This isn’t meant to be a substitute for proper diagnosis – always check service information and follow the recommended diagnostic procedures. But keeping in mind that tolerances on high-pressure diesel fuel systems are pretty strict can save diagnostic time. “Just about” 12 volts isn’t the same as “at least” 12 volts when diagnosing these vehicles.
If the batteries and charging systems are indeed OK, it’s good practice to “check the basics,” meaning making sure the starter is OK including testing for voltage drop on the cables and checking for power at the starter relay, checking that the glow plugs are OK and the glow plug control module operates (including verifying power and ground) and checking for power to the fuel pump circuit. If the vehicle cranks fine, but starts and quits (“no start” means different things to different customers) watching the exhaust pressure during cranking can help locate a plugged exhaust – if it gets high fast, the exhaust may be indeed plugged.
Most of the time these preliminary checks will give you a good idea of what’s causing the problem by now – but there are vehicles that need a bit more testing. No problem, here are a few more tips to help out.
Even though the diagnostic charts usually direct you to check fuel quality right away, from experience checking for Diagnostic Trouble Codes (DTCs) and programming updates early on in the diagnosis is a smart move that can really save diagnostic time. In my own experience I’ve seen far more problems solved by hooking up the scan tool early on and investigating the parameters, programming versions and DTCs than by immediately sampling the fuel quality (not that that doesn’t happen and isn’t a valuable check).
|This is how many no-start conditions are fixed—by charging and testing both of the batteries.|
Finding out if any codes relating to the fuel injectors, control sensors or modules are stored really doesn’t take long at all and leaves you smelling much better than sampling the fuel will.