Every modern vehicle is governed by computers these days. It’s an unassailable fact that you simply can’t ignore. Millions of lines of code go into algorithms that control our engine’s fuel delivery, turbo boost, fuel consumption, ignition timing, our transmission’s shift points…any number of things really.
The ECU takes information from around your vehicle and processes it to give you the best power delivery and fuel consumption it can. Pretty nifty eh? But how does the ECU gather this information? Yep, you guessed it: sensors. These sensors are dotted about your 4X4 all over the place, and they play a vital role in ensuring things are running as efficiently, powerfully and reliably as possible, so they’re an important part of the puzzle.
When a sensor fails, either due to being dirty or because you knocked it completely off on that last log you just drove over, it’ll ‘throw a code’. What this boils down to is that the ECU is no longer receiving the correct information and it throws up a red flag letting you know something’s wrong. While this sounds like a good failsafe practice, in reality it can be a massive pain in the backside, particularly if you’re out in the bush, wondering why your 4X4 has randomly decided to go into limp mode 500km from civilisation.
So here’s our brief rundown on six of the most important (and potentially annoying) sensors on your vehicle, and how to get out of trouble when one of them decides it’s after lunch on a Friday arvo and stops working.
The importance of diagnostic tools
Vehicle-specific diagnostic tools are available fairly cheaply these days and are worth their weight in platinum when it comes to code-clearing your ECU. If you own a late-model techno-rich vehicle, we can’t recommend strongly enough that you grab one before your next big trip.
MAF/MAP Sensor
Mass Air Flow (MAF) or Manifold Absolute Pressure (MAP) sensors are for reading the air entering the engine through your vehicle’s intake – as such, they’re usually found between the air filter and intake manifold. A MAF sensor measures the CFM (or flow) of the air and sends a signal to the ECU to adjust the fuelling to suit. A MAP sensor reads the pressure of the air – on a turbocharged vehicle this can be either positive pressure or a vacuum – and again send this down the line to the ECU which works out how much fuel to dump into your cylinders. Clever, but super annoying when you’re halfway down the Canning and your 4X4 starts sounding like a bi-plane or coughing like a thirty-year pack-a-day smoker. You glance down and yep, the check engine light is on. Bugger.
While both of these sensors do occasionally give up the game completely, you can often get them functioning well enough to get you back to town. First check the wiring running to the sensor, we’ve seen these things degrade over time, particularly if the vehicle sees a lot of mud or beach work, and it may be a quick soldering job until you’re back on track. If you’re driving in thick dust or have been mud-bogging your way through everything, the sensor may just be dirty. Remove the unit and spray it liberally with your MAP or MAF sensor cleaning spray which is available at just about every auto-parts store. In a pinch, rubbing alcohol will do the job too. Also, with MAP sensors, closely inspect all vacuum lines around the sensor, as a leaky one could cause a drop in pressure and mess with the sensor’s readings.
Oxy Sensors
Oxygen sensors are those weird little gizmos that stick up our of your exhaust pipe and have a couple of wires running to them. Many later model 4X4s can have a couple of them along the length of the exhaust. They’re there to measure (surprise surprise) the amount of oxygen in your exhaust so they can tell the computer if the engine is running efficiently or if some fuel levels needing ramping up or down.
There’s good and bad news when these go south. The good is that they’re an absolute breeze to change (a spanner or two will knock it over), the bad is that you’ll need a diagnostic tool to clear the code. Check engine lights, weird smells from the exhaust, plenty of smoke out the tailpipe and reduced economy are all telltale signs that your oxy sensor has given up on life.
Crank Angle Sensor
Also called crankshaft position sensors, these guys monitor the revolutions of your crankshaft and let the ECU know where the piston is through its range of travel at any given moment, so the ECU knows when to inject fuel, and on petrol engines, when to fire the sparkplugs. While they’re not known especially for failing, their position on the engine (right down behind the crank pulley) means they can be exposed to a fair bit of crud in an off-road environment.
If you do have a faulty one, you’ll usually be unable to start the vehicle (it’ll crank, but not fire). There’s usually three wires running to them: a positive, a triggering signal wire and a ground. Make sure you know which is which before removing or messing with it in any way. Test the triggering signal wire for a pulse and the positive wire to make sure it’s receiving power – if it ain’t, chances are it’s cactus. Replacing them varies from vehicle to vehicle. While it’s usually not overly difficult, they can be tricky to get to – so bone up on where it is and how to reach it on your 4X4 before leaving.
Throttle Position Sensor
As the name suggest, the TPS is there to determine how hard your boot is mashed down on the go pedal. Symptoms of TPS problems include erratic acceleration, intermittent check engine light flashing, stalling for no reason and difficulties in changing gears. While these guys are generally a throwaway part once they’re shagged, we’ve see some pretty ingenious bush-fixes on the mechanical parts that’ve broken. If the inner potentiometer is dusted though it’s time to swap in a new one – not that it’s difficult, they’re usually found right near the skinny pedal or throttle butterfly on petrol engines so access is pretty straightforward. But whether you think it’s risky enough to carry in your spares kit is up to you…
Wheel Speed Sensors
Wheel speed sensors work in conjunction with the anti-lock braking system and are there to…well, do we really need to tell you? The name is kind of a giveaway.
The first sign you’ve got a faulty one is the ABS light on your dash lighting up. Luckily they’re a fairly easy fix and are generally a pretty cheap (around the fifty-buck mark) part for most vehicles. Secure your vehicle on jack stands, pop the negative battery cable off and pull the wheel off. The sensor should be easy to spot and is often held in place with screws or small bolts. Pluck it out and pop the new one back in and you’re good to go.
Knock Sensor
Designed to pick up any undue vibrations from your petrol engine, knock sensors send a signal to the ECU to change the fuel and/or timing to prevent detonation. A bad one will usually exhibit symptoms under load or at highway speeds, losing acceleration and fuel economy thanks to the computer dialling everything back, often going into limp mode. The hardest part about fixing them is finding them. They can be found on the block, the head or the intake – so pretty much anywhere. Once you have it though, it’s usually as simple as disconnecting the wires and plucking it out with an appropriate sized socket, again, using your scan tool to reset the ECU and clear the code.
