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350matt, Mechanical Engineer
Category: Porsche
Satisfied Customers: 22896
Experience:  BEng Hons, worked in the car industry for the last 22 years
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I have a 1983 928 automatic and I have been trying to locate

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Hi I have a 1983 928 automatic and I have been trying to locate the speed censor, In the workshop manual on items 28 ignition system it shows it above the starting ring on top of the bellhousing, On my 928 there seams to be no place for the sensor, can you please give me any ideas for me to locate this device,
The reg is A965MPR Vin no WPOZZZ92ZDS841816 Engine no 82D6063
Kind regards ***** *****


on the early 928's they used the k-jetronic fuel system and a distributor so there is no crank speed sensor as on the late models

if you can tell me your issue I'll try to help further

Customer: replied 1 year ago.
Hi Matt My speedo has stoped working so I automatically thought speed sensor, I have had the clocks out they are all clean and no rust and look in good condition. the printed circuit is in good condition so last port of call was the sensor



if you look at this wiring digram

you'll see that the speedo gets its speed from an 'impulse generator'

see here

this is located on the transaxle / differential carrier

however if the cruise control works, the sensor and wiring to the Central Electric Panel are OK.

If both the speedometer and odometer are dead, but the cruise control works, the most likely problem is a faulty connection on the instrument panel foil or connectors.

The circuit starts at the speedometer, and a trace on the flexible printed circuit board runs from there to Terminal 1L. A brown/red wire runs from 1L to H1. H1 is the lower left connector on the 8th plug from the left end.The circuit runs behind the Central Electric Panel from H1 to T1. Count from Plug A on the left end up to T, skipping I and skipping the two short plugs in the middle. Again, 1 is the lower left terminal. There should be a brown/red wire in the terminal. The circuit runs from there to the sensor, changing color to solid brown at the sensor connector. The sensor is a simple magnetic switch, operated by a ring of magnets mounted on the diff carrier

The circuit goes thru the switch, and returns on a brown/red wire to Terminal T2 on the Central Electric Panel. T2 is next to T1. The circuit runs behind the panel from T2 to H2, and a brown wire runs from H2 to the instrument panel ground on Terminal 11R on the printed circuit board, then to Terminal 7R, and then on a brown wire to ground at MP 3.

When the sender switch is momentarily made (contacts close) by the passing of a magnet on the ring, the circuit from the speedometer is completed to ground, and both the odo and the speedometer receive a pulse.

You can start troubleshooting the speedometer/odo sending unit circuit by checking for voltage at H1 with the ignition switch on. If there is no voltage here, the problem is probably a faulty connection on the flexible printed circuit board. Could be at the speedometer or at the edge connector. If there is voltage here, move to H2 and check for pulsing voltage when the rear wheels turn. If there is voltage at H1, but no pulsing voltage at H2 when the wheels rotate, the problem may be a faulty connector or broken wire. Start by cleaning the connector in the spare tire well.

Customer: replied 1 year ago.
Thank you Matt, The information has saved me hours of fumbling around for a sensor and also give me more to go on with the speedo, I will give it a go on Saturday to try and resolve the speedo problem. Kind regards michael

glad to help

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