P0652 code and no boost

I have a 2004 1.9 TDI Beetle, 5spd manual. My daughter bought it a couple of weeks ago. I drove it home, 135 miles, with no issues except for an abnormal brake pedal. The brake booster tube had cracked. I replaced it and problem solved. It was performing well enough that we decided to take it on a kayaking trip, 235 miles from home. The car performed well until we stopped about 25 miles from our destination. We stopped a a gas station. I turned the car off and went inside for about 5 minutes. When I came back out and started the car, I noticed that the AC wasn't blowing cold. It was 95 degrees so I figured that once I got back on the road it would be ok. I pulled onto the highway and had absolutely no boost and the CEL was on with the P0652 code stored. (Low reference voltage on B circuit) Being at a primitive campsite with limited cell service, I wasn't able to do much research. Car is home, now, and I've begun to try and diagnose the issue. Here's what I've done so far, with no positive results.

1. Pulled vacuum on the turbo actuator to test it. It has full travel and holds vacuum.

2. Checked vacuum to the N75. 25HG. I cannot get any vacuum on the outlet side. I'm not sure if I'm doing it right. I connected the gauge to the outlet side and it reads a constant 0. Even when I rev the engine to about 3500rpm. No movement. I wonder if the code is keeping the N75 from activating?

3. Checked MAP for contamination. I also checked the reference voltage and it is 4.97. I'm not sure if that is low enough to throw the code.

4. I removed and cleaned the wiring harness connections to the ECM.

5. I unplugged the MAF and it didn't change anything.

I'm not sure which direction to go next. I know that whatever set the code is causing the issue. I'm just not real sure on what to check to find the issue.

4.97V should be voltage enough. Did you check the voltage reading right at the harness connector to the actuator? A common point of failure, if it is not the position sensor itself, is the wiring leading to it. Pull the corrugated sheath off the lead and inspect the insulation for bare spots.

I do remember the sheath crumbling when I touched it so it may very well be the culprit. I'd really like for it to be that simple.

I checked the voltage at the actuator and it was the same as the MAP: 4.97.

Today, I applied constant vacuum to my turbo actuator and drove far enough to hit 3rd gear. I still had no boost. I assumed that the constant vacuum would have caused the turbo to create boost. It did not. It seems logical that the actuator rod would not have any travel if the vanes were stuck but it seems to have the full 3/4' or so of travel. I'm now leaning toward the possibility that the waste gate is stuck open. Does anyone have any information on how to check and see if this is so?

There is no waste gate...only the vanes create the condition within the turbo assembly to make boost. If it does not have the actuator's position sensor for a reference it is likely putting the engine into limp mode by cutting fueling back to prevent creating boost and by disabling the N75 valve vacuum signal.

Sorry.....got my codes mixed up. Thought it was referring to the actuator position circuit instead of the MAP sensor. Not having a functioning MAP sensor will definitely put it into turbo disable mode by whatever means...cutting vacuum and fuel. The easiest way would be to swap out the sensor with a known good one. They don't often fail but they do fail. I've got a car here now that had one fail. Got to be either the sensor or the wiring...and perhaps a blown fuse due to bad wiring. If you've already check that you are getting reference voltage at the harness connector it is not likely that the fuse is out. And lastly, the ECU could have gone bonkers, but that really does not happen often.

My approach would be checking continuity from sensor to ECU on all the wires. If that checks out then swap out to a good sensor. If you can get one at a salvage yard then it would probably be OK. One from a BEW or BRM engine will work but check the part number, (038906051B if it is the original to the car or 03G906051D if purchasing new), they can get changed out to a different one when performance mods are done. Otherwise it is about $50...Bosch only, thank you.



Added info from those who know:
http://wiki.ross-tech.com/wiki/index.php/17036/P0652/001618

When removed the MAP to check it for contamination, I noticed that the thin metal strip, with the bulb on it, wasn't sitting center of the plastic piece. it was laying to one side. I'm not sure this would affect it but I ordered another one and will replace it because I already have access to the location and it's a pain to get at. Another thing I noticed, while inspecting fuses, was that the one for the injectors was badly corroded. I replaced it and cleaned the contacts with hopes for a good result. Nothing. I can erase the code and the CEL illuminates within 2 seconds of starting the car. I'm optimistic about the new map, though. Thanks, JETaah, for your suggestions.

I've replaced the MAP with no positive results. I moved on to the MAF and discovered that I only have 2.11v on the reference wire. I checked with ignition on/engine off and with the engine running both inside the connector and back probed it. My understanding of the 5v reference circuit is that it is used as a clean ground from the ECM. I plan on testing the leads at the ECM the first chance I get. At this point, I'm kind of hoping it is bad there. From the looks of things, it seems that replacing the ECM would be easier than tracing down a bad lead.

Today, I was trying to look around the Beetle for some obvious signs of wiring shorts. I crawled underneath the driver's side front and found what appears to be some kind of switch or sensor that is screwed into the transmission. Both wires that go into the connector are broken loose from the connector. I searched for VW sensors and switches that resemble this one and it looks a lot like the cooling fan switch. Whatever it is, I think I need to get a new harness and get this thing reconnected. Anyone have any info to help me identify this thing?

Manual trans next to the starter teerminals? Two terminal housing? That is the back up light switch. They all break eventually. Touch the wires together, ignition switch on, and see if they light up.
Just splice a new pigtail in there.