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CAN BUS - Intermittent ESP light

Scanning fault codes on a Audi TT Quattro revealed

intermittent CAN Bus errors. When the fault was

present, the ESP light would go out on start-up but then

illuminate as the car was driven. The CAN network was

inspected using PicoScope whilst the vehicle was driven,

and the captured data reviewed when the ESP warning

light illuminated, showing that the CAN bus signals were

incorrect. After a visual inspection of the wiring, we

scoped the bus lines with the ABS ECU disconnected. As

the scrambled signal was still present, we reconnected

the ABS and disconnected the Haldex module on the rear

diff; PicoScope displayed a good set of bus signals. As

the supply and earth were confirmed as OK, the Haldex

unit was replaced, and the CAN bus signals returned to

normal.

www.picoauto.com/tutorials/audi-can-bus.html

Diesel Variable Nozzle Turbocharger

-640mbar

-80mbar

-480mbar

-170mbar

Over

boost

Normal

Boost sensor is forced to hit its

operating limit; hence we see

the signal flat-line at the top.

Typical capture recorded during Wide Open Throttle (WOT) boost events. In this

example of an over boost condition, the control circuit reduces applied pressure to

a mere -80 mbar until boost eventually drops to normal. We can see during normal

regulation the control circuit maintains a healthy -170 mbar. The proof test here is

that if the turbo unit can adequately respond to -170 mbar, then it should quickly

reduce boost at -80 mbar; it doesn’t.

More importantly, these test results determine the ECM control circuit is not

contributing to this fault.

After a few years of steady use of a Variable Nozzle

Turbine (VNT), carbon deposits build up around the vane

mechanism causing it to seize, therefore allowing boost

to deviate from normal. A WPS500X was connected on

the solenoid output line to measure the negative pressure

applied to the turbo actuator. When the required

boost level is reached, we expect the ECM to release

and regulate pressure applied to the actuator whilst

maintaining a suitable boost level. If however the vacuum

level correctly falls yet the boost level continues to rise,

then we have a problem. This test allows us to see what

is happening at the end of the control circuit, and can

reveal leaking solenoid valves, and blocked solenoid waste

outlet.

www.picoauto.com/tutorials/vnt-pressure-control.html

Valve Timing Errors – Misleading MIL

An Alfa Romeo Spider 2.0L Twin Spark with MIL lamp

illuminated, but no drivability issues. Reading DTCs

indicated “CODE 8 PHASE SENSOR CIRCUIT” which

cleared at KOEO, but returned when the car was started.

As the auxiliary drive belt had recently been shredded,

we decided to check the valve timing. This test only

takes a few mintues and, with a known good capture as

reference, is as effective as stripping the timing covers

and visually confirming timing mark alignment. With cam

and crank reference marks, PicoScope can be used on a

running engine where any excessive slack can be seen as

‘wandering’.