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'91 740 speedometer quit 700 1991

posted by  v8volvo on  Mon Apr 13 23:08 CST 2015
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last visit: Sat Oct 18 11:54 CST 2014
[RELATED]

Hi all,

Wondering if someone knows off the top of their head how the car reads the VSS signal so that I can choose a starting point for chasing a sudden speedo failure. Speedo quit, and at the same time I got ABS warning light and at the next stop confirmed fault code 3-1-1 is stored (no speedometer signal). This is on a 1991 740T sedan.

Question is this: does the speedometer itself do all the processing for the VSS signal, and all the other devices (EFI computers, ABS, etc) read off the speedo? Or do some or all of those systems get their signal directly from the VSS? In other words, does the fact that the ABS and ECU lost their speed signals indicate that there's no signal from the VSS, or could a speedo failure create the same symptoms?

As you might surmise, this will determine whether I start diagnosis at the rear axle or in the instrument cluster. '91 is a bad year for clusters and speedos, so won't be surprised by a failure there, but the sudden failure seems to suggest possibly wiring too...

Thanks in advance!
--
George, Seattle WA






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