Question about 1990 Volvo 740
Check the connection at the speed o sensor.
It's in the differential cover, about 3" above the filler plug. Be sure
it's mechanically secure, and check the contacts (both sensor and
connector) for signs of corrosion or oxidation. As much as possible,
examine the harness for any signs of a break or tear, or for signs the
wire might have been cut. If it's not wiring, then it's either the
sensor or the speedometer head (which is electronic). Possibly, the
power supplied to the speedo head where it's mounted in the dash
cluster is intermittent.
The speedometer in my 745 with 160K has been
jumping around a little on the highway. The cruise control would try to
follow the erratic speedometer, then disengage. I suspected a broken
wire at the sending unit. What I found was a burnt wire at the sending
unit. The wire had grounded out on Volvo's safety wire. I suspect that
the safety wire slowly eat through the insulation on the wiring,
causing it to ground out. The plug was melted and an inch or so of
insulation on the green stripe wire. Make sure the safety wire is not
cutting into the two wires on the sending unit at the back of your rear
end. [Response: Scott] Check the instrument cluster grounds under the
dash and in the right footwell. In 88 there were some 700 series cars
that would register up to 60 mph while sitting still and the grounding
problem was the cause.
Posted on Jun 29, 2009
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Posted on Jan 02, 2017
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