About half.
My 90 currently ought to be off the red and beyond on a nice motorway drive. You can put a resistor in series to knock the resistance up and hold the guage back, get the engine warmed up a bit till the guage sits halfway. Then measure the resistance between the tip of the sensor and the main body of it. For easy counting lets say it reads 300Ohms, you then therefore want the sensor to read 300ohms at normal operating temperature. So get the engine up to temp, with the heater blowing nice warm air and measure it again, say it reads 40ohms now, so you need a 260ohm resistor, with the sensor "held back" that should sort your guage.
OR get a matched sensor, mine used to be a 2.5NA, so I am going to screw the NA sensor into the top of the block where the glow plug sensor sits (I have a manual glow plug button).
Or pay £45 and buy the right guage.
i've never paid more than a fiver for 200Tdi guage - the 200 and TD/NA sender has a different thread hence why you can't use the orginal sender
keep your eyes open on ebay and get the right sender rather than messing about with resistors
So get the engine up to temp, with the heater blowing nice warm air and measure it again, say it reads 40ohms now, so you need a 260ohm resistor, with the sensor "held back" that should sort your guage.
do a search on here to see how many people moan about how long it takes for their heater to get anything close to warm - you might then see how daft your method is
Haha well, fine then - touch the stat housing and when its too hot to hold your hand there measure again.
The resistor idea works perfectly well; and it costs about 5p to do... excellent...
Or some black paint.i bought my 300TDi gauge for £10 - the dial at the back differs, which is one way of telling what gauge is what.
Oh, if you have a disco engine, what colour is the plastic surround for the sender? If its green, then you need to buy a black sender for your Defender gauge.