Turbo solenoid voltage

Does anyone know what the supply voltage to the solenoid should be?
I have tested the red/white wire and get 12.15 volts supply and then tested the brown/white which shows 3.3volts. This is with red probe on wire and black probe to earth on each wire.
Reading across both wires is 8.75volts
When testing the solenoid on the bench it holds a vacuum which decreases when i put 12 volts to it.

Car runs well for some miles then slows with no power. The last time it did this all the dashboard lights came on and the speedo and rev counter stopped working. Engine was still running despite this!

Pretty sure the thing works on PWM, which is basically a fast switched voltage. At different duty cycles, the multimeter would show different (average) voltages.

Thanks Paul. I also have a problem with the wires. At some time the connectors have been pulled out of the plug. The plug is marked 1 and 2 and I'm not sure which way round the wires go.
I can't find a wiring diagram to show this.

As a standard two wire solenoid, it should be polarity agnostic. So it will be fine either way round. The closest I have been able to find in my Haynes is listed as a turbo boost control pressure sensor but shown as a solenoid on the diagram with red and white from fuse 1 (15amp) in the engine fusebox to pin one out to brown and white from pin two going to the ECM for a 2001 to 2003 model TD4. The next diagram shows a boost pressure sensor as a three wire module.

paul99 said:

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Once again thank you Paul.
If you have a normal solenoid would it not just have one 12 volt power lead and one negative? If so have I got some rogue voltage in the brown/white wire?

Yes, usually a solenoid is traditionally a fully on or fully off device often built into a relay or other two state device. However, in this case it is used to provide a smoothly varying output from off to maximum. To do this, there are two common ways to control a solenoid. One is by using a lower voltage for a partial output, and the other is by using the full voltage, but switching it on and off pretty quickly. That second is the Pulse Width Modulation (PWM) and is easier to use with digital electronics. So the solenoid is supplied with a constant 12 volt positive line and the ECU switches the other end of the coil to ground for a moment, then breaks the connection, and keeps pulsing the connection on and off. By keeping the ground connected for a little longer and breaking the connection for a slightly shorter time, the AVERAGE voltage will look like it smoothly alters from 0 to 12 volts. Your multimeter will show a voltage between 0 and 12 as it isn't reading fast enough to know that it is really seing a fast pulsed 12 volt line. Check out PWM and DUTY CYCLE on Wikipedia or elsewhere for a clearer explanation than I can give, with pictures.
Testing the solenoid coil on the bench, you will expect to see a voltage drop across the coil, as the windings of wire inside it have resistance to the current flowing through it.

Landanywhere said:

speedo and rev counter stopped working. Engine was still running despite this!

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Those two are probably supplied their data from one of the ABS wheel speed sensors. Any ABS faults showing up?
This following quote is from another thread, but may be worth checking out in your case:

Nodge68 said:

Crank sensor would be my guess. These can partially fail, causing odd issues like low power and/or cutting out.

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I think you need to get any codes read, too.

Most appreciated Paul, great explanation and just what I needed.
Have had three different code readers on it and they all come up with 'No Faults' so I will change the crank sensor as a matter of course.

I was wrong on the rev counter, by the way, that figure will be calculated from the crankshaft sensor data, with only the speedo data coming from the wheel speed sensors.