Real-Life Diagnostics – Diagnosing a no-start problem on a SEAT Arosa

27 Mar 2015 | Author: | Comments Off on Real-Life Diagnostics – Diagnosing a no-start problem on a SEAT Arosa

Real-Life Diagnostics

Take a Diagnosing a No-Start Problem on a Arosa

By Nick Hibberd


The vehicle was a 2004 Seat 1.4 TDI (3-cylinder) that had found its way to a diesel shop with a complaint. The car began by occasionally out, but progressively got worse and turned into a complete It would crank fine but wouldn#8217;t fire. Checking the with the scan tool, I no DTC s logged.

Ordinarily I#8217;d that the codes had already erased at some point the line, but after several I couldn#8217;t get any DTCs myself.

for the fault

Let us start at the beginning by cause and effect. Here, CKP and CMP are along with the output to the injectors.

Fig.1 shows a recording taken during There are a few anomalies, one being the of a CMP signal drop during an period that is released the injector is released. This cam had three unique groups of each relating to a particular so why the need for a fourth CMP pulse? the other cam identifiers, they all fairly short in duration, the shortest measuring 4.5 ms at cranking

Our suspect signal drop was significantly less at 0.7 ms, so it#8217;s a very narrow cam tooth or is in a problem. I#8217;m leaning the latter. There was something which didn#8217;t make on this system all injectors a power cable from the and my current clamp was monitoring cable to get an overview of all injectors.

to the capture only one injector is present.

A short time the investigation I had a change of fortune the engine started. Scrolling through the recorded captures, I Fig. 2.

The significant difference now, the engine running, was in the CMP sensor: the signal biasing that has from 5 V to 0 V. Even though the CMP had delivering the cam identifiers, it was the sequence of and low logic states that had incorrect. There are possible influences to be aware of when you see a CKP or CMP signal, but this was not a mechanical A faulty CMP sensor?

Possibly, but it explain our injector in Fig.1 with the CMP signal. And what the sudden change in signal at point X . It seems an odd coincidence immediately after this X the start-up operation changed.

to the CMP sensor

By now there are more than answers, and when happens I pick one diagnostic and stick to it. With each of the puzzle so far involving the CMP sensor at level, the next methodical is to see if it was being powered up correctly. the basics first.

On this model the CMP receives battery voltage from an control relay. This supply is shared amongst sensors and actuators, including the ECM in turn powers the injector

Fig.3 was taken with the at idle and just beginning to We have a problem! The power feeding the CMP sensor is suffering electrical load and it#8217;s that the injectors are having the impact, so there is more to problem than just the CMP

Even though the CMP signal being affected here it#8217;s easy to predict the problem will get much during cranking, and this offer an explanation of Fig.1: the CMP simply gave in. There is a relationship displayed here voltage and current. We can see that, as the supply deteriorates, so does the available to drive the component, but the high current demand is making the poor voltage evident.

Cause and effect, then to cause again. I picked out the supply voltage at a shocking 4.3 V, but a surprising 11 A being helped by the capacitor in this HDI system. without the idle stumble, it is clear that the same exists in the preceding injector where the injector current is a 17 A while the voltage supply sharply to 11 V.

The root fault is a high-resistance power source. I that all components connected to the supply were suffering to the extent, so the next logical was to look at the control relay

The relay was found in different of solder joint fatigue. Its weren#8217;t brilliant either, I have to say is surprising on a 4-year-old car 21,000 miles on the clock. The bad had introduced a high resistance in the path to several engine creating a basic voltage circuit.

The engine control on VAG drive systems is a recurring and can exhibit all kinds of symptoms. unwanted high resistance was unpredictable effects on the circuit, that not all relay problems are #8220;not working#8221; conditions.

Get a new

After the repair

The new relay was now full battery voltage, cranking duration and improving conditions in general.

I returned to a few observations to see if this poor problem could answer questions. I#8217;ve already the CMP sensor drop-out in Fig.1 and can conclude that the low supply available during cranking the problem, and the sensor just up as the injector circuit consumed all current.

Incidentally, even the boost circuit could only an injector current of 7.5 A. The mystery X in was most likely electrical from the unstable power In particular, the fractured PCB joint surface scorches as evidence of

Finally this no-start had no codes logged. I saw my first INJECTOR CIRCUIT THRESHOLD the engine started and only the rough idle period. during repeat cranking a poor system voltage, signal ranges, and incorrect outputs, I saw NO DTCS LOGGED . You may to re-evaluate how much your strategy relies on trouble

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