Change you can believe in

Auto boxes are often regarded as a black art, are they really that complex?

Published:  11 November, 2013

By Andy Crook

The only constant is change; this is certainly true in the automotive industry. However, the Vauxhall in front of me this month was bucking the trend - it refused to change gear at all.

Automatic gearbox faults can be daunting but with the correct tools, training and equipment, they are no more complex than any other vehicle system. So when a local garage asked if we could have a look at a Vectra with only 5th and reverse gears I was keen to have a look.

We always start by trying to find out as much as possible about the nature of the fault. This is what I call the 'who, what, where, when and how' of the fault or symptoms. In this case, the garage just handed over the keys as they had not taken the time to collect this information but on the road test it was clear the vehicle was in default, running limp with all the associated warning lights illuminating the dash. A quick visual check revealed no more than a replacement gearbox had been fitted in the past. So a global scan was performed and a number of codes were evident in the various systems. The only code that concerned me was stored in the gearbox ECU and this read: P1983 slc3 stuck linear solenoid valve clutch 3.

Wiring connector

The test procedure for checking the solenoid is not easy - the ECU bolts onto the gearbox with no external wiring or connector access. To test the solenoid, the ECU has to be removed to access the terminals. This has limited value as the circuit is not loaded during testing. However, comparing clutch solenoid three with the other clutch solenoids proved that its resistance was much higher (35? compared with 5.6 ?). The solenoid did not click when it was provided with a live and ground like the others either.

This appeared to be an open and shut case, an internal fault caused by the solenoid - another gearbox would be required. The fact that the vehicle had already been fitted with a replacement gearbox did little to inspire confidence in this diagnosis. Unsurprisingly, the customer wanted more proof before committing to another expensive replacement.

I suggested removing the valve block cover to access the solenoids; this would allow me to power up the solenoids and compare the readings using an oscilloscope and amps clamp. This would prove the failure with a loaded circuit; something I was more comfortable with. Doing this was well worth the effort, upon removing the cover it was evident that one of the electrical connections had considerable heat damage. Could this be the source of the high resistance? It seemed that it was as I replaced the terminal and then checked the resistance readings which were now an acceptable 5.6 ?. Bingo. The cover was replaced and the gearbox refilled with the correct fluid. The test drive proved the repair had been a success.

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