I've had a couple of cars in this week that I've had to diagnose electronic problems on,so thought I might take you through how I blunder my way to success.I haven't done much of this over the last couple of years,personal problems and now living 85km from work hasn't given me much time - and I normaly do this after hours,I could of knocked both these out over a glass of wine in 1/2hr each after work,but both were 2 day repairs.
We'll look at the 1992 Mitisubishi Lancer first,towed in as a no go and told by the AA that the fuel pump was faulty (yeah right) It starts and runs for a couple of seconds and stops,sounds like it's running out of fuel alright.The first thing I pick up for diagnosis is my Snap On Vantage,a Dual Channel Graphing Digital Multimeter (GDMM) with built in data base,it's very easy to use and uses a couple of D cells which last for months.I want to check the fuel pump first,so hook my Low Amp probe on and access the fuel pump wire,can't find the fuse to loop into,so rip out the rear seat and hook into the fuel pump wire with the clamp probe....the first image shows the amp draw on the fuel pump,each hump is a bar of the comutator.If I knew how many bars on the com I could calculate rpm,but don't see the need - the wave form looks clean,speed about right,and it runs all the way until the engine stops,so I reckon the pumps ok.If the amps went high and the pump slowed down I'd say it was siezing or the filter was blocked,if it speeded up with low amps I'd say it was running out of fuel...if it cut out as the engine stopped the pump would be faulty.
I also notice it's only running on 2 cyl,2 & 3 are out,the plugs are ok,leads ok,and both coils have the same resistance.Luckily we have a Mirage in with the same 4G91 engine for an alt repair,so we do the old swapparoo,change coils and ign module between cars - the Lancer coils and module work perfectly in the Mirage,the Lancer still has 2 cyls out with the Mirage parts.
That means the signal from the computer is missing for those cyls,so go off on another tangent to check the computer.Mitsi's have bad computers and it will pay to check before I go any further,we pluck the board out and see it has already had a couple of capacitors replaced,and repairs to the circuit board as leaking caps damage them,there is some signs of overheating and other signs of stress....we put it back in with that knowledge in the back of my mind.
Back on the case I have my scanner hooked up and clear the codes and check some inputs and outputs.Checking the injector pulse I find this is what's killing the engine - it runs up,then the injectors quit and it dies,confirmed on the scanner,it keeps bringing up cam angle sensor fault,so I check the cam and crank sensors.I check the cam/crank sensor and find the crank sensor good as in the second image,but just a 5 volt line for the can sensor - so that's it,fault found.
The cam/crank sensor is on the back of the cyl head where the distributor used to sit,so get a second hand unit and fit it - the motor fires up on 4 cyls and continues to run...fixed! The next image is the missing cam angle sensor wave form,both these are Hall Effect sensors and so show a nice square wave form,the crank sensor is even and tells the computer how fast it's going,the cam sensor tells where the motor is,and as it's a DIS 4 cyl with 2 coils each coil fires once a rev,so a wide and narrow wave form.
OK,so maybe I should of plugged the scanner in first,but it won't tell me if the fuel pump is faulty,so I felt I needed to eliminate the pump first,and I also needed to confirm a coil fault wasn't knocking out those 2 cyls.These images were captured on my Fluke 98,a DSO (Digital Storage Oscilloscope) it's much more powerful than the Vantage and I use it to narrow down on a fault.
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Originally Posted by bugjuice
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