Eaton speed sensor

This topic contains 4 replies and was last updated by trucklite 8 years 11 months ago
May 25th, 2012 3:56am
I've got a c12 in a 2000 freightler fld and the cruise and speedo suddenly quit. I've test continuity between the speed sensor and the ECM. It checks out fine. I'm not to sure how to test the speed sensor. Any body got any ideas how to check it and if the sensor would be the problem?
May 25th, 2012 8:58am
This type of sensor, like almost all speed sensors, will be a simple A/C generator. In other words, a magnetic pole passes by a coil of wire and creates a voltage signal in one direction as the magnet approaches the coil and an opposite voltage signal as the magnet moves away. To test this type of sensor, a simple ohmmeter will be used to check for opens, shorts and grounds.

If the sensor is good it will have a certain internal resistance, X amount of ohms, showing continuity and a level or range of resistance that the manual will indicate is correct. Check the sensor when unplugged, and be sure to only use a meter that has a minimum of 10,000 ohms per volt internal resistance. In other words a good digital meter.

As you get involved with vehicle computer diagnostics, you will find that you have only 5 different TYPES of sensors with the A/C generator being just one type. The A/C generator will create different voltages and ranges and current flows, but the testing for them will be basically the same.

The same thing occurs with the other four types of sensors. Have fun.
May 26th, 2012 3:43am
Thanks Jim, I will be trying that. But.... even if the sensor failed wouldn't the cruise still work?
May 26th, 2012 9:54am
Well, you did ask how to test the sensor.

If the sensor sends a signal out of range: too high, too low or not at all, the check engine light will come on and you will likely have set a code in the computer. Using a scan tool will determine what code you have and lead you to track down the problem. If you do not have a scan tool tool, your only option would be to use the flash codes displayed when you activate the proper sequences. If the check engine light and a code is set, you may very well lose the cruise control because the truck computer will not know how to compare the relative speeds.

The scan tool and/or flash codes do not pinpoint the exact problem, but does direct you to the circuit that needs to be checked. It still requires the a mechanic understand the components and how to eliminate the good from the bad. People who get good at it can make a lot of money.
May 26th, 2012 10:07am
It should also be understood that probably 90% of the codes and electronic problems are a direct result of the harnesses, plugs and wiring for that system. You said you checked them, but always keep the 90% factor in the back of your mind.

Also keep in mind that faulty grounds and corrosion at the junctions can create intermittent faults that cause you to consider pulling out your teeth, never mind your hair.

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