How do Volvo turbo control valves (TCV) work?

JDS60R » Its a pulse modulated solenoid. So it opens and closes like a fuel injector to get the right boost characteristics throughout the entire rpm range. Red and yellow are correct. Blue goes back to the intake tube before the turbo. (No air is lost in these systems)

When they fail, how do they fail?

They tend to fail with the red and blue open (no boost) but most are replaced far ahead of failure time as the response is lowered with the age of the solenoid components. It has been common, to maintain peak performance to replace the TCV and boost pressure sensor at about 60 K miles. Many of them continue to perform great far after 60K. When Volvo started with actual boost pressure sensors the first design would often go bad by 60K. This is why many folks replaced them both.

Several of the Volvo systems do not use a dedicated boost pressure sensor. They are using the rpm, MAF etc to calculate a boost pressure.

I think the boost pressure sensors came in around 1999. On your T5R if you have over 100K miles I would replace the TCV with a real Volvo one from FCP. If the MAF is in spec and the oxygen sensors have been replaced every 100K you should do fine.

Wastegate valve

The boost pressure may become excessive if the turbine speed is too high. This is prevented by the wastegate valve (5), which bypasses a proportion of the exhaust gases flowing to the turbine.

The turbo control valve (TCV) on my T-5R

TC control valve

The wastegate valve is controlled by the boost pressure by means of the TC control valve (6). An output on the ECM (7) supplies the valve with a pulsating signal, the duty cycle of which can be varied to control the pressure delivered to the wastegate valve servo. When the ECM determines that a higher boost pressure is permissible, the duty cycle of the signal delivered to the TC control valve is increased and a proportion of the pressure delivered to the servo is relieved to the air intake. The control pressure and wastegate valve opening are thereby reduced, increasing the flow of exhaust gases through the turbine and increasing boost pressure.

Control of TC boost pressure

The TC control valve duty cycle (pulsating signal) is controlled by a desired value computed by the ECM.

The ECM uses signals from the following sensors for this purpose:

TP sensor (8)
MAF sensor (9)
RPM sensor (10) In other words, the system is not equipped with a pressure sensor. The ECM uses the MAF sensor signal (a measure of the flow through the ACL) to compute boost pressure.

abscate » Here are the hose colours and assignments on the 1999 Turbo control valve, aka Boost Control Solenoid

Ive had this car since 2000, and I believe this is how it came from factory, so the color codings are probably factory too.

I made the assignments by tracing them – if someone with Volvo software could confirm that would verify this data.