Testing a component without a oscilloscope

I am not sure if this is in the right spot but here goes. First off this has nothing to do with home audio. What I am trying to do is test a component for my transmission in my car. The component is a transmission speed sensor and I need to test the output of said sensor to find out if its bad or not. I do not have access to an oscilloscope. Is there another way to measure the output wave form (I am going to guess no).
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
The sensor may need a bias voltage such as being fed via a resistor from 12v. You could always AC couple it to a small amp/headphone amp and see if frequency alters. Often the amplitude is critical with sensors so any of the above tests may not be very conclusive. Even the amplitude can be determined without a 'scope if you build a comparator that has an adjustable DC reference point... it's how far you want to take it.
 
I am coming to the conclusion the best method would be to spend the 200 bucks and buy a hand held oscilloscope. It would be a handy tool to have around since cars are getting more and more computerized. I have tested the sensor for continuity and resistance but that's as far as I can take it with my Dmm. I was looking at this model on amazon unless I can find one on Fleabay.

Edit: This one is REALLY nice *drool*
 
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speed sensor test

No scope is needed to test a speed sensor. Most if not all speed sensors work off what is know as the "Hall" efect. Just set your meter to dc voltage and spin the sensor by hand, it should produce a small voltage. That voltage is sent back to the trans PCM and or the engine ECU to give info on shifting (if auto). There are sometimes two sensors one at the trans and one on the spedo any one can cause a problem but most of the time its the trans speed sensor. The best thing you can get if you want to know whats going on with your car is a good OBDII scaner. That would give you all the info needed to diagnose your troubles.
 
Funny you posted about using a meter did that yesterday and determined its the only sensor I haven't replaced yet (figures) The input speed sensor depending on the engine RPM in D (with the brakes on) varies from 0-5 volts ( 5VDC being the reference or max) the output speed sensor holds steady at 5vdc regardless of engine RPM. According to component test procedure this should be changing. I still have to make sure the reason its not changing is because its shorted and not because the wheels are not moving. According to the testing as I stated above it should change since the transmission is engaged in drive and the output shaft is spinning the torque converter.
 
I would think that would go without saying as the sped will not give a reading unless your moving. That is why it is better to pull the speed sensor and spin it and check the reading. Not everyone has a lift to put a car on to test. The adding wires and running in the car is way more work than just pulling the speed sensor and testing. Belive me I know more about this then the average joe. The contacts to the molex at that speed sensor are very easy to damage also and can add more problems. Just pull the sensor and test it.