Can some one explain to me how out speed sensors work?

[TurboShadowES]

I have a dodge truck that uses a speed sensor just like our old td's, and the speedo doesnt work all the time. Im trying to trouble shoot it but i dont know exactly how our speed sensors work. I need to know what wire does what (signal, ground ect). I also need to know how they work... like how it "talks" to the speedo..

any help would be AWSOME

thank you
Dustin

[allmotor]

more information, year of truck, 2wd or 4x4, trans auto or manual.... I had a manual 2wd cummins truck that the speedo did weird things and would drop to zero then go to actual speed then drop 20mph it was weird and random when warm but worked perfect when cold. problem was the speedo gear on the output shaft was slipping. replaced gear and all is well.

[TurboShadowES]

90 4wd CTD

well, I dont think it is a mechanical issue. I kinda sorta bought the truck from the first owner and the truck has done this since it was brand new. Chrysler replaced the spped sensor and the adapator, along with the gauge cluser and it still didnt fix the problem. So, they gave the owner some money back to live with it. Not somthing I would have done.. but they did.

What Im wanting to do it figure out how to hook a DVOM up to the speed sensors signal wire, and see if the signal pukes out half the time, or if it is in the wiring somewhere. Thats the goal.

thanks
Dustin

[allmotor]

why not just connect a scanner and read data. on my situation, I saw the PCM was getting the same information that the speedo was showing. Same situation as you.. previous owner had the PCM, speedo, sensor, 90° adapter all replaced w/o success. I just took a stupid uneducated person (me) to figure out the slipping gear problem. sure glad I'm not an big buck dealer tech screwing over customers because I'm too lazy to actually solve the problem instead fo throwing parts at a problem.

[TurboShadowES]

that would work if i had a scanner... but I dont. But using my DVOM, I should be able to at least read the voltage.

my speed sensor is in the t-case thou, so I have a diff adaptor then you. I may take it apart and have a look thou, but ushally mine works or it doesnt.

Thanks
Dustin

[Phreakish]

the ones I've seen in cars have a 'flying magnet' ring that goes past a switch in a glass bulb. It opens/closes that switch 8 times per rev... at least, thats what I was told, so its just a 'square wave' pulse 8 times per rev... or should be.

[allmotor]

How can you own a turbo dodge and not have a scanner.. it is as important of a tool as bandaids, beer, and aspirn. it's like being a carpenter without a hammer or a proctologist without rubber gloves. if you want to "attempt" to do your own repairs, you need the tools.

[Una]

I use my meter to find VSS wires on all sorts of vehicles at work, when I need to install aftermarket cruise control. You need a good fast reacting meter, like a Fluke 78.. It has a nice little bar graph at the bottom. Usually I can ground one probe, and hook the other to the wire I think is VSS. Set the meter to DC volts. Then move the car forward or backward and watch for that bar graph to start a regular pulsing up and down. On some cars that doesn't work, and AC volts will work. Still others, that won't work either, and AC or DC Hz counting does the trick. Kinda weird.. But a meter is not going to troubleshoot an intermittant problem unless the problem is persistant enough that it'll stay broken for an hour while you track it down. If it's just random quick burps, you'll need an oscilliscope.. I usually don't resort to using that for cruise control installs, but it comes in handy for cruise control troubleshooting on older cars.