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, Fri, 12 Apr 2002 10:37:04
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Usually when the caliper is dragging, it is not only the caliper, but the brake disk and wheel that act as major heat sinks. With aluminum alloy wheels, the heat is radiated quite effectively, although the wheel will be hot to the touch if you come to a stop and put your hand on it. The brake fluid would boil, the wheel grease fry, and the rubber hoses and gaiters burn well before the caliper started to glow red.
The thermostat is filled with a wax that expands over a reasonably narrow (ca. +/- 2 degrees)temperature range. The stock 82 degree turbo thermostat opens up at 82 degrees give or take a few degrees. It's primary function is to remain closed during engine warm up. At operating temperatures above 82 degrees, it stays essentially wide open and the excess engine heat is dissipated from the radiator. The cooling fan kicks in because it is controlled by a temperature sensor. It tries to limit the upper temperature that the system reaches by increasing the convection across the radiator fins.
The temperature gauge is "calibrated" over an approximately 20 - 30 degree range from low to red line. Between where the thermostat typically opens (about 1/4 - 1/3 up the range for both of my '96 AEROs) (Aside: My '90 9K runs about mid range - the gauges are calibrated differently on the newer cars because SAAB was constantly questioned about their cars apparently running hot - I've owned SAABs for almost 30 years) and when the fan kicks in is probably no more than 15 degrees, although it may be less since the fan sensor (near the radiator) and the gauge sensor (on the block) are in two different locations. For the gauge to rise to mid scale is probably no more than 5 degrees. (I would welcome someone who is sufficiently ambitious to actually make these measurements with a calibrated thermometer.)
On a warm day, the radiative efficiency of the system is lower than on a cool day. Therefore, it is very possible that on a warm day the amount of extra work needed to over come the brake drag can result in a rise to mid gauge, or even higher. In fact, brake drag is often quoted as a source of overheating problems in diagnostic automotive troubleshooting texts - including SAAB manuals I've read over the years. On a cold day, the disipation of heat through the radiator is more efficient, so the rise may or may not be noticable and depends on the entire system.
I agree with Ari that the caliper (and probably the brake rotor as well) should be replaced immediately.
Steve B
posted by 165.170.12...
, Fri, 12 Apr 2002 10:37:04
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