This job exposes a worrying trend in a lot of automotive workshops.
It seems many technicians have unrealistic expectations of what the scan tool can do.
The Elantra came from another repairer, with the complaint that the air conditioning stops working after a short amount of driving.
‘I’ve checked everything,’ the technician explained. ‘The ale works perfectly for a short time, then for no reason it just blows warm air. The ale switch still lights up, the blower fan still works fine -just no cold air.
‘I’ve regassed it once, checked fuses, changed relays, changed pressure sensors and I’ve even scanned the computer, producing no fault codes.
Maybe the PCM (powertrain control module) is stuffed.’
The vehicle was left with me so I began with the usual checks – battery condition, charge rate, connections and ground circuits. All seemed in good condition.
The ale worked very well and when I plugged in the scan tool, there were no fault codes. In the test drive, the ale stopped blowing cold air.
At the workshop, the ale compressor was disengaged. Another scan showed no fault codes.
Scrolling through the PCM live data with the fault present, the coolant temp sensor was reading at 114°C which seemed a bit high.
At the radiator the fans were on high, but pulling warm air and the heater hoses were red hot.
Inside the cab the temperature gauge was above normal but not in the red. Watching the coolant temperature on the scan tool with the heater going flat out, when it dropped down to about 1oaoc the compressor re-engaged and started working again.
The problem was a stuck thermostat. It was slightly open, just enough to let a bit of cooling through, but obviously not enough.
A new thermostat and 0-ring were fitted, the cooling system was flushed and the problem was solved. A long test drive confirmed the fix.
Here’s where this story really starts…
I explained my diagnostic path and the repair process to the first repairer, who asked, ‘Why didn’t my scan tool give me any fault codes? Why did I spend money on this useless thing if it doesn’t tell me what’s wrong?’
I explained that there were no fault codes because the PCM was doing exactly what it was designed to do – to turn off additional unnecessary loads if the engine starts to overheat.
‘Well, it should still tell me that,’ he responded.
He was grateful enough, but the episode left me thinking about the advantages that humans have over scan tools, or any tools for that matter.
The scan tool can only read what the vehicle software has programmed into it and that’s it. And while we couldn’t operate without them, we have the advantage of being able to look at a problem from many different angles, and apply many different theories.
Once we study workshop manuals, circuit diagrams and what our scan tool is or is not telling us, we can systematically narrow the problem down to a diagnosis. Now show me a tool that can do that. You can’t buy one. But you can develop one of your very own.
I’ve made as many mistakes as anyone, but if I’ve learnt anything, it’s that when you get that sticky problem that’s driving you nuts, before jumping in the deep end and before throwing parts at it hoping for a fix, slow down, sit back and relax, talk to some fellow techs, get another opinion and really think the problem through from all angles.
In my experience they are generally simpler fixes than you might think.