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

having difficulty coping with the additional power produced from a faster moving
alternator, which was in turn messing up the ECU. The brushes were in good shape,
but the regulator was an aftermarket brand, so it could be an isolated problem.
Car Runs but Won't Re-Start; Bad RPM Sensor. I recently posted a starting
problem with a 1991 240, where it would start fine when it was cold and would start
fine hot if restarted immediately after cutting off the engine. However, if it were left for
10 minutes or more after cutting it off hot, the starter motor would just turn and turn
until it finally started - sometimes it did not and the battery just ran down. As it turns
out, it was the RPM sensor, which is also a crankshaft position sensor. The heat soak
after turning off the engine made the sensor open circuit most of the time - during
starting, apparently, the computer did not pick up enough crankshaft position
information to start the car. However, since the car ran fine if it did start, the sensor
must have been intermittently putting out a signal, enough to update the computer in
order to correctly operate the fuel injection and ignition systems after starting. After
replacing the sensor, I have had no further problems with starting. [Chris Herbst] If the
car stalls while turning corners,check that the rpm sensor wire isn't split open and
grounding on the rear of the engine somewhere when you turn corners or slow down.
Car Stalls When Brakes Applied: Vacuum Leak or FI Relay. [Inquiry:] I am having
a problem stalling under hard braking. I have had it checked for vacuum leaks and
they appear OK. It only happens when the car is warm and I apply the brakes hard.
[Response 1:] I would look in the direction of the power brake booster. Right after you
brake, you fill up the booster with air, which has to be evacuated by the engine. If
your idle is somewhat shaky, throttle plate is out of adjustment or you have a lazy idle
air motor the idle can drop enough to stall it. The check valve in hose from intake
manifold to booster has an arrow on it indicating the direction of installation Take it
out of the line and suck on it to see it opens and shuts correctly. [Response 2:] Check
the solder joints on the fuel pump relay. Cracked joints will sometimes cause a warm
relay to drop out when the engine speed drops and the alternator slows and the
system voltage drops.
Car Stalls at RPM: FI Relay or Hall Sensor. [Inquiry:] Car is 1986 740T with B230
engine and A/T. Occasionally, while accelerating, RPM at approx. 2500 the engine will
start to hesitate, choke, and die. I pull over to the side of the road where it will restart
on first attempt. I take off and all is well for days and weeks. I thought that once the
engine shuts off, and the car still moving from inertia I could move the shifter into
Neutral, and restart the engine. Starter will Not even engage. However, with the car
stopped, it will start in Neutral. Why is this ? Fuel pump relay is about a year old. Other
than this, car runs and idles fine. [Response 1: FI relay] It sounds like the fuel pump