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

the voltage reading drops to zero, you've pinpointed the electrical component with the defective
or missing ground. Since electrolysis might occur only when a certain component is energized,
have a helper switch each vehicle component on and off while you observe the voltmeter
readings. To check components or accessories that don't have an on/off switch, use a long
jumper wire connected to the battery's negative post to provide a temporary ground to each
electrical accessory. Ground each component with the jumper wire and watch the meter. If the
jumper wire restores a missing or faulty ground, the meter will drop to zero. Be sure to check for
intermittent voltage surges generated by the starter during cranking. To do so, watch the meter
as you crank the engine. Any jump in voltage during cranking indicates a loose, faulty, or
missing engine ground. Any electrical device with a huge current draw, like a starter motor or
radiator cooling fan, will chew up a cooling system far faster than a trickle of voltage from a
poorly grounded underhood relay or other low-amperage device… One telltale sign of
electrolysis and/or electro-chemical corrosion is discoloration of the affected area. Aluminum
components usually turn black and pitted, while copper/brass components are likely to develop
a blue-green corrosion byproduct. Unfortunately, bad coolant can cause similar discoloration, so
a blackened PTR core doesn't always mean electrolysis is to blame, but it should raise the
possibility in your mind during diagnosis. [Tip] You can solve the problem for particular
components by adding redundant ground wires of 16-gauge copper mounted with hose clamps
on the component and a screw to body sheet metal.
Gauge Shows Overheating. [Symptom:] My temp gauge started going almost to the red (I
always shut the car off to avoid a boil over) and gave readings (now that I think about it) that
were generally high and also sporadic. [Diagnosis:] I pulled out my Bentley manual and it was
plain as day (consistently high sporadic gauge readings) that it was the voltage regulator. I took
out the voltage regulator for the temp gauge cleaned the four contacts and voila...no more
"overheating."
Temp Gauge Shows Intermittent Redline. My temp gauge would almost redline for a couple
of days but then be normal for a week or a month and then repeat the symptoms. This doesn't
feel like an engine that's over-heating. [Fix:] Try loosening the block gauge temperature sender
a quarter turn then tighten. Slight corrosion at the sender's threads may interrupt circuit
grounding. Worked for me anyway.
Overheats with A/C On. [Inquiry: 1991 740 B230F station wagon. Normally the car can sit for
10 20 or 30 minutes at idle and the temp gauge will stay in the normal spot.
However, when I run the A/C the car will over heat even if I'm running at 70 - 80 mph.]
[Response 1:] Before you go any further you should wash out your radiator to clear it of any
accumulation of bugs and dirt. Many Volvo radiators are replaced for this reason and it does fix
them as a new radiator is obviously clean. It could be a thermostat, bad fan clutch, head gasket
problem, etc., etc. but the radiator being obstructed is an easy thing to try. Get your garden hose
out and wash from the fan side towards the front of the car. If you see a lot of debris then it is
worth the trouble to remove fan and fan shroud and continue to wash but more thoroughly. It
helps to use Fantastic or Formula 409 as that will get more dirt off the fins of radiator and