Lab Scope Hook Up

p51302 said:

Have any of your used an oscilloscope to diagnose your car ailments? If so, how did you do it? Did you have to place resistors between your probe and the component you were testing, i.e. O2 sensor.

Click to expand...

Never had to put a resistor on an inductive pickup... To check o2 sensors I use an MT 2500 and just check the cross counts. If you need more info feel free to PM me.

I use the 97 all the time. I never use any resistors... it makes it really nice diagnosing sensors

My backrgound is computer electronics, so maybe I can help.

Most of the sensors are simple thermistors or variable resistors that don't produce an output that is useful on a scope. The O2 sensors put out a changing voltage that switches 2-3 times a second. That's too slow to give a good display on anything but a storage scope or one of the LCD display instruments like the fluke.

Although most oscilloscopes will generate sweep rates as low as 2 seconds, unless it is a storage scope or has a phosphor that paints and holds an image, the display is very hard to read since it doesn't repeat very often.

The things that will put out a useful display are the pip sensor, the VSS sensor, the TFI module, the EGR vacuum regulator, the IAC control voltage. They all generate a pluse that switches often enough to make a readable scope display. I remember getting an interesting reading off of the EGR sensor one time.

The MAP or BAP sensor puts out a 5 volt square wave that changes frequency with variations in atmospheric pressure. The base is 154 HZ at 29.92" of mercury - dry sunny day at sea level, about 68-72 degrees. You need an oscilloscope or frequency meter to measure it.

Baro or MAP test using frequency meter - run the test key on engine off. The noise from the ignition system will likely upset the frequency meter. I used a 10 x oscilloscope probe connected from the frequency meter to the MAP/BAP to reduce the jitter in the meter's readout.

There may be more but that is all I can think of at this time.

More thoughs...

The high voltage ignition system will work with an inductive pickup. Use one inductive pickup for the #1 spark plug wire and connect it to the external trigger input. That will trigger the sweep to start with #1 cylinder. Connect the other inductive lead to the coil output wire and to the #1 channel or vertical input. That will allow you to watch the cylinders fire in order. The inputs from the inductive pickups may be too much voltage for the scope to deal with, so proceed with care.

I use my oscilloscope to test alternators. I also hook right up to the source.

Mario

right to the sensor.... a breakout box if the sensor is reading right but the computer doesnt seem to be getting the right information.

by the way... I hate ford for making us use a b.o.b to see what the computer is reading. This is one area where gm was actually leaps and bounda ahead of ford.