ALH turbocharger boost issues

There is a lot of time and effort being spent by a number of people troubleshooting manifold boost issues and "Limp Home", with the ALH engine.
I am now retired from a long career in the diesel engine field and began to get interested in turbocharger issues on my Jetta when it went out on loan to a family member but returned on a trailer, months later, not driveable.
In my research on how the system works, the VNT turbocharger, the vanes and actuator, vacuum source and lines, the N75 valve. For some reason, my Bentley manual does not show the MAP sensor. in the text. I found it mentioned in the component locator section of the manual.
On this forum and on You tube i see a number of people trying different things, changing parts, even replacing the turbocharger but I have never seen the MAP sensor being discussed as a possible problem.
The MAP sensor is a Euro version of a manifold boost sensor, and while I have yet to remove, test, clean this one, I am sure it is the cause of the problem. In this case, all the other components check out OK. During operation of the engine, if the ECM is receiving a faulty signal from the MAP then it supplies a different voltage to the N75 to dump the vacuum to the actuator. The VNT vanes then move from the closed position, exhaust gas is routed around the turbine wheel, the turbocharger turbine and shaft slow down, the compressor wheel loses centrifugal force and boost pressure goes down. With reduced boost pressure, the ECM also signals the fuel injection pump to reduce delivery and "Limp Home".

ebeday said:

On this forum and on You tube i see a number of people trying different things, changing parts, even replacing the turbocharger but I have never seen the MAP sensor being discussed as a possible problem.

Click to expand...

I think the reason you don't see much discussion on MAP sensors is because failures and/or problems with them are very rare. Actually, N75 valve failures are also rare, but they often get replaced during troubleshooting anyway.

If you're having boost issues, the problem almost always goes back to a vacuum issue, or the turbo itself...either the VNT mechanism inside the turbo, or the VNT actuator hangin' on the back of it.

What kind of boost issue are you having? Under-boost, or over-boost? What have you checked on the system and ruled out so far?

If by chance you decide to remove the MAP sensor, there's a good chance the heads of the two screws will snap off. Just FYI. Maybe not as big an issue on the west coast, but if you're east of the hundredth meridian, those screws will likely be rusty.

ALH boost issues

I have listened to your post and while searching for the MAP sensor to remove it, I discovered that a hose clamp on the pipe from the intercooler to the intake manifold had failed, the boost pressure pushed the pipe out of the joint, into the air conditioner compressor pulley which rubbed a big hole in the pipe. This was the cause of the immediate problem, low boost pressure.
The engine has a history of the overboost, limp home mode and for this reason,I suspect the MAP sensor. I don't know if the map sensor will log a code.The car is equipped with a Webasto engine heater so no cold starts, the car was used as a commuter 30 miles each way. Carbonned up turbocharger vanes are very unlikely.
I know that the N75 valve, the actuator, the VNT vanes move freely and that I have good vacuum. All vacuum lines are sound. The actuator operation was checked with the engine running using a hand held vacuum pump. I know that the N75 valve checks out, resistance thru the coil is in spec.With the engine running I can check the N75 valve operation with the Vag-Com and hear the sound of the engine change. The N75 valve holds vacuum ok.
I am not left with many options except the MAP sensor and yes one screw was seized and I had to grind the head of one screw away to remove the sensor. I don't see any electrical specs for the sensor. Maybe someone knows where to look.

You do not see much talk of MAP sensors because they don't often fail.

The ALH's turbo system is VERY simple. Easy to inspect, easy to diagnose.

Overboost problems are almost always a sticking VNT mechanism/gunked up vanes, or an N75 valve that cannot properly vent the vacuum, or some type of restriction on the vent circuit. Since the vent circuit is also teed into the EGR control system, a blockage from the tee to the air cleaner would also cause an EGR flow problem.

ebeday said:

......................... I discovered that a hose clamp on the pipe from the intercooler to the intake manifold had failed, the boost pressure pushed the pipe out of the joint, into the air conditioner compressor pulley which rubbed a big hole in the pipe. This was the cause of the immediate problem, low boost pressure.
......................................

Click to expand...

Yes, this is another common issue, loose/disconnected piping. With all the other stuff in good nick, you may find your limp problem solved.