You didn't hurt anything. In fact, I taught Automotive Electrical for nine years and I forbid my kids from poking holes in wires to take readings. Those allow moisture to get in and cause corrosion. A lot of GM vehicles had aluminum wiring too and that would corrode really quickly, as in a few weeks.
Back-probing through the rubber weather pack seal is perfectly acceptable, and it is often necessary when unplugging the item will cause false or inaccurate readings. I suspect what your instructors were afraid of is the test probe going in sideways and touching two adjacent terminals. The other problem is sometimes the terminal is kind of recessed and the probe doesn't make good contact with it. That's always my first thought when I see 0 volts or an open circuit when testing for resistance or continuity. If you see any voltage, you know the probe is making good contact.
There's another problem you can locate when you use a voltmeter AND a scanner. I don't know what your level of electrical expertise is so I can explain further if necessary, but in your training, did you ever hear of a "pull-up resistor"? Basically that is a resistor in the Engine Computer that is so big it has no effect on the circuit, that is until the sensor's connector is unplugged. Then that resistor puts 5.0 volts on the sensor's signal wire to force it to go to a non-acceptable condition and set a fault code. There's two things that go wrong here. First, if there's a break in that signal wire, the pull-up resistor puts 5.0 volts on it and that is what the scanner will display, but depending on the type of sensor, you will usually find the correct voltage on it. A throttle position sensor is a perfect example. As long as the ground and 5.0 feed wires are connected, you will find between 0.5 and 4.5 volts on the signal terminal, depending on throttle position. The break in the wire prevents that voltage from getting back to the computer. What you measure at the sensor is not the same what is displayed on the scanner. The two different voltages tells you to suspect a break in the signal wire.
The second potential problem, as far as I know, only applies to some older GM models. When a fault is detected for a sensor on any car brand or model, the Engine Computer has the ability to inject an approximate value to run on. It may not run well, or in some cases it runs better. A good example of that would be a break in a wire going to the coolant temperature sensor. Many systems default to minus 40 degrees, and that is what would be shown on most scanners. The computer knows, though, the coolant can't possibly still be minus 40 degrees after the engine has been running for a few minutes. Based on intake air temperature and how long the engine has been running at idle or under load, it may assume the coolant is at least up to 180 degrees, and it will use that value in its fuel metering calculations. Where the problem comes in on some older GM vehicles is the scanner will show that "injected" or "assumed" value, not the actual signal voltage from the circuit with the defect. In a case like this, where the acceptable signal voltage range is 0.5 to 4.5 volts, you would have a fault code related to "coolant temperature sensor voltage high" but the voltage shown on the scanner would be that injected value of, lets say 1.5 volts. Not knowing this about GMs, you'd scratch your head over how there can be a fault code that keeps coming back as soon as you erase it, when the signal voltage is correct. Measuring the voltage at the sensor would show the true 5.0 volts, and explain the reason for the fault code.
In the case of your map sensor, with the ignition switch on and engine off, one wire will have 5.0 volts, the ground wire will have close to 0.2 volts, and the last one is the signal wire. Again, the acceptable voltage range is (approximately) from 0.5 to 4.5 volts. GM and Chrysler do things very similarly so I'm going to switch to my Chrysler experience. You can expect to find around 4.2 volts. That represents barometric pressure. As soon as the engine is started, as manifold vacuum goes up, the voltage will go down. At idle you'll typically find around 1.2 to 1.5 volts. Beyond that there are tests only the Engine Computer can run on that sensor. Again, this pertains to Chryslers, but they bought their map sensors from GM years ago, the original junk ones and the improved ones, so you know they have to work the same way. A code can be set related to it taking too long to switch from its "engine-off" to "engine-running" readings. The code is "no change in map from start to run". Typically that was caused by a failing sensor that hadn't quite fully failed yet. A pinhole leak in the diaphragm that tugs on the sensor element would prevent it from generating the proper signal voltages. The same thing could happen if gas condensed in a low spot in the rubber hose going to the sensor. That would dampen the signal and make it respond too slowly. Chrysler had a recall for that to add a second hose. Now most manufacturers plug the map sensor right into the intake manifold to eliminate that hose and the potential leaks that go with it.
To my knowledge no one does this, but the map sensors are sensitive enough that they could be used to measure engine speed. Every time a piston goes down and takes a gulp of air, vacuum increases for just an instant. If you watch the signal voltage on an oscilloscope, you can see those little pulses of voltage. Count the number of pulses per minute, divide by the number of cylinders, multiply by two, and you have engine rpm. While they don't actually do that, the computer will detect a lack of those pulses and set a code related to "pneumatic failure". If you experiment with a small adjustable DC power supply and try to apply 1.5 volts on that signal wire, the engine might idle just fine, but the computer will recognize something is wrong.
Don't try that on a Ford product. As usual they do everything backward and different from everyone else. Their map sensors put out a square wave signal that varies in frequency, not voltage.
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Thursday, January 30th, 2014 AT 7:07 PM
