Blinded by the light

This got me thinking. The Mk 5 golf with an error message for lamp failure.
Simple enough at first glance. (You can read more about the fault in an earlier post).
It turned out to be the side light/brake light that was inoperative.

I checked the lamp visually it appeared fine, so I measured it's resistance it showed 0.6 ohms.

This is the bit that got me thinking..............is that right? I compared it with the working side light and it too had a resistance of 0.6 ohms. So it is correct. Or is it?

These cars and many newer vehicles have done away with many of the fuses and use the ECU's to monitor faults, if any faults are detected it simply switches off the outputs. Then a warning is displayed in the dash panel. The joys of vehicle networks.

I set about some calculations to find out if the measured resistance was correct.

I knew the bulb rating 21W 12V therefore I could work out the current flow.
P= I x V
I =P/V = 21/12 = 1.75 Amps

Ohms law states I = V/R
R = V/I = 12/1.75 = 6.9 Ohms

So why was the resistance of the lamp filament so wrong?
Lamps convert heat into light, heating a conductor will increase its resistance. A lamp will show around 1/10th of its operating resistance in a "cold" measurement.

This can be clearly seen in the scope trace above, this was taken after I repaired a high circuit resistance in the wiring to the lamp. (the cause of the lamp failure warning you can read about in an earlier post).
Notice the "cold" resistance is much lower than the "hot" resistance seen as a high current 11 Amps falling to the operating current of 2 Amps. Notice how the Body ECU warms the filament with a short burst of current before switching the lamp on this stops current spikes and prolongs filament life.

Noble part 2

This car has proved difficult to diagnose, an intermittent miss fire, normally under load had proved elusive. Weather conditions, lack of rolling road time, along with fixing other basic problems had turned this into a bit of a saga.

Having checked the inputs/outputs of the ECU, everything appeared fine. However the car would occasionally cut out. Once this happened it would re-start and perform without fault for ever decreasing amounts of time. The ECU was inspected and tested and was found to be ok.

I turned my attention to the coils, these showed good current ramping (below), but it was during these tests the car cut out. The output just vanished.

This means the ECU has stopped grounding the coils, but why? Could it still be an ECU fault?

I decided to investigate further, a check with the local Ford parts department proved fruitful.

The coils have been super seeded.


We tried a new coil to see if there is any noticeable difference. The current trace (above) shows the subtle but measurable difference. A further 5 coils were ordered and the car returned to the rollers. We now had closer to the rated output and no miss fires!

Have you seen the light?

This could be one of the most difficult faults to diagnose on modern vehicles.
A Golf MK V has a side light/brake light failure, however the local garage and the main agent have come to an expensive conclusion. It must be the body system interface. This computer controls many of the body and lighting functions on the vehicle. Replacement is costly not just the parts but the time plugged into the dealer diagnostic interface "coding" the unit.

This vehicle has few relays and even fewer fuses. The circuit at fault has no apparent circuit protection. If a fault is detected by the body interface it simply turns off the supply.
This in turn puts a fault light and message on the multi function display on the dash.

This vehicle had no such message or warning light. No fault codes were found in the body system interface. The lamp had been checked and passed both visual and resistance checks.
Yet when the side lights were switched on, or the brake pedal depressed the lamp failed to light.
The previous repair shops had checked for voltage at the lamp, and the output of the BSI.
No voltage means no output from the BSI, with no codes the garages had assumed this meant an ecu fault.

Is this diagnosis correct?
The main agent was unsure and requested we checked the system. The BSI checks for faults two ways termed hot and cold monitoring.
Cold monitors run every time the ign is switched on, hot monitors run while the circuit is in use.
A quick check with the scope showed the cold monitor was indeed running. So why was there no output, and if the was a circuit fault why no code? Or warning light/message?

You will often hear me banging my drum about Voltage Drop testing, however this can only be carried out with the circuit in use. ( Dynamic Testing) in this case this is not possible, so we tested using resistance checks, first continuity to ground (short to earth) then short to live with no fault error message we expected these simple checks to be fruitless. So it proved.
However the resistance between the BSi and the lamp showed 57 Ohms. Bingo this test was then repeated along the loom until the section at fault was identified. A loom connection proved to be the culprit showing signs of corrosion, and once cleaned and protected from further water Ingres normal operation was restored. It appears the fault is outside the fault code parameters but within the fault limit. This results in no output and no warning light/message on the multi function display. This has to be the worse case for the technician.

ABS Diagnosis

Customer complaining of noise and brake judder, booked into a local garage for pads and discs.
These were fitted and the fault remained!
The garage checked for codes but as the ABS light was not illuminated they were not surprised to find a clean bill of health. No codes found.
I was asked to check the braking system and report my findings.
A quick test drive proved the ABS to be operating, but not when you expected.
In cases like this I like to remove the ABS fuse to disable the system, then perform a road test. This careful road test, remembering the ABS will not operate can prove if it is an ABS control or operating fault, or a mechanical fault in the braking system.
In this case the car now performed faultlessly. Proving an ABS fault.
But without fault codes how can it be diagnosed quickly?
Start with a visual check, and sure enough the reason was clear to see a cracked reluctor ring.
Not easy to spot, but easy to see with the scope.

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!