I am trying to troubleshoot a non-working speedometer/odometer problem on a '91 S15 Jimmy 2wd with a 5-speed manual transmission. I have tested the speed sensor voltage output while it was removed from the vehicle and it shows to be good. The strange thing is that when I re-install it, put the rear axle on jack stands and test the sensor with the wheels turning, the sensor shows no voltage output. That makes me wonder if the "trigger" inside the transmission is missing or damaged. Can somebody tell me exactly where and how the "trigger" is located in the tranny? I can look in the opening where the speed senor is located and see a gear. Is there a magnet attached to this gear that serves to trigger the speed sensor to produce it's voltage, or possibly is the magnet mounted on the rotating shaft? Any help or info is appreciated. Thanks!

 

the gear you are looking at is actually a reluctor ring, just a piece of round metal with notches in it..... heres an example of one.... http://img.auto555.com/photo/product...UCTOR-RING.jpg .... the magnet is inside of the speed sensor and when ever a tooth on the ring goes by the magnet it created a small AC current. the faster the spped the higer the AC voltage... so set your DDM to AC not DC and see what you get

 

Thanks for that info...I think it will help in solving my problem. Previously, when I took the sensor out, I checked the output voltage by holding the sensor near a spinning magnet and did get a voltage reading. At the time, I knew the sensor had a magnet inside it, but didn't know if the reluctor ring itself was magnetic also, or non-magnetic. I will try testing the sensor again using a non-magnetic piece of metal and see what kind of reading I get. Thanks again for the help!!!

 

It might not even be AC, it could be pulsed DC. If so, you'll probably need a multimeter capable of measuring duty cycle or an oscilloscope to measure it properly.

 

I think you are correct R2P....I remember reading somewhere that the output voltage from the speed sensor to the DRAC is not AC, but rather DC. I'm not quite sure what pulsed DC is....my voltmeter set at DC would not measure pulsed DC correctly? Thanks for your help!!!