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MG MGB GT V8 Factory Originals Technical - False reading
|My Capillary driven temp gauge started reading half way between N - H and purging,refilling and so on had no effect so installed new leads to fans via Kenlowe thermostat ,with a new lead from the starter solenoid ,new switch to a relay with another lead still no joy . Bought new thermostat -82C but before installing ito 3.5 Stage 2 engine I obtained a Seeley laser thermometer . Instant cure of Angst ! Readings go 77-80 at thermo housing across top hose and top of rad. With it you can check engine temp anywhere but I suspect with different materiels you get different heat transfer to the surfaces . But it is very easy to check either side of the T/Stat or top and bottom of the rad -just point the laser and read cent /Fahr as needed not -cheap but better than heart attack .|
The reading on the gauge starts at stone cold half way between C and N any ideas on how to get the needle to read to C when or an easy way to get the gauge out (72 roadster )
|You need to be careful with those 'laser' thermometers, the later dot only shows roughly where the sensor is pointing, it isn't reading the temperature at that spot. Even the better gauges only have a 10 to 1 aspect ratio or thereabouts, which means even positioned 10" away the sensor is reading an area 1" in diameter. OK for a rad, but any further away than that on hoses and thermostat housing etc. gets progressively more inaccurate, averaging the point of interest with its surroundings. Cheaper instruments only have a 1:1 aspect ratio which really means they need to be touching what you are measuring unless it is very large.|
Any road up. Normally the gauges read low when they are on the way out as the tube cracks and the gas or whatever leaks out, and they have to be exchanged. I suspect with yours it is something to do with the gearing between the Bourden tube and the pointer, and so potentially fixable by realigning the gears.
Don't know about an 'easy' way to get the gauge out of the car altogether, the only way is to undo the big nut that secures the bulb into the head or inlet manifold, and that is the trickiest bit - getting the bulb out without breaking the tube. After that it is a matter of undoing the little thumb nuts on the back of the gauge studs that clamp the U-bracket to the back of the dash, and withdrawing the gauge and full length of the tube through the dash hole. If it is a dual gauge you have to disconnect the oil pipe from the flex pipe or adapter coming off the engine as well.
But if you are just investigating the gauge mechanics you may be able to pull the gauge forward from the dash far enough to work on it, leaving the pipes connected, if they have enough slack.
|Thanks Paul -as ever a reasoned reply with good info. However I read the instuctions first and kept the instrument head within 3to 4 inchs of the spot I wished to check . Although after a 5 mile trip the gauge indicated almost on the H the rad reading was 78/82 at the top and 80/84 at the thermo housing. At the base of the rad 65/70. As a cross check I put a t/stat into hot water and heated it -at 82 it opened when the laser was focussed on the stat -but if aimed at the base of the pan the water temp carried on rising to an indicated 100/203 . So now a careful try to remove the gauge -thanks for the tips on this and many other MGB subjects .|
This thread was discussed between 05/04/2009 and 06/04/2009
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