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Re: OK, now what?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Thu, 24 Oct 2002 13:27:42 Share Post by Email
In Reply to: OK, now what?, R.T., Thu, 24 Oct 2002 13:04:48
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The turbo gauge reads manifold pressure. That's pressurized (above atmospheric) under boost, and vacuum (below atmospheric) when not boosting - at idle, for example.
When the boost gauge need is pointing right at the gap between white and yellow, that is atmospheric pressure. If your gauge reads ABSOLUTE pressure, it would show 14.7 psi, or 1 bar. If your gauge reads RELATIVE or DIFFERENTIAL pressure, it would say 0 psi or 0 bar. Relative or differential is in relation to atmospheric. I'll assume it's a relative/differential pressure gauge, so it'll show 0 with the engine off, + values when the engine is boosting (turbo gauge in the yellow) and negative values at idle (turbo gauge in the white.)
You want to connect your pressure gauge to the same line that goes to the turbo gauge which does run to the instrument panel.

As to temperature - yes, two gauges WON'T work with one sensor. The sensor is just a variable resistor. The gauge works by putting a voltage through that resistor, and measuring the current. The current is proportional to the resistance, which is proportional to the temperature.

When you put two gauges in parallel - the existing one and the new one - half the current goes through the new one, and half the current through the old one. So it reads low. The solution is to (1) use only one gauge - after all, why have two? or (2) get a new sensor with two outputs.

Unless the new VDO gauge is designed for the particular temperature sensor in the Saab, it won't read right. The sensor is a resistor that changes resistance with temperature. There a lots of them out there, each with a different resistance change for a given temperature. Unless the gauge is matched to the sensor it'll read wrong.

The VDO people should be able to tell you what type of temperature sensor/values should work with that gauge.


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