Pride before a fall and rise

 So I decided to try sort out my temperature gauge - it hasn't been working for months.

I'd checked the wiring and sensor and I knew both were OK so I decided to try dredge up some knowledge from my electrical engineering degree 30 years ago (I went into software and forgot most of my electrical stuff). 

Working on the fact that the sensor is just a load in the circuit that includes the gauge, I decided I needed to lower its resistance to allow some voltage to move the gauge needle. I recalled that placing another resistance in parallel with the sensor should achieve that. Not knowing what resistance I needed, I bought a £3 variable resistor, wired it in parallel with the sensor and stuck the resistor on a convenient surface:

Now, before fitting it properly, I jury-rigged the wires to make sure the theory worked and, Holy Shit but it did - with the ignition on, I altered the resistance and the gauge needle moved accordingly! I was seriously impressed with myself and I almost fired off an email to my university to congratulate them on their teaching abilities. 

Anyway, I then wired the resistor up properly, which involved snipping off the old spade terminals on the earth and sensor. While doing the earth, I noticed some dirt under the terminal, which i unthinkingly removed and crimped on a new terminal. 

Once wired up, I connected up the laptop so I could read the coolant temp and adjust the rheostat so the gauge matched it. Once started and running for a minute, I could already see the needle rising on the gauge. 

Hugely proud of myself, I went off for a drive to check it worked at all temperatures. A mile down the road, however and the gauge read 110 while the laptop showed 95 so I stopped to adjust the rheostat. But I found that no amount of twiddling would get the needle below 80 - I should have been able to set it to anything I wanted, in theory?! 

And then it dawned on me - the sensor was working correctly, without the need for the rheostat. To prove it, I cut the wires and voila! A working temperature gauge! 

So all I had needed to do months ago was remove the earth and clean it (and maybe the new, shiny spade terminal also helped). 

Doh! 

So a lesson learned - the simple things first before dusting off the old grey matter.