Testing ECU Inputs with a Multi-Meter

Fuels and emissions

This topic was all about diagnosing an engine showing symptoms of faulty or failing ECU inputs or outputs.  These symptoms generally include abnormally high fuel consumption, smoke coming from the tailpipe, knocking sounds in the engine, weak running engines and sometimes no start. 

This process will require either a multi meter or an oscilloscope.  Having both will give you high advantages and make this process a lot easier.

Since this process involves taking readings from sensors, you may have to refer to manufacturers specifications.  Otherwise sensors that have variable resistors or any sort of changing sending voltage or resistance we can (not fully) safely assume that if they are changing according to what they are sensing, they must be working.

As usual, any repairing will start with listing up the symptoms that the customer has told us.  (I like to think of it as being a doctor, doctors are trained to ask as many questions as they can about what symptoms a patient is feeling including what, where, when, how, and what happens when this happens etc.)  This will give us the opportunity to give us a rough idea of what may be the problem.  And of course like any repair, start with a basic 'visual' check,  I like to keep an eye out for cables being stressed or anything that looks like it has been tampered with (Teens that come in usually overhaul  their cars and end up messing things up!)  If anything looks dodgy, we can use our knowledge to test the cable by following it and seeing what it is for.

We were given booklets in this case,  which gave us the general instructions for testing a sensor.  All sensors have the same purpose, to send voltage signals to the ECU let it know what's going on, but they all work differently.  We need to find out if the sensor is in a good working condition by looking at it visual, if you must, move its parts around, give it a wiggle around, etc. 

Now we need to collect some readings.  We took a multi meter and set it to ohms.  Our multi meters have an automatic ranging system which ranges itself to the most appropriate range.  These are handy, but be careful that when you are recording readings down, note that the range might change (eg. from K-ohms to M-ohms).  Calibrate it by touching the two leads together until the multi-meter reads its lowest possible reading.   Depending on what multi-meter it is, it should give some type of signal like a beep to tell you it's ready.  Check the resistance of a sensor by putting the negative terminal on ground.  Find some bare metal closest to the sensor, like the bolt that attaches it to the block, or if it is attached to plastic (like in most cases, the MAF sensor) then back probe the terminal to see which one it is.  (or just put it on some other bare metal, since we are looking for changes in signal, not the really the readings themselves.)  Now we need to use our knowledge to figure out how to change its reading and see if its signals change according to its environment.   For example, If we are testing a TPS (throttle position sensor) then we will test the resistance at idle and compare it with the reading we get at full throttle (full blat!) .

Now that we have the resistance readings, turn on the ignition and check the readings.  Of course, we need to record the readings and compare them.  Now turn on the engine and find your readings.  Compare these readings and they should be different, depending on what sensor it is.  Using the example of the TPS,  we got readings of 0.8V at idle and 3.83V at a wide throttle.

We have now taken readings from sensors.  But wait, don't stop if you have found a problem!  Always keep looking for more problems.  In a workshop,  customers are paying you to get their cars back on the road, so if you find a problem, it may not be the only one.  If you can, use your brain power to try in any way to jump the problem (eg. broken terminal, make your way around it by running a small wire past the broken area).  This will help tell you what happens when the parts that your customer has paid for will fully solve the problem.  (Remember, customers first!)