Mercedes Sprinter Turbo – Limp Home – Diagnosis and Fault Finding

 
Mercedes Sprinter Turbo – Limp Home (LHM) Diagnostics.

You will probably read a great deal on the internet regarding the issues surrounding your Sprinter lapsing into limp-home mode without showing any EML (Engine Management Warning Lamp).  I have also read these web entries with interest and have concluded that there is a lot of confusion around what exactly is going on so I have decided to add my own input so that hopefully you can sort out this irritating and troublesome issue.

The early Sprinters with vac controlled turbo actuators are quite basic in operation and there are only a few elements to the control loop.  These can all be checked out systematically and the faulty element identified and replaced/repaired.

Let me try and explain in basic terms how the turbo control system works so that you may get a better understanding of what is going on:

VNT Turbo and Vacuum Actuator (sometimes known as a VGT Turbo)

Sprinter Vacuum Boost Actuator

Sprinter Vacuum Boost Actuator

The turbo itself is a Garrett VNT variable vane turbo, the details of which you will find elsewhere on this site. ( http://www.mercedes.gen.in/WP35/diesel-mercedes-sprinter-turbo-rebuild/ ) It is controlled by a vacuum actuator that moves a set of mechanical vanes within the turbo to vary the turbo charge rate – there is no conventional dump valve on these turbos and many times you will wrongly read people refer to it as the ‘dump valve actuator’.   The charge actuator resides bolted to the turbo body on an extended bracket, its push rod connects to a small lever that enters into the turbo body casting, moving a small lever shaped rather like a dog-bone.  This lever engages in an annular ring that in turn interlaces with a set of similar levers attached to each variable vane. A single action of pushing or pulling the actuator rod moves all the vanes inside the turbo in unison, developing more or less turbo charge dependant on its position.  Inspect the external lever that the actuator push rod attaches to, ensure this moves smoothly as you accelerate / rev the engine.  Look for any misalignment of the lever throughout its stroke as it passes through the turbo body, as the bearing surfaces on this shaft can wear or oval allowing the internal lever to ride over the annular ring inside resultantly not moving any of the vanes.  A quick and dirty test to indicate any problems with the variable vanes is to remove the air box or air filter housing and listen as the engine is revved.  Once the engine is started, the vac actuator rod should move down slowly as vac builds, this in turn should apply force to the lever moving the vanes into a fully charged position.  As you rev the engine you should hear a very loud and strong tornado type howl from the air box, if you do not, and the actuator has moved pulling the lever downward there is a chance that the turbo itself is faulty.  Normal operation is as follows.  Stationary engine, rod fully extended, vanes in no charge position. Start engine, rod pulls lever downward slowly, vanes full charge position.  Rev engine, rod moves in and out during the rev cycle, vanes apply variable charge through rev cycle – Audible howl from air box.

Turbo Actuator Electric Vacuum Control Valve

Sprinter Boost Actuator Valve

Sprinter Boost Actuator Valve

If the vacuum actuator is not pulling down once the engine is started check for vac at the pipe, if vac is present, the actuator could be seized or faulty (internal diaphragm split) rectify this.  If you follow the vac pipe back from the turbo you will see that it goes to an actuator that is bracket mounted on the inner wing below the air box at the back of the off side headlamp (UK Vehicles)  This can be pulled upward off its bracket mounting rubber and moved to a better place for visual inspection.  This electrical valve actuator is popularly at fault with the Mercedes Sprinter model and your dealer will have sold hundreds of this item over time.  It has a single two wire electrical plug connected to it and three rubber pipes; in some cases its pipe locations are even marked: IN – OUT – VENT.  Check vac supply to this device observing and rectifying any split or leaking pipework.  Once good vacuum supply has been determined, with the ignition off (no power to the valve and no vacuum present) the route of vacuum should be blocked by the valve to the turbo actuator.  The turbo actuator supply pipe should be vented to atmosphere via the electric control valve to a small filter (looks like in-line fuel filter) mounted above the engine and sitting by the side of the brake servo.

sprinter vacuum diagram

Turning on the ignition places a constant 12v to the electrical controlled vac actuator valve, (This voltage pulses during normal engine operation)  vac should now be routed from actuator valve inlet to the turbo actuator (outlet) and the vent should now be blocked.   Vacuum switching can be verified by using  a hand vac pump (Mytivac or similar) Simply sucking on the pipes will not prove anything, as there is an internally sprung pressure control diaphragm inside the electronic valve so that operation will not occur until sufficient vac is present –  the only thing you will achieve by sucking with your mouth – is a red face!  An old Mercedes engineer once shared that there are two versions of the electric vacuum valve available, one with a blue cap and the other with a black cap, only the matching type should be used as a replacement.  I cannot verify this information as I have only ever seen the black cap type, but best practice would suggest replacing the unit only with the correct version to be 100% on the safe side.

Relevant Service Manual Extract View Here

ECU Control

The ECU control loop circuit is quite simple for turbo actuation.  The electric vac valve is controlled by simply switching the 12v supply on and off by the ECU, this is called PWM or Pulse Width Modulation – it’s easy to test this control voltage is present with a volt meter at the valve electrical plug/connector with ignition on.  If no control voltage is present look for a break in the wiring from the valve to the ECU.  Trace back the wires to the ECU connector and prove continuity exists from the ECU connector to the valve.  Pin outs and wire colours are given in the included diagrams.  There has been commonly reported breaks in the valve supply wiring as it is routed in front of the intercooler behind the grille and also in the area of the near side headlamp where the loom kinks back into the engine bay around the radiator. The wire colours to look for here are white and blue (on my model) but please check the included diagram/chart for correct model notation.

Relevant Service Manual Extract View Here – Wiring Identification at ECU – Turbo Boost

ECU General

Remove the ECU from its slide tray under the passenger dash area beneath the glove box area.  It is quite common for the bulkhead grommet to displace where the main ECU loom is routed to the engine bay allowing water to enter and run down onto the ECU connectors.  Remove all the connector attachments to the ECU and check for pin corrosion, if this exists clean up both the pin and connectors as best you can.  Importantly – reseat the bulkhead grommet.

Charge Pressure Sensor / Intake Air Temperature Sensor

Sprinter Boost Pressure Sensor Location

Sprinter Boost Pressure Sensor Location

There are two sensors in the control loop, a pressure sensor and an air temperature sensor.  They are identified as one having three or sometimes four wires (commonly three) this is the charge pressure sensor. The second is intake air temperature sensor having two wires.  They are both located in the upper section of the intercooler discharge hose, on the near side of the vehicle (UK versions) on the hard moulded plastic section of ductwork before the final supply hose to the inlet manifold, just to the side of the radiator.  The pressure sensor is attached to the housing with two small bolts.  The connections to this device are again simple to understand and test.  One of the three wires is ground, the second is 5v+ve supply and the other connection gives an approximate variable voltage output +0.2v to +5v referenced to ground dependant on charge pressure.  The other sensor (air temperature) is found slightly lower down the plastic section of pipework, this measures charge intake air temperature. This has a two wire connection that is connected across an internal thermistor bead that feeds back a change in resistance value relative to air temperature inside the inlet pipework, the two connected wires are fed through the bulkhead, directly back to the ECU along with those from the pressure sensor.  The temperature sensor should measure between 400 and 500 Kilo-Ohms out of circuit.  In the information given it is possible to plot the output from the charge pressure sensor to give an indication of charge pressure in the system, from this you can derive if it functioning correctly or if to suspect it as faulty.  If you need to measure what is going on in-circuit without disconnecting these components, take a number of drapery pins and push them through the insulation into the conductor cores of the required wires thus allowing you to connect a test meter and take measurements without the need to disconnect or cut any wires.

Relevant Service Manual Extract View Here – Sprinter Boost Pressure Sensor

You can disconnect any of these components without lighting the EML warning lamp as no indication of a fault is given by the removal of any of these components from circuit.  That is why the dreaded turbo limp home fault often occurs without any indication on the dashboard.

Rev Limiter Function

Normally Sprinter engine revs are ECU limited whilst stationary to around 3500 rpm. Depressing the clutch and then further releasing it should now allow revs to increase above this range to the rev limit. If there is a limp home fault, engine revs will not progress above 3000 rpm even when dipping/releasing the clutch when stationary. Once the fault is rectified the rev range and limiter function as described above will return to normal.  It is a good idea to check out both of the necessary brake and clutch pedal position switch sensors (on the pedal box above the clutch and brake) and where possible always get hold of a compatible diagnostic code reader as often faults are stored and not indicated with a EML lamp.

I hope this helps you to better understand the function of the turbo boost circuit on early model Mercedes Sprinters and that it assists you to fault find and rectify any problems you may have.

709 thoughts on “Mercedes Sprinter Turbo – Limp Home – Diagnosis and Fault Finding

  1. I have an RV on a 2007 Ram Sprinter chassis. We hit a large pot hole and on the next hill we lost power. I believe it went into limp mode. I pulled off the highway and revved the engine in neutral and it would only go to 3000. I shut it off and restarted, revved the engine and it went up to high revs. We got back on the highway and all was well for awhile and then again we lost power. If we are in town it seems to run as designed, but soon after we get on the highway it does it again. At one point when idling in traffic something made a ticking noise like signal lights do but no lights visible. (Maybe no connection to the problem). What should I look for before I take it in for a code reading?

    1. Harry,
      There could have been a jolt to the wiring loom as it runs behind the left hand headlamp, this is a common area for it to develop a fault because it kinks back sharply and in some cases causes internal wiring to the boost controller to fracture and become intermittent. Do check the intercooler hoses and make sure you cannot see any signs of oil staining at the joints or side tanks of the intercooler. This would indicate a boost leak.
      Hope that helps
      Steve

      1. Steve;
        Thanks for the reply. I see no oil leaks near the intercooler. All looks well in the harness behind the left headlight but it could be an internal problem. The vacumn holds at 26.5 (70). The plunger above the cooler opens fully with my vac pump at 18 to 20 and holds vacumn. The reason I tested it was because its operation seems to be erratic. It is extended when cold. Sometimes it pulls in when I start the engine, sometimes it doesn’t.
        When in limp mode it is fully extended.
        I had a local country mechanic test the system with his old code reader. He got a PO299 code. I will crawl under it again to check intercooler and hoses for leaks.
        I am 78 years young and we are on SS so I cannot afford any costly repairs. I have been a mechanic all my life so I hope I can solve the problem with everyones help.
        Thanks;
        Harry

  2. My 2015 906 sprinter 2.2 done 26,000 miles DPF filter kept going into regen mode to the point I had to have it forced regen done at Mercades, a week later it was back so I opted for DPF program filter to be removed and filter to be removed. I now don’t get any codes however it goes into limp mode after a short 1/4 of a mile it won’t rev above 2,800 rpm until I switch off remove key and wait for approximately 3 minutes it then does the same again the guy who did I the remap is at his wits end as I am any help please

    1. Phil check the exhaust manifold for leaks especially at the back, if its leaking here it will trip out on underboost. This may have been disturbed when removing the exhaust/cat.
      Regards
      Steve

  3. Your website has become a bible for me. I have a 2000 313 as a campervan which I have owned for about 4yrs and travel to VERY remote places. As with all my vehicles I have a Haines manual. I had problems with limp mode for about the first yr. I would stop switch off for maybe 10 sec start and it would run ok until the next time. Had it in with the ‘experts’ with no cure. Then one day it just cured itself. That in itself is a worry as I don’t know what it was. Another problem I had which I saw coming with harder starting, was the fuel pressure valve O rings. I was only a couple of hours from home so I opened the air filter and gave it a good dose of brake cleaner. It started and I drove home. The third issue was ‘ERROR’ relating to the key lighting up in dash display. the Haines manual says to lock lock and unlock the door with the key, which I did, and its been ok. Hope my dramas are uesful to someone else and thanks for a great informative site.

  4. Thanks for your help with the turbo limp mode this is my problem on and off.
    Do you know if you can fit a engine data scan to 2000 sprinter .Thanks for your help.
    Tony

  5. Steve Ball;
    The problem with my 2007 Sprinter going into limp mode turned out to be a crack in the inter cooler hose.
    Harry Thorn

  6. Is there a way to tweak the cruise control? When I am travelling on an interstate here in SC and GA at 70 MPH and hit a fairly steep hill while on cruise it will drop a few miles per hour before I hit the top. but if I override the cruise and push the pedal down it will hold 70.
    Harry

  7. May be a bit late to the party but here’s my £.02…. 2006, 165000 miles. Losing turbo intermitently. Pulled every sensor connection, cleaned and applied dielectric grease. Found the ground to boost sensor had been compromised as well as on the sensor on coolant reservoir (which shares the same loom). They both had a spot where the insulation was missing. I simply covered them each with electrical tape and gave it a good days test drive. No more drops in boost. In fact, it seems to have more power. I do show signs of oil at many joints. I’m going to clean everything well and keep a weather eye. Thanks for the great write up!

Leave a Reply

Your email address will not be published. Required fields are marked *