 WWW.PNWR.ORG 17 The electrical cable to the sensor goes through a formed opening in the rubber cover. A photo of the arrangement is shown in Figure 1 from the factory shop manual. Access to this area requires the car to be on a hoist or jack stands that will allow the left rear wheel to be removed. The forward-facing sheet metal where the access cover is located is inboard of the left rear shock strut between the cam tower and the engine flywheel. The electrical sensor cable is permanently attached to the top of the sensor so that a slotted, long neck socket wrench is needed to remove the sensor. The early sensors had a hex head of 13 mm and a single connector pin at the end of the cable. The later models have a 14 mm hex head and a two-pin connector. There is a factory tool (no. 9222) that provides a correct 14 mm slotted socket for this task as shown in Figure 2. Some owners of the oldest models have had trouble removing old sensors that have been in place for many years. For a stripped threads emergency work-around, it has been reported that a new sensor with an aluminum bracket attached to a near-by crank case location will often achieve nearly the same results as the original set up. The head temperature sensor cable is routed upwards from the sensor cover to the rubber grommet on the engine shroud (shown in Figure 1) where it is routed into the engine compartment along with the two flywheel sensor cables. In the engine compartment there is a vertical bracket mounted on the number three cylinder that contains three plugs on the left side of the engine as shown in Figure 3. The number one top plug, which is white, is the cylinder head temperature sensor, while the number two black plug is the engine speed sensor and the bottom number 3 plug is the engine reference mark (TDC) marker. Note that both the early cylinder head temp sensors with a single pin, as well as the later model sensors with two pins, both plug into the same two plug socket in the cable connector that goes to the DME. The later sensors apparently added a much better ground than was originally used. From the vertical bracket location, all three cables go into the main Motronic cable, which goes forward to the system 35 pin connector on the ECU box under the driver's seat. Replace- ment of the sensor requires removal of the sensor unit from the cylinder head, disconnecting the cable connector in the engine compartment, and pulling the sensor lead out of the engine compart- ment into the forward area where the sensor has been removed. 911 Carrera owners of early models with intermittent running problems who have installed new cylinder head sensors report greatly improved running characteristics with smoother acceleration and no hiccups or intermittent cut-outs. References 1. Caldwell, A., The Early 911 Motronic Engine Systems, Porsche Panorama, February 2003. 2. Caldwell, A., Early 911 Carrera Motronic Problems, Porsche Panorama, April 2008. 3. Porsche 911 Carrera Workshop Manual, PCNA publication WKD 482 020, 1984, with updates. 4. Porsche 911 Carrera Service Manual, Bentley Publishers, 2000. Figure 3. 911 Engine Compartment Head Sensor Cable Plug (Reference 2) 1. Cylinder Head Temperature Sensor 2. Engine RPM Sensor 3. Engine Reference Point (TDC) Sensor

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