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Testing Engine Control Components (sensors)

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Testing Engine Control Components (sensors)

Postby binarypunk » Thu Apr 25, 2013 1:40 pm

The bare minimum you will need to carry out any of the testing detailed here, apart from standard workshop tools, is a basic multimeter. You can pick one up for around a tenner from Halfrauds or less from fleabay, but if you intend to use it regularly you will regret spending that little. By spending a little more, as well as getting a better-made product, you will also get some additional features which will prove very useful...indeed, essential for certain testing. Look for a meter which is able to test the usual voltage and resistance, but also that can test temperature and frequency.

Personally I use an Amprobe AM-240 for this kind of thing. In addition to the above, it also features max/min/relative data hold, audible continuity tester, capacitance and diode testing, a backlight and built-in stand. It isn't cheap, but it is a great investment if you will be doing regular electric/electronic work.

The chances are, you might have got here because you have a fault code. Never trust a fault code to tell you that a sensor is definitely faulty - it only means that the ECU is not getting a correct response from it. Often this could mean faulty wiring, or even a faulty ECU, so before you start splashing the cash, test the component first. If you happen to have a known-working one lying around, then you can swap them out, sure...but many people have swapped out a working item and unwittingly cleaned the connector in the process, solving the problem and being mislead into believing the item was at fault. None of these tests are tricky, and most can tell you conclusively whether the item is faulty.

Some faults have no code at all, or the item might be working just well enough to keep the ECU quiet, but not well enough to keep the car running well. In these cases, only methodically testing every component will work it out......although the symptoms or an experienced mechanic may give you a place to start.

In all cases, the first thing to do is to disconnect the electrical connector and inspect it for dirt and corrosion. A dose of Electrical Contact Cleaner may well fix your problem. While you are there, ensure that the wiring is intact and that the insulation is not cracked or worn through - many a component has been replaced, when in fact a trapped or broken wire was the true problem.

On with the testing!

Airflow Meters
These measure the amount of air coming into the engine, to allow the ECU to order up the right amount of fuel to go with it. If it is very broken, there will be a fault code. If it is slightly broken, you will have over/under-fuelling resulting in lumpy performance, hesitant acceleration, and/or nasty fuel smells.

(AFM) - 1.6 NA
The NA 1.6 AFM is different to all the rest, and to most other cars, so I'll cover it separately:
1) Disconnect connector (do NOT unscrew, it unclips) and the pipework and remove AFM from car
2) Ensure AFM body is free from cracks or damage that might cause air leaks
3) Check that the vane/gate moves smoothly
4) With your multimeter, check the following resistances with the gate at different positions:

AFM NA 1.jpg
AFM NA 1.jpg (17.97 KiB) Viewed 2027 times

AFM NA 2.jpg
AFM NA 2.jpg (18.4 KiB) Viewed 2027 times

If there is a problem, it is potentially solvable - I will cover servicing this item in another post.

AFM/MAF - other models
All other models, and most other cars, use a hot-wire system for airflow measurement. These, unfortunately, are less apt to be tested.
However, you can inspect the hot wire by removing the sensor and checking that the wire is intact, and clean. If it is, then there is not a lot else to go wrong. If it is dirty, a very careful squirt of contact cleaner, benzene, or carb cleaner might fix it. Do NOT attempt any mechanical cleaning of the wire, except as a very last resort, as it will very likely break.

Crank/Cam Angle Sensor
These tell the ECU where in the cycle the crank is (or the cam, which is much the same), to allow it to fire the spark plugs at the right moment. Most faults with this component are logged as fault codes in the ECU, but if not you might find problems starting or idling, or hesitant, lumpy acceleration.

The output should flip between 0V and 5V once or twice (depending on model) per revolution of the engine. The best way to test this is with an oscilloscope while the engine is turning, to verify that there is correct timing and clean voltage changes. If you have a multimeter with frequency function, this can also be used to ensure the output is flipping at the correct rate for the engine revs. If you have neither, it is possible to use a multimeter while a colleague turns the engine over by hand with the ignition leads removed to prevent loss of hands.
Generally speaking, these sensors will either work, or not. However, there are some subtle faults that would cause rough running, that might be invisible unless you use an oscilloscope.

Coil Pack(s)
These turn small voltages from the battery (12V), into big voltages for the spark plugs (~100kV). They are notoriously weak on these cars (even weaker than all other cars). Faults do NOT cause fault codes, but will stop your car from starting or running properly...sometimes only when hot, or damp. You will find the car misfiring or backfiring, lumpy or hesitant acceleration, or even stalling or failing to start at all - often when attempting to restart when hot.

1) The primary coils are on the 'car' side of the coil pack, not the HT lead side. With the connector disconnected, test the resistances of the primary coils, between the input and ground. The resistance should be very low (for the NA: 0.78-0.94 ohms) - be aware that most consumer testers are nothing like this accurate at very low values, so just look for a figure in this ballpark...very low, but not zero.

2) Measure the resistances of the secondary coils - between ground and the HT output.
The resistance should be fairly high (for the NA: 11.2-15.2 kohm - similar for others). Again, not zero and not extremely high or infinite.

3) Measure case breakdown resistance. This requires specialist kit - a 500V 'megger'/megaohm insulation tester (good quality calibrated ones go for around ÂŁ200-300 although for these purposes and if you'll not use it much, a Chinese ebay special can be yours for around ÂŁ20).
Use your megger tester to measure the resistance between the primary coil and the outside of the case. It should be 10 Mohm or more, otherwise a lot of your juice will be escaping to ground and not getting to your plugs.

Without the kit it can be deduced that the case is breaking down if the car exhibits coil-pack symptoms only in hot or damp conditions. Also, a careful inspection of the case can reveal tiny cracks or deterioration of the plastic.

Idle Air Valve (NA)
This is essentially a helper for the ISC valve (below) on early cars to provide additional airflow when the engine is cold. Coolant flows through little tiny easily-clogged pipes and heats up wax which expands/contracts to open and close a valve to control the airflow. It is integral to the ISC on later cars. There is no fault code for this item, but if you have problems starting or idling when cold, or have uneven idle when warm, it is a suspect.
1) First ensure your base idle is correct. Then go away and come back later...
2) With engine stone cold, bridge TEN and GND in diagnostic box with a jumper wire (or paperclip) and start the engine - leave the throttle alone. Engine revs should start high and gradually decrease to base idle as the engine warms. If the engine stumbles or stalls when cold or the revs change abruptly while warming up, the IAV is faulty. You may have some luck and find it is just dirty, rusty, or clogged. Otherwise it is very difficult (and rather pointless) to service.

Idle Speed Control Valve
This item, as the name suggests, controls the idle speed according to engine temperature, engine load and electrical load. Uneven or droopy idle and stalling are symptoms of problems here.
1) With engine up to temperature, disconnect the ISC's connector. The engine revs should immediately climb to approx 1200rpm and drop back to idle upon reconnection. If so ISC is good, if not proceed to step 2:
2) Measure the resistance across the ISC's terminals. (12ohm +/- 1ohm @20C). If outside this range, replace ISC. If not, check wiring.
If you find the revs change slowly, or not at all, and the solenoid resistance checks out ok and the wiring looks good, you probably have a sticky gate/valve. This can often be cured with a careful cleanup of the gate and its hinges using carb cleaner and a cotton bud.

Oxygen/Lambda Sensor
This works out the fuel/air mixture from the exhaust gasses and the ECU adjusts fuelling in response. Failure results in the ECU entering 'guess mode' and usually means over-richness.
1) Engine warm and idling, disconnect oxygen sensor connector
2) Connect voltmeter between connector output and ground
3) Hold revs at 3000rpm, check that meter reads 0.55V approx
4) Increase/decrease revs suddenly, check that meter shows 0.5-1.0V during increase and 0-0.4V during decrease
If not, replace oxygen sensor
O2 sensors on early cars have only 1 wire (they do not require power). Later cars have additional wires for a pre-heater (they only work when warm, so the output is ignored on early cars when cold...)

Water Thermosensor / ECU Temp Sensor
This tells the ECU how hot the engine is so that it can control the fuelling and idle speed, essentially the equivalent of an automatic choke of days gone by. Symptoms of faults here include poor starting and idling only when cold, or hot.

First off...be very careful with the connector on this item as it is made of cheese and easily broken. If you have recently been fiddling with your coil pack on an early car, I'll bet you've broken or dislodged it so check that first.

Preliminary test:
With engine stone cold at approx 20C, resistance between sensor and ground should be approx. 2.5kohm. If wildly out, replace.

Complete test:
Remove thermosensor and place in a container of coolant and place in freezer. Come back later...

With a thermometer and an ohmeter, measure the resistance of the sensor at various temperatures and compare to the table below:

Thermosensor.jpg (10.85 KiB) Viewed 2027 times

Emissions Purge Valve
There is a black canister on the offside inner wing - this is a charcoal filter that traps fuel vapour when the emissions people test the car, and releases it into the engine when they aren't looking and you're hooning it. The valve that controls this is frankly rubbish and can cause an air leak at idle when (and not if) it perishes. This will cause poor idle performance and you'll find it difficult to set the base idle correctly. It is an item included for the sake of emissions only and is not essential for the running of the car.

1) With engine warm and running at idle, disconnect the vacuum pipe from the charcoal canister, at the purge valve end
2) Place finger over purge valve opening and ensure no vacuum is felt
3) Disconnect purge valve electrical connector – ensure there is still no vacuum felt
4) Apply 12V across purge valve terminals – ensure that vacuum is now felt
If not as specified, repair or replace purge valve...or remove the blasted thing and block off the pipes.

Throttle Position Sensor
This item tells the ECU how hard you've buried your foot in the carpet. In the case of the NA 1.6 manual, it is basically a switch...but it must be correctly set so that the ECU knows when it's either closed, part-open, or buried - if incorrectly set, you will find the car idling badly, or stumbling when you touch the throttle gently (in which case the ECU doesn't know you've touched the throttle and sticks to idle fuel for a second or so before waking up). All other models have potentiometers which are less sensitive to calibration, but have the weakness of getting dirty/worn tracks that might confuse the ECU at certain points of travel, particularly the first few mm which are the most-used.

NA 1.6 (manual)
1) Disconnect TPS connector and connect ohmeter / continuity tester
2) Insert feeler gauge between throttle stop lever and stop screw
3) Check continuity/resistance as specified in the table below:

TPS 1.jpg
TPS 1.jpg (20.94 KiB) Viewed 2027 times

TPS 2.jpg
TPS 2.jpg (14.45 KiB) Viewed 2027 times

Later potentiometer types can be tested by checking the resistance between the output and ground while the throttle is moved slowly from idle to full open and back. The resistance should change slowly, steadily and smoothly - any sudden jumps, areas of flat-response, or low/high spots are a sign of a worn or dirty potentiometer. They are potentially serviceable if you can get them apart to clean the track - if you've ever had an old stereo with a volume knob that crackles and pops when you move it...that's the exact same problem.

Working on a test that doesn't involve the Mazda diagnostic SST - if you know of one please shout :???:

If you have a fault code, or symptoms, that point to one of the sensors being faulty, but it tests ok and the wiring is good, then it is time to suspect the ECU itself. This is particularly so if you have several codes that change, come and go, or seem to not make any sense.

I've tried, but I can't think of a much sillier place to put an ECU than where Mazda did on these cars. If you get a bit of a water leak into the cabin, or leave the roof down in a downpour, and it'll be sat in a puddle getting rusty and shorting out. Pull up the carpet in the passenger footwell, remove the cover plate, and have a good look for signs of water damage....particularly the connectors, which are made of many wafer-thin bits of copper. If in doubt, test the resistance from the wire side of the loom connectors to the pins on the other side - it should be essentially zero, or else there is a problem. Also inspect the ECU connectors for green gunge, which is a sign that it is time to cry. However, it is probably easier (but not cheaper) to replace the ECU than the loom connectors.

Unless you are very handy with electronics, the ECU itself isn't user-serviceable. You can find people that will fix them for you, but it is usually cheaper to pick one up from a breaker. However, you might be lucky and find that it is only gunged up, and a dose of contact cleaner might fix it. If it has shorted out and blown internal components, you're pretty much stuffed.

If you have any feedback or suggestions for this guide, feel free to send me a pm. If you have other general questions, please post a thread on the open board.

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