Determining if an N75 solenoid is working...

[capthook]

Hello all...

I'm having issues with an 04 Jeta TDI (BEW) that may be a result of a faulty N75 solenoid... The VW manual I have (Bently - Two Volume set) indicates to test the resistance on the N75 and that the proper range is 14 to 20 ohms for the 1.9 litre BEW engine. I get a measurement of 16.4 on a cold engine. Does that result definitively indicate that the N75 is working properly or not?

 

[wonneber]

"working properly or not?"
It will show what the ECU want's it to do.
The N75 plug (not plugged in) should have 12 volts on 1 pin and the the other may have low voltage if any.
The 'other' would come from the ECU and be pulsed as needed.
The 16.4 ohms is in the range.
The N75 resistance should not change with temperature.
Yours is in the range.

You did not mention what problem you are having.

 

[P2B]

Does that result definitively indicate that the N75 is working properly or not?

No, it only indicates that the solenoid coil resistance is within the expected range.

To test N75 operation, run the N75 output test in VCDS (engine idling) and observe the turbo actuator. If it cycles through the full range (about 3/4") N75 and actuator are working. If not, tee a vacuum gauge into the N75 output hose to determine if the issue is N75 or turbo.

 

[capthook]

P2B... When you say "tee a vacuum gauge into the N75 output hose" do you mean cut the existing hose, install a tee fixture and then running a vacum test? If so, my follow-up questions are (1) where does one get a tee for either a 3mm or 5 mm vacum hose, and (2) is the tee installed permanent or is it removed after the test and if so how is the repaired? Assume I'm not the sharpest pencil in the box!

 

[oilhammer]

Back the Fail Train up a bit. What problem do you have or think you have that you are trying to diagnose?

The N75 is just a simple duty-cycled solenoid. ECU uses it to control boost. Did the ECU tell you that you may have a have a boost control issue?

 

[capthook]

Oilhammer et al...

My car's symptoms were rapid onset of low power on hills and/or passing, inability to get above 3300 RPM and copious black smoke (mostly in 1st, 2nd and 3rd gears while accelerating, and a Boost Pressure Regulation error (P0299) using a VCDS diagnostic tool. To date I've replaced the air filter, fuel filter, and MAS with some improvement but no fix. I'm now focusing on the N75 and the actuator but I get a measured resistance from the cold N75 of 16.1 Ohms which, as I understand it, is within the normal range. So I want to test the vacuum (recommended by another TDI Club member) to isolate the problem between the N75 and the actuator... Thoughts?

Back the Fail Train up a bit. What problem do you have or think you have that you are trying to diagnose?

The N75 is just a simple duty-cycled solenoid. ECU uses it to control boost. Did the ECU tell you that you may have a have a boost control issue?

 

[Vince Waldon]

Have you seen the low-power troubleshooting step-by-step linked in my Sig?

 

[P2B]

You have an underboost code, most commonly caused by a boost leak in the piping between the turbocharger and the intake via the intercooler.

Look there first, and run the N75 output test to check that the ECU is able to control the turbo. Best place to get a tee and spare hoses for vacuum testing is the junkyard.

16683/P0299/000665 - Ross-Tech Wiki

wiki.ross-tech.com

 

[shoebear]

P2B... When you say "tee a vacuum gauge into the N75 output hose" do you mean cut the existing hose, install a tee fixture and then running a vacum test? If so, my follow-up questions are (1) where does one get a tee for either a 3mm or 5 mm vacum hose, and (2) is the tee installed permanent or is it removed after the test and if so how is the repaired? Assume I'm not the sharpest pencil in the box!

The way I've done this is to get a short piece of vacuum hose, disconnect the regular hose from the turbo, connect the short hose to the turbo, and install the tee between the regular and short hose. Then connect your mityvac to the tee to complete the test setup. I got the tee by buying an assortment pack of them at a local auto parts store (O'Reilly?), and finding a size that works. This can be set up on either end of the regular turbo vacuum line -- but if you set it up at the turbo, it's easier to observe the actuator movement. When you're all done, just disconnect everything and plug the regular line back in.

While you have everything out and set up, I suggest also test just the turbo actuator. Connect your mityvac to the turbo actuator vacuum port. Find the actuator rod and keep a finger on it as you apply vacuum. It should 1) move with vacuum and 2) move smoothy without jerking. If it doesn't mover, your actuator is probably bad; you can replace this for under $50. If it moves but is jerky or gets stuck, your turbo is dirty. The quick way to clean a dirty turbo is with oven cleaner; for a more complete clean, remove the turbo and clean manually on the bench. There are complete instructions for this on this forum. I may be wrong, but I think the bench clean method is documented in a PDF by DriveByWire.

 

[oilhammer]

Your N75 is likely fine... you have a boost leak somewhere.

 

[PakProtector]

You have an underboost code, most commonly caused by a boost leak in the piping between the turbocharger and the intake via the intercooler.

Look there first, and run the N75 output test to check that the ECU is able to control the turbo. Best place to get a tee and spare hoses for vacuum testing is the junkyard.

16683/P0299/000665 - Ross-Tech Wiki

wiki.ross-tech.com

I would say that the underboost code is most likely caused by an actuator with a leak, or a hose connection that leaks.
Couple things to test, all worthy of a 'First' category, but since there is more than one, you do them all.
-with a vacuum source pump down the actuator and make sure it moves, and that it holds the negative pressure

you will need a MightyVac, and a short length of 3mm hose
Could be hose ends have cracked and leak on their nipples. For the main run to the actuator, you can probably trim 1/2" off each end and be OK until new hose arrives.
-cycle the actuator with VCDS
Requires a functional vacuum pump to do, so either N75 or vacuum pump wear. WIth said Mighty Vac, plug in in place of the reservior ball and confirm the pump is working.

This is all( I believe) in the limp-mode fix thread.

Douglas

EDIT: IFF the top intercooler hose blew off, it shifts the intake piping into the power steering belt pulley and will shortly saw a circular slit into it. That won't be rerpariable w/o a new piece.

 

[P2B]

I would say that the underboost code is most likely caused by an actuator with a leak, or a hose connection that leaks

That would typically cause an overboost code since the ECU is unable to consistently move the vanes from their default full boost position.

 

[J_dude]

That would typically cause an overboost code since the ECU is unable to consistently move the vanes from their default full boost position.

I thought default was no boost, I bumped a vacuum hose off accidentally and had no vacuum and car was gutless and smoky

 

[P2B]

I thought default was no boost, I bumped a vacuum hose off accidentally and had no vacuum and car was gutless and smoky

I stand corrected, no vacuum is no boost. Despite that, I'm used to rusted out actuators causing overboost codes, and boost leaks causing underboost.

 

[shoebear]

I thought default was no boost, I bumped a vacuum hose off accidentally and had no vacuum and car was gutless and smoky

I think the TURBO's default is no boost, but the ECU applies full vacuum to the turbo at idle/no load. So the ECU's default is full boost. I'll check this when I do my next oil change -- coming up in a few hundred miles.

 

[oilhammer]

That is correct. Moves VNT to full boost and then farts away the vacuum via the N75 to the air cleaner as necessary.

 

[J_dude]

I think the TURBO's default is no boost, but the ECU applies full vacuum to the turbo at idle/no load. So the ECU's default is full boost. I'll check this when I do my next oil change -- coming up in a few hundred miles.

Oh, yes that is true.

Edit: Oilhammer beat me lol

 

[capthook]

Gentlemen...

When I use the selective test Solenoid Valve for Boost Pressure Control (N75) with the engine NOT running I get the following results:

Mass Air Flow (actual) = 310.0 mg/str
Atmospheric Pressure: 897.6 mbar
Boost Pressure (actual): 887.4 mbar
Accel. Pos. Senor 1(G7): 0.0%

When I use the same selective test with the engine RUNNING AT 2000 RPM (but with the vehicle not moving) I get the following results:

Mass Air Flow (actual) = 335.0 mg/str
Atmospheric Pressure: 897.6 mbar
Boost Pressure (actual): 918.0 mbar
Accel. Pos. Senor 1(G7): 20.0%

During this test the actuator rod did not move....

I have also checked the resistance on the N75 and read 16.2 ohms which I understand is in the normal range for a cold test.

Additionally I have phyiscally manipulated the actuator rod and it seems to move smoothly... I have also tested the actuator with a Mityvac and it holds vaccum. And the acuator rod moves as expected when vacuum is allied and increased. I also tested the vacuum on the line feeding the actuator and got 23.5 inHg. I also checked that the acuator held vacum at 23.5 and it did. Additionally, the acuator rod moved as it should have when I used the Mityvac to pressurize the actuator, starting around ~3 inHg and bottoming out at ~ 18 inHg

I also tested the vaccum out at the N75 and got 23.0 inHg... and the vacuum holds steady after i shut down the engine...

I have also run a vacuum line from the "t" junction on the hose between the fuel/vacuum pump and the brake servo directly to the N75 and then took the vehicle for a test drive (up to 110 kph) - that test resulted in the complete elimination of black smoke emitting from the car's exhaust but did not change the turbo function (at least from what I could tell).

Any advice from these result to find a solution to my problem? What is my next step?

 

[P2B]

I'm surprised the MAF is reporting airflow when the engine is not running, on the other hand it wouldn't occur to me to run the test with the engine off - no point.

You should run the test as follows:

Engine running at idle.
Select the ENGINE Controller
Choose BASIC SETTINGS -04,
Choose GROUP 011
Click Go
The N75 will cycle from OFF to ON every 5 seconds or so. (about 10% duty cycle to about 90% duty cycle). The turbo actuator should move through it's full travel (about 3/4") with each cycle

 

[capthook]

I'm surprised the MAF is reporting airflow when the engine is not running, on the other hand it wouldn't occur to me to run the test with the engine off - no point.

You should run the test as follows:

Engine running at idle.
Select the ENGINE Controller
Choose BASIC SETTINGS -04,
Choose GROUP 011
Click Go
The N75 will cycle from OFF to ON every 5 seconds or so. (about 10% duty cycle to about 90% duty cycle). The turbo actuator should move through it's full travel (about 3/4") with each cycle

P2P...

When I run the test everything works as expected other than the actuator does not move.

Having said that:

- when I test the actuator with 25 inHg vacuum (with a Mityvac) it works fine and responds as expected...
- when I test the vacuum upstream of the N75 I get 25.0 inHg (either at the N75 valve itself or at the junction of the vacuum line to the brake servo)

When I run the VCDS test (described above) with the Mityvac on the vacuum line going to the actuator i get no vacuum.

My conclusions are that (a) I have proper vacuum and no leaks in the vacuum system; (b) I have the correct Vacuum going into the N75; (c) I have an actuator that works as designed when I apply ~ 25 inHg vacuum to the vacuum line going into the actuator; and (d) when running the VCDS test I get no vacuum from the N75 to the actuator.

In light of all of this I think the N75 is faulty and should be replaced... Have I missed anything?

 

[P2B]

In light of all of this I think the N75 is faulty and should be replaced... Have I missed anything?

You have pretty much narrowed it down to the N75 or it's wiring harness.

Try back probing the wires at the N75 with a multimeter (preferably analog) set to DC volts while running the test to see if the ECU signal is present. It's a PWM signal so the actual voltage isn't relevant, but you should see a step change in the reading each time the test cycles.