My 1991 XJ40 had a leaking radiator that was repaired unsuccessfully with epoxy. The rubber flaps in the fan shroud were as hard as plastic and broke off when touched. I was given the car because of the overheating problem. I found that the harmonic balancer was sheared and the radiator fan viscous coupling had leaked all it’s silicone oil out so was in freewheel mode only. All these cooling related faults should have cooked the motor but miraculously the motor is undamaged.
The quote obtained for making a new radiator was R12,000 (South African Rand). I decided to try plastic welding to repair the inlet pipe which was cracked nearly all the way round with a big 1cm hole at the top, and it looks like I got it right on my second attempt. I used a 25 Watt soldering iron and black cable ties and made sure all the edges were blended in properly. On the shroud I used 1.5 mm sheet rubber to cut the pieces needed to replace the flaps and fitted them with small bolts and nuts rather than pop rivits.
On the harmonic balancer I removed the trigger wheel and in the land behind the pulley I drilled and tapped four holes and fitted 6 x 25 mm bolts to make the pulley solid (no rubber damping). The fan coupling I also drilled and bolted but as a temporary measure until I raised the R2000 that a new one cost.
All these repairs I carried out have changed the driving experience from a nightmare to a pleasure but the last problem was the temp. gauge. I was not happy with a temp. needle fixed on N in the middle of the gauge. I want a temp. gauge that tells me that my motor is getting hot, not a temp. gauge that tells me that my motor is overheated. I checked the resistance range on the temp. sender unit and it appeared about right. The fuel gauge appears to read correctly so I assume the voltage stabilizer is putting out it’s correct 10.5 volts.
Using a few resistor values across the sender unit to earth I established that a 200ohm minipot from the sender unit to earth would enable me to use the range above N to show actual increases in temperature so now the needle on N is cold and needle at 3/4 is operating temp. and an overheat condition would put the needle in the red and actuate the red overheat warning light on the dash, long before your engine has burnt out.
Adjusting the potentiometer allows one to move the needle to the 3/4 hot position and makes small corrective adjustments easily possible