Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

Car Stalls, Lights Die: Electrical Ground Fault. [Inquiry:] Does anyone know if the
bulb out sensor box can cause a dim headlight intermittantly. Sometimes if at a stop
light or parked, if I turn on the brights the engine will die. Also sometimes the rt. turn
sign and bright indicator on the dash appear to be dim when they shouldn't be lit at
all. [Response:] There is a ground bar on each inner front fender. Make sure all of
these wire connectors are clean and tight. I forget what else grounds here but a dirty/
loose ground at this grounding point will cause the engine to die and also dim
headlights. Check both ground points for the inner fender area for being loose and/or
dirty.
Car Over-Revs RPMs While Under Acceleration. [Inquiry] I occasional get an overrev
when accelerating hard. The car is a 940 Turbo with a manual transmission and
overdrive. Typical symtoms appear when I accelerate in third or fouth; there is an
increase in engine revs without the increase in speed. [Response: JohnB] The
condition is called "flare"; you have clutch slip or clutch slip in the overdrive unit. Bring
it up to 50mph in 3rd or fourth (somewhere around 3500 rpm, your choice/speed) and
use full throttle with your other foot on the brake to maintain speed....engine speed
should not increase.
LH-2 Cold Idle Problems -Bad ECT or O2 Sensor and Wiring Harness Notes (BB)
The block temperature sensor (ECT engine coolant temperature sensor under the
intake manifold) plays a big part in cold running decisions and could pre-maturely
allow the O2 sensor signal being used before the engine is warm. Because of this a
cold idle problem rarely involves a faulty O2 sensor, but may involve the temp sensor
and more specifically the wiring at the temp sensor. My experience is that this usually
leads to quite rough running at all times, but fast idle and poor acceleration are known
symptoms. With the ignition on and all wiring in place, you should see voltage at the
temp sensor terminal connected to the blue (or orange) wire that goes back to the
ECU. See Diagnosing ECT Failures for more information. No volts means broken
wiring or bad ECU. A bad ECU may actually just be a bad ground at the ECU, so be
warned. A cold engine should read 2-4 volts, a fully warmed engine 0-2 volts. If you
see the full supply voltage of 4-5 volts (the reading you should get from the ECU when
the connector is pulled off the temp sensor) then the temp sensor or its ground wire
are faulty.
The temp sensor wiring goes into the wiring harness and along the firewall before
joining with the O2 sensor wiring and going through the firewall to the ECU behind the
right side kick panel. If you have a general wiring harness deterioration problem then a
fair bit of digging and careful tracing may be required to isolate the problem. You can