# Testing a speed sensor
**URL:** https://community.electricforum.com/t/testing-a-speed-sensor/6420
**Category:** GEM Forum
**Created:** [October 14, 2016, 11:31am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420 "2016-10-14T11:31:17Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@minglis](https://community.electricforum.com/u/minglis)
#### Post date: [October 14, 2016, 11:31am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/1 "2016-10-14T11:31:17Z")
I’ve seen several posts about bad speed sensors. I’ve found one place that will let you send yours in, they’ll test it, then if it’s bad, “rebuild” it & send it back for $20. Not a bad deal, but they seem so incredibly simple that I don’t know why one couldn’t test it oneself; if I knew what to do. I haven’t found a post with instructions on how to do this.
Isn’t the sensor simply some wire loops in the plastic cap that are excited by the magnet passing them, which then sends that charge to the controller, which counts the rate of the surge? What am I missing? If so, I’m unclear what the third wire going to the cap is. It makes sense to me that the “test” of the sensor would simply be continuity, or maybe the resistance, between the appropriate wires. But I don’t know for sure, and I don’t know why there are three wires. Does anyone know how to test this?
I assume if I measure no continuity between any of the three wires in my connector that mine is bad. Does that make sense? I’ve tried resetting the controller. I assume the large gray one is a voltage controller and the black one is the “controller”? - I disconnected both and let them sit, then reconnected them.
I’ve also read that one can just disconnect the speed sensor and the car will run fine, obviously without speedometer. That’s not true of mine. I still get the error codes 81 & 82 and it behaves the same.
Thoughts?
Thanks in advance.
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### Author:  [@Inwo](https://community.electricforum.com/u/Inwo)
#### Post date: [October 14, 2016, 12:35pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/2 "2016-10-14T12:35:26Z")
The sensor is an open collector hall effect device.
The Green wire is held to 5v-12v by a pull up resistor in controller.
The red powers the hall at 5v-12v.
Black is common B-.
To test, connect meter from black to green. Roll car slowly. Meter will change from +12 to 0v 4 times for each revolution of motor.
If it pulls down but doesn’t rise to match voltage of red wire, you can add an external pull-up resistor.
If it doesnt drop to near zero, needs adjustment, stronger magnet, or new hall.
My MM device has it’s own circuitry that is more sensitive than controller.
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### Author:  [@wrascal](https://community.electricforum.com/u/wrascal)
#### Post date: [October 14, 2016, 1:31pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/3 "2016-10-14T13:31:13Z")
We used hall effect speed sensors on our sander chains. If you pull the sensor out, plug it into its power and wipe the end with a metal screwdriver, you should get a pulse that will momentarily read on your digital speedometer. If it displays a speed for a moment, your sensor is good. Turn the drive slowly until the tip of the tooth is centered in the hole. Thread the sensor in until you can just feet it touch the tooth and back it out 1/2 turn. Then try it.
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Russ
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### Author:  [@Old\_Houseboater](https://community.electricforum.com/u/Old_Houseboater)
#### Post date: [October 14, 2016, 4:30pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/4 "2016-10-14T16:30:36Z")
82 code is sensor problem
81 code is magnet probably came off the shaft or broke or melted.
Truly, I have never seen an 82 code unless the sensor was toast. I have a few in the parts box. I have had 2 in the last 4 years that were bad. Very infrequent failure item.
Rodney
Rodney
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### Author:  [@MARKO](https://community.electricforum.com/u/MARKO)
#### Post date: [October 14, 2016, 5:44pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/5 "2016-10-14T17:44:41Z")
New to this forum. I own a R&T 2009 Hunter 4x4. Mice ate up wiring harness. Can any one give me the wiring diagram for the 2–30 amp relays in the dash that control the transfer case and 4x4 lock?
Thanks Mark
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### Author:  [@minglis](https://community.electricforum.com/u/minglis)
#### Post date: [October 15, 2016, 8:51am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/6 "2016-10-15T08:51:22Z")
Thanks guys for your rapid responses.
Russ, your’s didn’t make sense to me, maybe because I’m not familiar enough with my GEM or maybe you are referring to a different vehicle? I don’t know what the “end” of my sensor might be and I don’t know what tooth in what hole. There is no tooth near my sensor.
Thanks to all again!
Mark
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### Author:  [@minglis](https://community.electricforum.com/u/minglis)
#### Post date: [October 15, 2016, 8:53am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/7 "2016-10-15T08:53:40Z")
Oh, I was going to ask: the magnet on the shaft looks in good shape. There’s a gap of just under 1/8" in the magnet, which means it’s not QUITE a complete circle. I assume that’s normal and part of the spring tension that holds it on the shaft. Is that right?
Thanks,
Mark
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### Author:  [@Old\_Houseboater](https://community.electricforum.com/u/Old_Houseboater)
#### Post date: [October 15, 2016, 10:15am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/8 "2016-10-15T10:15:43Z")
1/8 gap is not normal might affect reading
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### Author:  [@wrascal](https://community.electricforum.com/u/wrascal)
#### Post date: [October 15, 2016, 6:52pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/9 "2016-10-15T18:52:52Z")
[quote=minglis;33073]Thanks guys for your rapid responses.
Russ, your’s didn’t make sense to me, maybe because I’m not familiar enough with my GEM or maybe you are referring to a different vehicle? I don’t know what the “end” of my sensor might be and I don’t know what tooth in what hole. There is no tooth near my sensor.
Thanks to all again!
Mark[/quote]
I had assumed most of these sensors are similar. We used hall effect sensors to sense teeth on a sprocket attached to a spinning shaft and the computer can be programmed to know how many teeth on the sprocket to achieve one revolution. Ours uses a very small air gap to sense the passing tooth. I guess GEM uses a slightly different system with a attached magnet. Sorry to confuse you.
```
Russ
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### Author:  [@minglis](https://community.electricforum.com/u/minglis)
#### Post date: [October 16, 2016, 8:19am UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/10 "2016-10-16T08:19:00Z")
Russ, No problem.
Rodney, ok; that’s helpful. sounds like it could register the sensor as defective and create the problems I have. I guess I need to replace it. We’ll see if I get one with the used, tested, guaranteed sensor I ordered. Otherwise, I haven’t seen them online. Any suggestions?
Mark
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### Author:  [@cargarci51](https://community.electricforum.com/u/cargarci51)
#### Post date: [July 23, 2019, 8:26pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/11 "2019-07-23T20:26:02Z")
Does the sensor need to be connected while taking measurements with voltmeter?
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### Author:  [@Inwo](https://community.electricforum.com/u/Inwo)
#### Post date: [July 23, 2019, 9:24pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/12 "2019-07-23T21:24:37Z")
Yes, you can use B- from battery or controller as common.
Then use a safety pin to pierce red wire. 10-12v.
Pierce green. Volts swing from +5 to 0v when rolling car. ~50 times per tire rev.
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### Author:  [@gem-care](https://community.electricforum.com/u/gem-care)
#### Post date: [July 30, 2019, 4:14pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/13 "2019-07-30T16:14:24Z")
It sounds like you’re describing a broken magnet.
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### Author:  [@Laslonimne](https://community.electricforum.com/u/Laslonimne)
#### Post date: [June 15, 2022, 2:09pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/14 "2022-06-15T14:09:55Z")
> [@minglis](#):
>
> I’ve seen several posts about bad speed sensors. I’ve found one place that will let you send yours in, they’ll test it, then if it’s bad, “rebuild” it & send it back for $20.
I realize this post is quite old. But you you use this “place” for a speed sensor rebuild? Do you have contact information for them?
Thanks much,
D
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### Author:  [@Laslonimne](https://community.electricforum.com/u/Laslonimne)
#### Post date: [June 15, 2022, 6:49pm UTC](https://community.electricforum.com/t/testing-a-speed-sensor/6420/15 "2022-06-15T18:49:20Z")
Never mind. Found a new one for under $20.