Gauge issue

[lundabo]

Ran my car today. Stopped to replace the brake lamp, and when I restarted, both my temp and fuel gauges were reading at half of what they should be reading. After driving for 15 minutes, both then came up to normal reading simultaneously. Any idea what is happening? Thanks in advance.

[SRK]

I'd start by checking the instrument voltage regulator. If that's going weird however I would also expect the fuel gauge to also react.

Steve

[lundabo]

Thanks...makes sense. I'll test it.

[jagwit]

I'll test it.

I wonder how.... Connecting a volt meter to its output simply shows either 12V or 0V in a cyclic manner.

The way I understand this "regulator", its really an on/of switch with a preset duty cycle, so I presume it must have a minimum on vs off ratio. Assuming we know what that ratio is, testing it would require an oscilloscope to measure the duty cycle?

[Heuer]

The regulator should output 10v with a 12v+ input. Put a digital voltmeter set to AC on it.

If in doubt make a new one: viewtopic.php?p=35094#p35094 Only cost a £1 in parts from Maplin or Radio Shack.

[cactusman]

Unless the digital volt meter is a true RMS meter it will mis read. Cheap dvm's expect to see a sine wave. Any wave form other than sinusoidal will result in an inaccurate reading. The 10 volt supply approximates to a square wave.

Better solution is to connect the 10 volt supply to be tested via a 1000 ohm or so resistor that then charges a small electrolytic capacitor...say 100 microfarads would be ok. The RC combination will smooth out the on off pulses. Then use a cheap dvm set to D.C. to measure the voltage across the capacitor. Not too precise but ok.

If you have access to a traditional meter with a damped movement....an AVO Mark 8 is the best...then this set to D.C. will show the correct voltage.....an example of old technology being better...and probably what Jaguar would have used back in the day...

[MarekH]

You can check that the regulator is working by connecting a suitably resistored LED to the output spade. You will see it strobing. When it is lighted, the voltage will be the same as the input voltage and when the light is out, it'll be zero volts.

You can then use your meter to measure the output voltage and thus deduce the on:off ratio. To change that ratio (and thus the average output voltage), you'll need to adjust the adjustment screw in the base of the unit.

Because the gauge needles only move very slowly, they are not affected by the strobing nature of the output.

A solid state replacement does not strobe.

As you reported that the gauges took time to reach their expected values, it implies that the voltage passed into the regulator was low. This means that the current passing through the regulator was low. This can happen if the circuit connected is a low current circiut (i.e. most of the gauges are "disconnected", pull only very little current and thus warm up the regulator heater wire very little. This may be by way of a poor wiring connection. Alternativley, if the alternator didn't get going, a similarly low voltage may have been present.

kind regards
Marek