Today I had a look at the Smiths (Caerbont) instrument voltage regulator that is fitted to the back of the speedometer. This regulator takes in the 12V and regulates it to ~ 10v for the fuel and temperature gauges so that they don’t fluctuate when the car’s voltage goes up and down.
It is a very simple circuit; it uses a resistor (4k7), zener diode and a NPN Darlington transistor (FZT600). Anyway I replaced the transistor a couple of times, but it kept on blowing.
So, I obviously had too much current being drawn, but from what, one of the gauges? Access to the back of the speedo and especially the temp gauge was difficult at the best. But eventually I found the culprit, the +10v feed from the regulator had fallen off the terminal on the temperature gauge.
The cure was a pain; I couldn’t get my hand to the terminal, so what I did was to loosen the locking bracket so I could rotate the gauge around so the wire could be easily reached and the wire connected, then rotate the gauge back again.
Started the car and let it warm up, both gauges read correctly. Yeah!
14/01/2016 Went for a long drive today, around 300+ kms with no problems from gauges.
- Fixed instrument voltage regulator with new ZFT600 transistor
- Enjoying the car!
- Hopefully nothing for a fair while!
Nice to do, but not required!
- Fit the new heater. (Not doing until I have car on the road and tuned in… it is summer after all!)
- Get a wheel balance and alignment.
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