05-12-2011, 09:21 PM
How to test engine sensors
Crankshaft position sensor (CPS)
Location: right on top of gearbox above the flywheel. Could be hidden by thermostat housing on some engines.
If your car won't start on the starter motor but will start when bumped, the chances are good that your CPS has had it. Failed CPS can also be a common fault of no spark or injection. Although CPS sensors arnt hugely expensive, there is no point in just replacing it unless you know it's at fault!
Tools required:
Multimeter
Crocodile leads for multimeter
The CPS is very simple. It consists of a magnet with a coil wrapped around it. When they fail, most of the time it's because the coil has broken up due to heat, causing a open circuit.
On the 306, they use a 3 wire CPS. 2 wires for the coil and magnet and 1 for the screening that is around the cabling. This screening protects from electrical interference which could lead to false readings and non-starting.
Firstly we need to determine which wires are the coil and magnet wires. You can't damage the sensor by hooking your leads up the wrong way so it's simple trial and error till you start getting readings.
Set your multimeter to AC voltage (yes I do mean AC and not DC) and connect your two crocodile clips to the pins on the plug. You now want to attempt to start the car. You should be getting a reading somewhere in the region of 0.4v to 1.0v. If your getting anything slightly outside of this, chances are the shielding in the wiring is deteriorating. If your getting a few millivolts outside of this range, don't worry. Chances are this is not the problem and the sensor is working fine. If your getting readings of a couple of volts or nothing at all, sensor is buggered.
Note: if your doing this with the sensor still plugged in and the car starts, you should start to see readings anywhere up to 3.5-4v
ABS wheel speed sensors
Location: on each hub. Will be pretty obvious as it will be the only wire in that area other than pad wear sensor.
If your ABS light comes on permanently on your dash, chances are a wheel speed sensor is at fault. These sensors are not that cheap so you will want to test before replacement
Tools required:
Multimeter
Crocodile clips for multimeter
A wheel speed sensor works in exactly the same way as your CPS does. A magnet and coil make up the sensor which is energised when the toothed wheel on your hub spins. This information is sent to the ABS ECM which will work out how many RPM the wheel is doing by how quickly the sensor is energised and then returns to its normal state.
On the 306 (not 100% sure this applies to earlier models) there are 4 passive sensors (1 on each wheel I case you couldn't guess. You probably won't find a active sensor on any car before 2005ish which is good because they are harder to test. ). If any of these go wrong, the ABS system is automatically disabled as a varying signal from 1 wheel could mean that you get a random lockup from a wheel. Not what you want!
To test, once again it's the same as the CPS. Unplug the sensor, take your two crocodile clips and connect to the pins on the sensor. With your multimeter set to AC voltage, spin the wheel. You should start to see fluctuating readings. Unlike the CPS sensor, the wheel speed sensors use a massive range. You could almost say that it doesn't matter what readings they give. As long as they give a reading they will work. Anything below 0.2v ish probably won't work though!
I'm unsure as to how accurate the 306 system is so this may not apply but it's worth noting.
On some systems you can have a massive range of readings however the system will pickup a fault if one sensor gives a different range to the other 3. For this reason, if you suspect a faulty wheel speed sensor, test all 4. If 3 are reading 0.2 - 0.4v and one is reading 0.2 - 0.8v, it's obvious that one is at fault even though it is still giving a reading.
Lambda Sensor
Location: on exhaust very near to cat
A faulty lambda sensor might not be obvious. It can be hard to notice as they rarely "just go". They can go over a long period of time meaning you might not notice the changes In performance and economy. The chances are they will not bring on the EML unless they give up altogether (although it might register a code in your ECM). The first you realise of it could be mot time when they give you a red sheet for failed emissions.
Signs of a failed lambda are low MPG, poor performance/flat spots, sounds a bit rough, runs rough and, if it starts to over fuel, it may smell very rich.
Some 306s use two lambda. One is pre cat and the other is post cat. This is so that the ECM can check that the cat is doing its job properly. If your car is older than V reg, this probably doesn't apply to you!
Tools required:
Multimeter
Wire piercing probes
If you don't have wire piercing probes, you can cut back the casing on the wires but you must make sure they are sealed up properly after
You will find 4 wires on the lambda. The colours SHOULD be 2x white, a grey and a black. The two white wires are for the lambda heater. The idea of the built in heater is that the sooner it's up to temp, the sooner it's giving optimum readings meaning better MPG and lower emissions. The grey and black are your signal wires. These are the two we're interested in. Use your wire piercing probes to pierce the wires. Connect to your multimeter and set to DC voltage. Turn the car on. You will notice that the voltage will fluctuate between two voltages. If it doesn't, sensor is at fault.
The sensor should fluctuate between 0.2 and 0.8v. This might not mean anything to you but it does to the ECM! 0.2v means to lay off the fuel and 0.8v means inject more fuel. As the sensor gets older and less sensitive, you should find that this range gets smaller or wider. A few millivolts out isn't a major problem but if your reading 0.1-2.0v for example, the sensor is faulty. depending on how good your multimeter is, you should see if fluctuating fairly quickly. A cheaper sensor might not show this that quickly. As long as it is fluctuating, your off to a good start!
There is no way to clean a lambda sensor which you might hear on the Internet you can. Even if you clear out all the soot, it won't make a difference. A new lambda is about £80. You will probably find that it will make a large difference to how your car runs when you replace the original one but remember to reset the ECM when you replace the sensor by taking the battery off for about half hour.
This doesn't apply to 306 but is worth noting
Most lambdas fluctuate between 0.2 and 0.8v but some don't and this is why generic lambdas are crap. You could fit a generic sensor and it might clear the EML but it might not be running right still!
I learnt this the hard way when the lambda went on my old Saab 95. All 95s were fine with a standard range lambda sensor except for the HOT aero. Guess what one I had....
The EML was cleared but there wasnt really a difference in running and performance and it still failed its mot. The range on the original aero lambda was infact, 0.4-0.6v. The generic sensor was making the ECM over and over fuel even though it was a brand new sensor.
I will try and add more sensors to this guide and will also try and add pics if I get a chance to take some. Hope this helps some people!
Crankshaft position sensor (CPS)
Location: right on top of gearbox above the flywheel. Could be hidden by thermostat housing on some engines.
If your car won't start on the starter motor but will start when bumped, the chances are good that your CPS has had it. Failed CPS can also be a common fault of no spark or injection. Although CPS sensors arnt hugely expensive, there is no point in just replacing it unless you know it's at fault!
Tools required:
Multimeter
Crocodile leads for multimeter
The CPS is very simple. It consists of a magnet with a coil wrapped around it. When they fail, most of the time it's because the coil has broken up due to heat, causing a open circuit.
On the 306, they use a 3 wire CPS. 2 wires for the coil and magnet and 1 for the screening that is around the cabling. This screening protects from electrical interference which could lead to false readings and non-starting.
Firstly we need to determine which wires are the coil and magnet wires. You can't damage the sensor by hooking your leads up the wrong way so it's simple trial and error till you start getting readings.
Set your multimeter to AC voltage (yes I do mean AC and not DC) and connect your two crocodile clips to the pins on the plug. You now want to attempt to start the car. You should be getting a reading somewhere in the region of 0.4v to 1.0v. If your getting anything slightly outside of this, chances are the shielding in the wiring is deteriorating. If your getting a few millivolts outside of this range, don't worry. Chances are this is not the problem and the sensor is working fine. If your getting readings of a couple of volts or nothing at all, sensor is buggered.
Note: if your doing this with the sensor still plugged in and the car starts, you should start to see readings anywhere up to 3.5-4v
ABS wheel speed sensors
Location: on each hub. Will be pretty obvious as it will be the only wire in that area other than pad wear sensor.
If your ABS light comes on permanently on your dash, chances are a wheel speed sensor is at fault. These sensors are not that cheap so you will want to test before replacement
Tools required:
Multimeter
Crocodile clips for multimeter
A wheel speed sensor works in exactly the same way as your CPS does. A magnet and coil make up the sensor which is energised when the toothed wheel on your hub spins. This information is sent to the ABS ECM which will work out how many RPM the wheel is doing by how quickly the sensor is energised and then returns to its normal state.
On the 306 (not 100% sure this applies to earlier models) there are 4 passive sensors (1 on each wheel I case you couldn't guess. You probably won't find a active sensor on any car before 2005ish which is good because they are harder to test. ). If any of these go wrong, the ABS system is automatically disabled as a varying signal from 1 wheel could mean that you get a random lockup from a wheel. Not what you want!
To test, once again it's the same as the CPS. Unplug the sensor, take your two crocodile clips and connect to the pins on the sensor. With your multimeter set to AC voltage, spin the wheel. You should start to see fluctuating readings. Unlike the CPS sensor, the wheel speed sensors use a massive range. You could almost say that it doesn't matter what readings they give. As long as they give a reading they will work. Anything below 0.2v ish probably won't work though!
I'm unsure as to how accurate the 306 system is so this may not apply but it's worth noting.
On some systems you can have a massive range of readings however the system will pickup a fault if one sensor gives a different range to the other 3. For this reason, if you suspect a faulty wheel speed sensor, test all 4. If 3 are reading 0.2 - 0.4v and one is reading 0.2 - 0.8v, it's obvious that one is at fault even though it is still giving a reading.
Lambda Sensor
Location: on exhaust very near to cat
A faulty lambda sensor might not be obvious. It can be hard to notice as they rarely "just go". They can go over a long period of time meaning you might not notice the changes In performance and economy. The chances are they will not bring on the EML unless they give up altogether (although it might register a code in your ECM). The first you realise of it could be mot time when they give you a red sheet for failed emissions.
Signs of a failed lambda are low MPG, poor performance/flat spots, sounds a bit rough, runs rough and, if it starts to over fuel, it may smell very rich.
Some 306s use two lambda. One is pre cat and the other is post cat. This is so that the ECM can check that the cat is doing its job properly. If your car is older than V reg, this probably doesn't apply to you!
Tools required:
Multimeter
Wire piercing probes
If you don't have wire piercing probes, you can cut back the casing on the wires but you must make sure they are sealed up properly after
You will find 4 wires on the lambda. The colours SHOULD be 2x white, a grey and a black. The two white wires are for the lambda heater. The idea of the built in heater is that the sooner it's up to temp, the sooner it's giving optimum readings meaning better MPG and lower emissions. The grey and black are your signal wires. These are the two we're interested in. Use your wire piercing probes to pierce the wires. Connect to your multimeter and set to DC voltage. Turn the car on. You will notice that the voltage will fluctuate between two voltages. If it doesn't, sensor is at fault.
The sensor should fluctuate between 0.2 and 0.8v. This might not mean anything to you but it does to the ECM! 0.2v means to lay off the fuel and 0.8v means inject more fuel. As the sensor gets older and less sensitive, you should find that this range gets smaller or wider. A few millivolts out isn't a major problem but if your reading 0.1-2.0v for example, the sensor is faulty. depending on how good your multimeter is, you should see if fluctuating fairly quickly. A cheaper sensor might not show this that quickly. As long as it is fluctuating, your off to a good start!
There is no way to clean a lambda sensor which you might hear on the Internet you can. Even if you clear out all the soot, it won't make a difference. A new lambda is about £80. You will probably find that it will make a large difference to how your car runs when you replace the original one but remember to reset the ECM when you replace the sensor by taking the battery off for about half hour.
This doesn't apply to 306 but is worth noting
Most lambdas fluctuate between 0.2 and 0.8v but some don't and this is why generic lambdas are crap. You could fit a generic sensor and it might clear the EML but it might not be running right still!
I learnt this the hard way when the lambda went on my old Saab 95. All 95s were fine with a standard range lambda sensor except for the HOT aero. Guess what one I had....
The EML was cleared but there wasnt really a difference in running and performance and it still failed its mot. The range on the original aero lambda was infact, 0.4-0.6v. The generic sensor was making the ECM over and over fuel even though it was a brand new sensor.
I will try and add more sensors to this guide and will also try and add pics if I get a chance to take some. Hope this helps some people!
Team Eaton
1999 China Blue 306 GTi6 - Eaton Supercharged - 214.5bhp 181lbft
1999 China Blue 306 GTi6 - Eaton Supercharged - 214.5bhp 181lbft
