Speed and current

James Dillon investigates a Volvo with cranky cylinders

Published:  01 February, 2013

When the phone rings at Technical Topics HQ with someone in the trade looking to book in a job, I often hear the words, "I've got a really interesting one for you to take a look at". I'm never sure whether to be pleased (because the interesting jobs give me something to write about for the magazine), or if the caller can hear my inward groan as I consider the prospect of entering close-hand-combat with yet another nightmare job. Everyone loves easy jobs don't they?

I take my newly prized pen and begin to write up the vehicle's diagnostic assessment form. Invariably, the vehicle has been touched by the hand of another and may already be sporting new parts. The customer will report what's been done already and I'm ready to go. True to form, the Volvo V70 D5, which appeared recently had no trouble codes stored and presented an intermittent symptom. Nice!

The vehicle's reported symptom was difficult starting, occasionally failing to start altogether. It had been in trouble for a while and, according to the other workshop, because of periods of extended cranking the starter solenoid had burnt out and had been replaced. My task was to get the vehicle to exhibit the symptom, while collecting the relevant data.

My first decision was what and where to measure. There are several possible causes for this Bosch EDC system failing to start, amongst them a lack of compression, poor fuel quality, low fuel pressure, cross flow EGR/intake - no or poor fuel delivery were high on the list of suspects. It goes without saying but getting to the root cause in the most straightforward manner and taking the least amount of time are key objectives in reaching a quick and accurate diagnosis.

Figure 2

One issue with this method is that it is a relative test. If all cylinders are equally worn (or, for example, the cam timing is off) there will not be a difference between cylinders. A rough rule of thumb is that the starter current will be in the order of magnitude of 3 times the Ah rating of the battery, too much lower than this will highlight issues. The cranking speed should be between 200 - 250 RPM (check the vehicle specs).

During our training courses, we investigate the rules of the motor and the relationship between speed and current (torque and rotational speed). Understanding this relationship prevents misdiagnosis. Motors are designed and matched to their application to provide adequate torque at the desired rotational speed. If the motor spins faster than it should, the torque (and current) will be lower than desired (the opposite is also true). The exception to this is electrical resistance.

This is where understanding the relationship pays dividends. A low speed and a low current means electrical resistance (power or ground, or internally in the motor), a high speed and a low current means a low mechanical resistance, a low speed and high current means high mechanical resistance, or an electrical short. The mechanical resistance can be caused within the motor (seized) or within the engine.

Figure 4

The engine management system needs to see relevant rail pressure, a speed and position signal and, in some cases, a required minimum RPM during cranking. If any of these are outside of the expected values, the system will not attempt to introduce fuel into the cylinders as it will potentially fail to start and cause elevated exhaust emission levels. So we could conclude that emission control, or the potential threat of excessive emission levels, can cause a non-start situation.

Figure 5

Using the cursors, measuring the cranking speed (using the cam sensor as a reference) showed that the engine was achieving 212 rpm during non-start and 211 during start, so the motor speed was/ wasn't the root cause. The level of electrical noise being generated by the worn starter was also a major contributing factor. We have enough evidence to condemn the starter motor.

The technician was unable to detect a lower than 'normal' cranking speed or the noisy electrical signal. So, when the call was made to report to the original garage what was required, they wondered if a starter which appeared to be operating effectively could be at the root of their problem. This is a classic case of a component failing but not in the right way, so the diagnosis wasn't obvious or easy, without the aid of an oscilloscope and an understanding of component operation. A new motor cured the symptom.

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