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

1993
+
tank: right
side of
pointer
should
touch red
part of
gauge
Non-tubular type fuel
gauge with:
60 litre
tank: 1/4
tank full
80 litre
tank: 1/5
tank full
Empty tank:
131ohms
LED "low gas"
lit: 113 ohms
Full tank: 2
ohms
Measure resistance between
brown and gray-white sensor
leads at sensor side.
1. Fuel Sender Unit Failure. [Tip from Dave Stevens] For the 940's, the early symptoms are a
fuel tank gauge that appears to stick, often reading too high. Then it progresses to the point
where it won't always go below a certain point or above a certain point. Driving over bumps or
filling the tank will often cause the gauge to jump and read more correctly. Eventually the
problem gets so bad the gauge will rarely move. No matter how bad it gets, you can probably
still get it to move if you take a mallet and carefully pound the top of the tank around the sending
unit (under the access plate). I would say that is the ultimate diagnostic for this particular
problem. Based on reports here, the problem did show up on a few cars during the warranty
period, but is now starting to show up more and more as these vehicles age. The problem likely
correlates better with mileage and rough road usage. Quite simply, the fuel level sending unit in
the tank is shot. Specifically, the internal sliding contacts are totally worn out. This only affects
the sending units in the later 940's with the enlarged ~73 litre tank. It is fundamentally different
from the earlier 900's and 700's with the ~60 litre standard tank or the ~80 litre expanded tank.
It's about 1.5" longer and uses a different pre-pump attachment. Additionally, the resistance
values probably don't match so a different or modified gauge is likely required (probably just a
resistor or jumper wire). The 940 also needs the maximum sender resistance to correspond to
the proper tank level as it triggers a bright low fuel LED on the dash. It's an expensive problem
to fix. It almost goes without saying that you can't get the little contact strip worth only a few
cents. You need the whole sending unit. Apparently Volvo only sells the sending unit with the
entire tank pickup assembly including the pre-pump for approximately US$500. There are two
alternatives: Find a working used sending unit and have it installed, or fix it yourself.
Repair of Sender Unit. To do the latter, remove the tank sending unit, remove the sender barrel
(fast, high heat de-soldering), crack it open without damaging anything (non-trivial), carefully