Serial Data analysis for DUMMIES.

The ability to graph serial data makes analysis easy.

In this example the CTS output is being monitored to check for the cause of a 17704 fault code.

The code relates to a cooling system malfunction, and the ECU monitors the CTS for this code so its a good place to start.

We were expecting an error in temperature reporting or thermostat failure.

We were not disappointed.

Fuel Trim in action

There are plenty of articles and much is spoken of Fuel Trim analysis.

However the message has got confused, after attending seminars by 'experts' I was alarmed at the level of miss information.

Looking at oxygen sensor readings will show trends, however the Trim will continue to adjust fuelling so fuel problems are effectively masked, ensuring correct catalyst operation. So much so that the O2 sensor could show normal response, despite abnormal conditions.

One 'expert' suggested measuring response time of the pre cat sensor after over run as a method of measuring short term fuel trim, utter nonsense. The response time will depend on many variables, the biggest being the distance from the exhaust valve that the sensor is positioned. The quanity and temperature of the exhaust gas content will also effect the response time. How can a 1.1 N/A expect to have the same response time as a 2.0 turbo.

Another misleading piece of advice was the true condition is always reported by the post cat sensor. While this has some foundation, the post cat sensor will report on the effectiveness of the catalyst, so should show less oxygen than the pre cat in a correctly fuelled engine. If the catalyst is faulty the post sensor may indicate a lean condition, that simply does not exist.

The best way to evaluate the trim is to use serial data, comparing long and short term trim at different engine load and speed sites.
Forcing the engine lean (on overrun) and rich (on snap throttle) will show oxygen sensor response but little about trims.

Burnt Out

I was called in to look at a Ginneta G50 race car that was burning out relays.

Used in the British GT Championship it would start and run fine but would burn out its relay before the end of the race resulting in a DNF.

A new pump and a few different relays had been tried, but the fault remained.

I used a amps clamp to test current draw, on the control and output sides of the relay.

I discovered that the control side of the relay had abnormal current draw, but where was the resistance, classic cause and effect diagnosis.

The effect = burnt out relay.

The cause = excessive resistance in the circuit.

A quick check with the scope(image shown above) showed higher than acceptable volt drop on the earth circuit.

This after prolonged used caused the relay failure, replacing a section of loom had this GT4 back on the grid.