MAP Sensor issues

[Moshmeister]

Hi everyone, I'm a newbie with a problem I can't get to the bottom of; I am hoping someone can offer some pearls of wisdom. My vehicle is a 2011 W639 Vito 2143cc.

About 10 months ago I accelerated up a typical Sheffield hill to overtake a cyclist, causing the engine management light to come on and reduce power. I've had various people look at it since and no-one can get to the bottom of the issue. Last weekend I had it read on a STAR machine with the proper MB software by and ex Northfield Merc employee. As with previous scans, it shows P0108 (MAP sensor circuit high) and P2463 (DPF restriction - as obviously it won't regenerate).
I replaced the MAP sensor twice, once with a Bosch and today with on original part. However, the codes still immediately return, even after deleting. There is continuity between the sensor and ECU, however the voltage in is 5v but the signal is 3.64v which is apparently too high. One of the things that keeps popping up is that if the voltage is too high, there may be 'no ground on the power supply'; as a non-auto electrician I'm not sure exactly where to start with that...
From your experiences, does the ECU need to be reprogrammed to accommodate the new sensor? (Merc said not though).

In addition I have noticed a couple of other potential anomalies. I ran a live data stream with the engine running and at idle the Absolute Throttle Position reads 87.9% and the Relative Throttle Position is 100%
These seem very high to me - I would have thought at idle the figures should be somewhere between 5 and 20% on average to keep it ticking over.

Fingers crossed that you can shed some light before I pull out all my remaining hair...
Mark

 

[supernoodle]

Petrols run at 14.7:1 air to fuel ratio, so they need to control air flow and fuel flow together.
Diesels are fundamentally different and don't operate at a fixed air fuel ratio. The throttle is fully open during normal operation to allow cylinders to be completely filled with air. Engine speed is controlled by varying fuel flow only.
So the live data you see for throttle is normal.
For info the throttle is used during engine shutoff to prevent shutoff shake, safety device in the event of an overspeed and to increase EGR flow under low exhaust pressures.

MAP sensor is just an analog sensor, 5v supply, signal ground and signal.
I would expect a circuit high fault to be flagged if the ECU was seeing a voltage close to 5v, not 3.64v. In the ECU the signal pin will be pulled up to 5v or down to ground, this is allow it to detect failures when the sensor is open circuit. With the sensor disconnected if you get the same circuit high fault, it suggests the ECU isn't seeing the signal.
What do you measure at the sensor plug with sensor disconnected? 5v,5v,0v? Or 5v,0v,0v?

 

[Moshmeister]

Thanks for the quick response. With the sensor disconnected it reads 5v, 0v, 0v and it flags up P0107 circuit LOW after running.

 

[supernoodle]

Ok, so there is a pull down in the ECU for that pin. At least that proves it's not an open circuit that's causing the fault.
Are you measuring all those voltages at the plug, ie what ground are you using?
Given that the ECU is just measuring a voltage on the signal pin I would use an independent voltage source to simulate the MAP signal. I use a potentiometer for this, but you could use an automotive test probe to do it too.
Sweeping the voltage should give some plausible values in the live data. For MAP you know it should be atmospheric (1 bar) so useful to know what voltage would give that. On better diag setups like Xentry you can see the sensor voltage as well as the engineering value in physical units.

 

[Moshmeister]

I measured the voltages at the plug and also probed the wires at the ECU connector. All readings were consistent. I used a basic Draper multi-meter using the earthing points on the wheel arch. Other than that, things get technical beyond my skillset...

 

[Moshmeister]

I think I may have found the issue.
I stripped the wiring loom at the back of the engine where I had noticed some chafing previously. Taking it back further, I found a broken wire in the section that spurs off to the lambda sensor.
Soldered it back together, taped it up and the P0108 code has gone and it runs well.
The only issue now is that it still won't regenerate as it has the P2463 'DPF blocked' code live when running - could a forced regen be necessary?