BEW MAP mystery - please help.

[jptbay]

Having trouble with this car (A4 BEW Jetta Tiptronic), and I can’t figure it out. Been trying as time will allow, but the pressure is on now as I was hoping to have the car running for my daughter who is starting college soon.

Car starts and runs, but no power.

No codes.

Fault showing in MAP sensor. VCDS shows 2193 mbar at all times – running or off. Group 22 Shut-off status shows 002 system malfunction. So, this implausible high boost signal is triggering Limp Mode.

Tried new 3 bar map, no change. (car is tuned)

Tested wiring continuity from map plug to ecu. No wiring faults in this circuit.

Read as much as I could find about related problems. Some reported other faults in unrelated harness’ can give wrong reference voltage. Usually O2 sensor or N18. See thread here

N18 passes function test in VCDS. Turbo actuator responds as expected. Visual inspection of N18 harness looked OK.

I took apart the front engine harness, all the way back past the battery. Could not find any faults.

Inspected O2 harness from junction under car, up firewall to main harness. No obvious issues. Bought new O2 sensor to try. No change.

Any ideas about where to look? I’m just about ready to buy a whole new engine wiring harness.

I don’t have much hair left to pull out.

Thanks for your help, John.

 

[Vince Waldon]

You've been very thorough for sure! The MAP reading frozen in place is a bit weird for sure.

Couple random additional thoughts, around the theme of "why is the MAP reading static?":

- +5 and ground showing up at the MAP socket pins, sensor disconnected?
- V reading at the MAP sensor output pin, sensor disconnected?
- V reading at the MAP sensor output, socket back-probed... and does it change with boost?

 

[jptbay]

You've been very thorough for sure! The MAP reading frozen in place is a bit weird for sure.

Couple random additional thoughts, around the theme of "why is the MAP reading static?":

- +5 and ground showing up at the MAP socket pins, sensor disconnected?
- V reading at the MAP sensor output pin, sensor disconnected?
- V reading at the MAP sensor output, socket back-probed... and does it change with boost?

Will go out and check. Back in a while. Thanks.

 

[jptbay]

You've been very thorough for sure! The MAP reading frozen in place is a bit weird for sure.

Couple random additional thoughts, around the theme of "why is the MAP reading static?":

- +5 and ground showing up at the MAP socket pins, sensor disconnected?
- V reading at the MAP sensor output pin, sensor disconnected?
- V reading at the MAP sensor output, socket back-probed... and does it change with boost?

Curious, voltage at both g31 pins, sensor unhooked.
Pin 3 violet/red 4.98 v
Pin 4 yellow/black 5.63 v

 

[Vince Waldon]

There may be a pull-up... what happens to those voltage readings when the sensor is plugged back in, I wonder?

 

[jptbay]

^^ Will check again shortly.

VCDS with Map unplugged still showed very high, just slightly different at 2196.0

Which circuit should be showing no voltage? I wonder if I try unplugging other sensors if I can get narrowed in on a fault?

 

[Vince Waldon]

I lent my Bentley to someone and it never came back, so going off Google at the moment:

Pin 1 (brown): ground
Pin 2 (gray/green): IAT out, 0-5V signal
Pin 3 (violet/red): MAP out, 0-5V signal, should be a volt or two at atmospheric, has a pull-down so should drop to ground with sensor unplugged
Pin 4 (yellow/back): +5V supply

The fact that:
- the ECU is reading a constant non-zero voltage from pin 3, regardless of boost
- pin 3 measures close to +5, sensor out of circuit

suggests a short between pin 4 and pin 3 somewhere in the harness between the ECU and the sensor, or at the ECU itself.

If you get the more or less the same voltage reading on pin 3 (aka nearly 5V) with the sensor plugged in OR unplugged that pretty much confirms it in my mind.

 

[jptbay]

I lent my Bentley to someone and it never came back, so going off Google at the moment:

Pin 1 (brown): ground
Pin 2 (gray/green): IAT out, 0-5V signal
Pin 3 (violet/red): MAP out, 0-5V signal, should be a volt or two at atmospheric, has a pull-down so should drop to ground with sensor unplugged
Pin 4 (yellow/back): +5V supply

The fact that:
- the ECU is reading a constant non-zero voltage from pin 3, regardless of boost
- pin 3 measures close to +5, sensor out of circuit

suggests a short between pin 4 and pin 3 somewhere in the harness between the ECU and the sensor, or at the ECU itself.

If you get the more or less the same voltage reading on pin 3 (aka nearly 5V) with the sensor plugged in OR unplugged that pretty much confirms it in my mind.

That makes a lot of sense. Would explain how continuaty test of these two wires still passes as well. I will dig deeper into this particular harness.

Thanks!

 

[Mozambiquer]

That makes a lot of sense. Would explain how continuaty test of these two wires still passes as well. I will dig deeper into this particular harness.

Thanks!

Look for a map fooling resistor or continuity between the violet red and supply with the harness disconnected

 

[jptbay]

Checked with my tuner and he suggested the ECU might be fried.

Bought a used ecu, plugged it in, and sure enough all the boost errors are gone.

Sending out the ecu's Monday to get cloned and immo delete.

What a relief!!

Thanks @Vince Waldon & @Mozambiquer !!