Current Affairs

Technicians often use technical Data to help diagnose components.
More often than not the data given for components is the resistance value.

I have a saying "resistance is futile".

This is because you need to disconnect any component to measure its resistance, this is a problem. We need to test under the conditions that the fault occurs.

So why is it, resistance is the only information given to technicians to test components?
The resistance is important information, using ohms law we can calculate the current in the circuit and test under loaded conditions using an amps clamp.
These inductive clamps can be used without interrupting the circuit so testing takes place under the same conditions as the fault occurs.

E.G. A typical petrol injector has a resistance of 14-16 Ohms.
The supply voltage is 12-14 Volts
Therefore the current draw should be in the range 1 - 0.75 Amps.

Compression Test

During miss fire diagnostics it is important to establish good compression readings early in you testing procedure. This does not present to much of a problem for most petrol engines, however it may still require considerable amounts of strip down.

Diesels present a far bigger challenge with countless different adaptors and special gauges required.

A much quicker method is using an oscilloscope with a high amps clamp. Just clip over the battery lead and crank. The relevant compressions of each cyl will show as peaks in the cranking current. A quick measurement will show cranking speed, critical on common rail diesels.

Plus you can observe the condition of the starter motor. 3 for 1 speeding up diagnosis and increasing profits.

Tour de Corsa

We received a call to confirm or deny diagnosis of the fuel pump on a common rail diesel Vauxhall Corsa. The owner was expecting the worst after diagnosis by the local main agent.
However using some quick serial data tests showed normal rail pressure, pressure regulation duty cycle and using the scope the pressure build and decay all looked to be with normal limits.
This was a good example of an all approaches diagnosis.
Good serial data, backed up by live scope readings, however the vehicle still had a miss fire once started from cold or after touching the accelerator if started from warm.
A quick look at exhaust manifold temps using an infra-red thermometer showed cylinder 4 to be the culprit. Suspecting poor servicing in the past an engine flush and service restored normal operation.
Much cheaper than a high pressure fuel pump!

ECU diagnosis

An increasing number of jobs refered to us are for ECU diagnosis.

This is often due to the expense of the new unit, or to confirm the diagnosis of another garage.

We can't test ECU's on site, but there are a number of tests that can be carried out, these tests simply prove what its not, until you are left with what it is. In many cases the ECU.

Due to the coding and immobiliser functions within modern ECU's it is sometimes better to repair the old unit.

A few simple procedures such as a global scan, for codes in all modules, not just the suspect ECU or modules as they are now known.

Check power supplies, grounds and primary signals if these are satisfactory then it's a good chance the ECU is at fault.

Many of the vehicles we test for module failure have wiring and connector faults, not ECU failure.

In this case a Landrover was refered to us for a second opinion, the owner did not trust the diagnosis or the the repair estimate.

A quick scan of the modules (results shown above) produced a number of codes.

Checking grounds, connections and lives proved satisfactory, all the primary signals to the ECU were present however there was no injector output.

As it was the holiday season, the ECU repair facility sent a un-locked ECU for the owner to use over the holidays. The repair will prove less costly than the new ECU the owner was quoted by the previous garage, plus having seen for himself that a replacement unit has fixed the intermittent non start he was more than willing to trust our diagnosis.

Not so Noble

Had a call to look at a Noble M12 GTO 3 R, not sure what this is.....nor was I.

A bit of reseach and I was in the picture, a British performance car built in small numbers.

Powered by a ford v6 as found in the mondeo but with 2 small differences twin turbo's.

The car in question had a running problem.

It was impossible to test drive the vehicle effectively due to the weather conditions, but it appears to be over boosting.

A new turbo has been fitted, this is the most likely cause, incorrect waste gate pre-load.

We suggested the shop check both turbo's and adjust accordingly.

They reported back informing us they had found a seized wastegate, however the car is booked on the rollers to confirm diagnosis.

More to follow......

Non Start after repairs

We are often called upon to advise other garages when they can't fix vehicles.
This reinforces the training need.
It is often impossible to diagnose vehicles over the phone but we can suggest procedures to garages.
This was a classic example;
The vehicle in question had been repaired in another shop.
After remedial work the car refused to start.
The only fault code 52 related to the ECU permanent memory.
The shop had sent ECU's away to remove immobiliser function, replaced transponders and receivers for the keys. But now was at a dead end.
We suggested checking fuses, however after a short time we were informed all the fuses were found to be ok.
So we suggested checking EARTH paths for the engine ECU.
The next call thanked us for our help, and enquired about courses in the near future.

Common Rail

Diesel common rail has been around for a while now and we are starting to see more and more problems with these systems.
This being the case we use a number of different techniques to diagnose problems.
The pump is the heart of the system, one quick test we perform is cranking pressure generation.
These pumps have to deliver around 250 bar during cranking or the engine will not start.
So this test can be carried out using a scan tool, or scope.
I prefer the scope as I like to measure the time taken to develop this pressure.
The different systems have similar rise times and it can be a good indication of system functionality.

This example from a cp1 Bosch system fitted to a Peugeot Hdi shows a pressure rise of less than 1 sec. You can see the pump strokes contribute evenly this is a good pump, with good primary supply and no excessive leakage from the high pressure components.