Date: Tue, 18 Feb 2003 12:33:46 -0000 From: "Andy Hookins" <andrew.hookinsnopsamamnopsamodata.co.uk> Subject: Re: heat question
"Grunff" <grunffnopsam.com> wrote in message news:b2t2um$1g74f7$1nopsam52899.news.dfncis.de... > Andy wrote: > > Sounds to me like your sensor (thermostat) is on its way out and isn't > > correctly picking up the temperature correctly. There are two sensors to be > > aware of and both interact with each other from memory, to find a > > balance/ensure correct reading. If one is damaged.... no big cost, probably > > around £50 at a main dealer, cheaper if local garage. > > The thermostat and temperature gauge sensor are two totally > different things, which don't interact with each other in any > way (other than via the temperature of the coolant). > > The thermostat is a mechanical valve, which is operated by an > expanging chamber filled with wax. It opens when hot, allowing > coolant to flow through the radiator. > > The temperature sensor is a thermistor whose resistance varies > with temperature, and this controls the level of the needle on > the gauge. > > There is a third 'sensor'; the fan thermo switch. This is a > bimetallic electrical switch which switched the radiator fan on > when the coolant temperature exceeds a pereset value (82/87/92C). > > The OP's symptoms are characteristic of either a sticking > thermostat or air in the coolant system. Both are easily rectified. Sounds like thermostat is working OK, the thermostat is a 'three-way' device. The third mode of operation is that the heater matrix is cut-off when the engine coolant gets really hot and water only circulates around the engine and radiator - that is why you're not getting any heating inside the car. Have you checked that the fan runs when the engine gets hot? When did you last change the coolant? Did you give the radiator etc a good flush? (they tend to silt up over time). > > Also worth noting that thermostat/temp gauges are 'dampened basically for > > drivers piece of mind so when an engine is at correct temperature, the gauge > > is usually about half way - it would normally be nearer the top in reality, > > but customers don't like to 'see' this as they worry too much. So they > > regulate the view of the actual temp, hence why I think the > > sensor/thermostat is at fault. > > As explained above, the sensor which feeds the temperature gauge > and the thermostat are two separate things, and not related in > any way. As for the damping, the gauge is damped, but damping > prevents rapid fluctuations, rather than reducing the temperature. > > -- > Grunff > The gauges are damped, also some temperature sensors are composite devices constructed with NTC and PTC thermistors such that they have a more or less constant resistance over the range of temperatures that the engine manufacturer considers 'normal' hence the indicated temperature will appeat to be the same over this range. AndyH.
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