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September Tech Notes 1974 PORSCHE STARTER PROBLEMS Allan Caldwell During the past summer, several inquiries were received related is engaged. The small numbers on the wires indicate the wire cross to inoperable starters on 1974 Porsches. 1974 was a banner year for section in square millimeters. The yellow wires to the starter are 4.0 starter problems as a result of new U.S. government safety rules. The square millimeters in cross section, which is one of the larger sizes Feds, in their infinite wisdom, decided that all cars sold in the USA used except in the main power circuits, and give an idea of the imporwould have a starter interlock that would disconnect the starter circuit unless the front seat occupants had their seat belts fastened. This led to seat belt attachFigure 1. 1974 911 ments with switches, and seat cushion sensors to Starter Current indicate if someone was sitting there, so you could start Flow Diagram the car when you were driving alone without fastening the passenger seat belt. Of course, if you happened to put packages in the passenger seat, the car wouldn���t start until you fastened the passenger seat belt. Both the 1974 911s and 914s were equipped with a 14 pin solid state relay that read a number of different switch voltages and signals and decided whether or not to allow operation of the starter solenoid circuit. In the 911, the relay is located behind the combination instrument on the left side of the dash and on the 914, the relay is located under the passenger seat. This system was a major contributor to the fame of Murphy���s Law, and even the Feds gave up on it in one year because of numerous problems and complaints, so the 1975 and later models didn���t have it. Later models retained a relay operated by various seat belt and door switches but all it did was operate the buzzer and seat belt light and was not connected to the starting system. In Reference 1, we reviewed a number of summer starting problems including the 1974 starter interlock systems, but we are tance of this circuit. The solenoid must get at least 8 volts in order to still encountering 1974 911s and 914s that never got fixed. activate the starter mechanism and turn on the main battery power to the starter motor. Plug connections in the wiring diagram are 911 Starter System denoted by ���T,��� followed by a number indicating how many pins are Figure 1 shows the 1974 911 starter system current flow in that connector and a letter indicating where it is located. The T6f diagram. The diagram has been simplified to show just the connecconnector coming out of the ignition switch is a six pin connector and tions critical to ensuring that the starter runs when the ignition key is located below the instrument panel. Any connector in the starter circuit that causes increased resistance due to corrosion or a bad connection when hot is potentially a possible cause to a malfunctioning starting system. The starter solenoid input to the J34 relay from the ignition switch is at the terminal marked C, and the output to the starter is at the J34 terminal 50. If there are any 1974 911 owners who have not yet done so, they should check to make sure that the yellow wire coming from the ignition switch bypasses the J34 relay and connects directly to the yellow wire going to the starter solenoid. A fairly simple way to do this is to disconnect the two yellow wire plugs on the luggage compartment floor (noted in Figure 1 as T1e) and connect the one coming from the ignition switch to the one 36 September 2011