TD5 D2 engine - Wastegate Modulator (2)

Looking through the latest Nanocom test data, the results are not conclusive. Still don't believe Hx is developing full boost, and reasons for that are not so obvious...

-  The Wastegate Actuator can be further adjusted to deliver more boost.
-  The Wastegate Modulator valve may be 'sticky' or leaking internally.
-  The MAP sensor could be dirty...

On a positive note though, the general performance of the engine has improved. There is a definite progression to the turbo when it gets applied and the acceleration and cruise at speed is good. I guess I just want more of that performance available lower down the rev-band.

Suspicion has turned again to the Wastegate Modulator mounted on the engine block. It can be seen to receive the signal from the ECU to operate at the various 'Duty-Cycles' of up to 40% which is the MAX value. Although the signal might be getting through I can't be sure that the internal valves in the Modulator are performing as they should.

There is a Nanocom enabled test setting for the Modulator that will send a test signal pulse to the Modulator with the engine off. It should be possible to hear the solenoid inside the Modulator clicking or indeed 'feel' it doing so by physical contact. I did try this the other day, but I was unable to hear the solenoid clicking and the physicality of the location means an assistant is needed to operate the Nanocom while I get physically close to the Modulator or use the old stethoscope trick.

There is a text message on the Nanocom about 'testing complete' but I don't know whether it will throw up a fault message or not.

There is a need to fully understand the operation of the Modulator. The data logs show signals are sent to the Modulator and Hx never overboosts so I am tempted to say the Modulator works, but the data also shows Full Boost on Hx is not quite what it should be!

MODULATOR OPERATION .

Here is what I understand about how the Modulator works. Numbered references are from the Rave diagram 'M192564' that depicts the Modulator on the engine block..

Boosted air pressure from the turbo outlet pipe is bled off through a small-bore flexible tube that enters the Modulator from below (2-lower) and exits at the top right (2-higher) to supply the Wastegate Actuator with pressure.

This will allow the increasing turbo generated boost pressure inside the thin-bore pipe to increase and begin to operate the Wastegate Actuator on the turbo unit. 

This is essentially the same as having the Defender Td5 set-up where the pipe from the turbo outlet goes direct to the Wastegate Actuator. This set-up then depends on how the Actuator is adjusted to determine when the Wastegate valve opens and that governs the level and duration of Full Boost available.

It can lead to the condition of 'Wastegate Creep' where the turbo never delivers full boost because its Wastegate valve is operated too early in the boost generation cycle.

It can be seen that adjusting the Actuator's internal spring will determine at what pressure level the Actuator begins to open the Wastegate Valve. 

Increasing the spring tension on the Actuator will delay the opening of the Wastegate and increase the availability of boost pressure in the inlet manifold. It is a delicate balance to achieve, but it is easily done with care. The big question is knowing what boost pressure it should be developing and comparing that to what it is developing now.

Back to the Modulator.

The Discovery Td5 engine has the Wastegate modulator fitted in order to allow the engine to safely develop maximum full-boost turbo pressure for longer. This should have a positive performance gain over a non-regulated Td5, allowing the full performance of the engine to be used when needed under load for overtaking or hill-climbing etc. In doing so it also eliminates 'wastegate creep'.

The Modulator has a third small-bore tube fitted (1) to its top surface that is connected to the Turbo INTAKE downstream of the MAF sensor. This provides an internal solenoid/valve controlled 'escape' route for the Boost Pressure inside the Modulator to be reduced by venting it into the lower pressure turbo intake stream. 

This is the act of Modulation and it ensures that small amounts of the boost pressure are released to reduce the pressure on the Wastegate actuator and thereby delay its opening and thus maintain full boost pressure for longer.

The Modulator is not a sensor of any kind. It does not measure the air pressure passing through it in any way and it's timed operation is governed by the engine ECM that can process signals from the various engine sensors to determine exactly when, how much and how long to open the solenoid. 

It is not a simple 'open and shut' signal either, it is a 'pulse wave modulation' (PWM) that rapidly opens and closes (feathers) the valve to achieve the desired control on the air stream. I can understand the desire to bleed rising pressure above a base level whilst also maintaining pressure at that base level.

On the test runs/data logs, various values have been recorded for the Wastegate Modulator. It should only be operational under boost conditions and has varied from 20% upto its max reading of 40%. At all other times it should read 0%.

Seeing these figures recorded at least shows the ECM and sensors are functioning, but can I really be sure the modulator responds to the command signals?

THE DUTY CYCLE.

That % reading is an expression of the 'duty cycle' meaning the amount of time the valve is open during a given period. The Nanocom data log will record new data every second, so I guess 0% means it is not open at all during that second and allowing Boost pressure to build. 40% means it is open for 40% of that second and allowing the increasing Boost pressure to be diverted away from the actuator thereby maintaining full boost pressure at the manifold for longer...

The ECM in the Td5 has a much higher sampling rate then the Nanocom and can process the signals many hundreds of times per second.

Don't forget that as the turbo spins faster, the boost pressure increases and a point is quickly reached where the turbo will generate too much boost pressure for the engine to handle safely.... Hence we have the whole idea of Wastegate valves and boost Modulators to control the generated boost energy levels.

Performance wise it really does come down to everything being adjusted 'just-right'...

POTENTIAL PROBLEMS.

So, assuming all the rubber hose connections are secure and not leaking and the electrical loom connections/wiring are fine, the only internal faults are to do with the sticking valves. If the valve in the modulator fails to open then the turbo boost pressure will not be modulated and it will flow through to the wastegate actuator. This is the same as on a Defender and can be considered a fail safe condition as it promotes wastegate creep.

Likewise, if the Modulator valve fails to shut fully then the building boost pressure will 'leak' away early and Full Boost may never be achieved.

Depending on how the Wastegate actuator is set up, this could lead to the condition where the Wastegate valve is opened before Full Boost is developed.

In the past I have driven various turbo-charged vehicles including some that 'whistle' as the turbo spools up. This has always been tracked down to a boost pressure leak somewhere, and the engine performance was noticeably down.

That kind of pressure leak is an extreme form of boost modulation. Hx is nowhere near that bad but even a little leak, even not to the atmosphere, would reduce the boost pressure available and be nicely disguised...

TESTING THE MODULATOR.

With the Modulator on the bench, I propose to attach tubing to all 3 connections and submerge the upper two tube ends in water so that when I blow through the remaining tube, air can be seen to emerge from them. If my theory is correct I would expect NO bubbles to emerge from the upper outlet (1) and lots of bubbles from the other outlet (2). This proves the internal link to the wastegate actuator. 

If any bubbles emerge from (1) then it points to an internal leak and a replacement modulator should be purchased..  That's the theory anyway..

REPLACING THE MODULATOR.

As there are no user serviceable parts inside, removal and replacement is the only real option available...

Replacement Modulators are readily available for around £20 from a wide variety of sources.

The photo above is from JGS4X4 and shows all three of the hose connections. The 'Pierburg' brand is the one to go for and on their corporate site I found a comprehensive data sheet about the operation and test data for the Modulator....

The above graphic shows the airflow through the Modulator when the solenoid valve is either 'de-energised' (closed) or 'energised' (modulating) ..

Glad to see my own musings confirmed!

Further reading...

' DISCOTd5.com ' - has a great in depth write-up about the Wastegate Modulator.