A few mornings ago I went into the garage to get my 85o wagon and head to work. As I approached the car I noticed a humming sound coming from the engine compartment (and I didnt' have a good feeling). I opened the hood and determined the sound was coming from near the battery. When I attempted the start the engine, the dreaded "er" followed by the worse "click." I disconnected the battery and took alternate transportation to work. Since then I've charged the battery on trickle and this evening reconnected the battery leads. Immediately the humming sound returned, with vigor since the battery was fully charged. I disconnected the battery leads again and bowed my head.

If anyone has a suggested cause and cure I'd sure appreciate some guidance.

Thanks all, and keep pedalling.

ECT sensor

Thank you, but what is an ECT sensor and how difficult is it to change? Sorry, but my auto mechanic skills are a bit rusty.

The engine coolant temperature sensor sits just under the thermostat on the passenger side, right above and behind the alternator. Usually, when the sensor goes, the engine fan stays on trying to cool the antifreeze.
Go ahead and start your car, get it up to operating temperature and check the temp guage on the dash to see if it is pointing to 3 o'clock. The problem could also be one of the fan relays, but the ECT is easy to swap out. Just make sure you drain some of the coolant before you start taking the large coolant hose off. Replace the thermostat at the same time while you are there.
For good pics on the procedure:
http://www.volvospeed.com/Repair/thermect.php

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

It is the sensor that plugs in (actually screws in) to the front of the thermostat housing. It is what actually measures the temperature of the coolant and sends the information to your gauge on the dashboard. When it is shot all kinds of wrong temperature readings are seen on the dash. Many people routinely replace it when they do a thermostat change, although it isn't always necessary. However, in your specific case, it probably is, as if this sensor is reading a high temp, it will keep your fan on even when ignition is switched off (power from your battery and eroneous information from the ECT). Good luck.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

Just to close the loop one this (for now), I want to share the status. I cleaned the battery and leads, reinstalled the battery and drove to the local Volvo shop. While they had the car (2 weeks, at my request) the problem never recurred. After the first week they wanted to send the car home, but I asked them to keep trying and to use the car for errands, etc. After 2 weeks of "good health" I accepted that I'd just have to take my chances driving the 90 or so miles a day. (BTW, I'm up to about 174,000.) The only work they performed was to pull the "possible" parts, clean the connections and return the parts. I've now driven the car another three weeks without the problem recurring. Keep your fingers crossed that things stay that way. Thanks to all for your guidance.

Also, I have a new (and hopefully unrelated) problem. If you're interested, see the new post titled: To Cruise or Not To Cruise

Be safe...and Keep Pedalling