05 Rear Dif. ABS Sensor

Sergio del Castillo · **Posted:** Mon Sep 18, 2017 8:40 pm

Hi you all,
Does any one knows how to check the rear differential ABS speed sensor? posting.php?mode=post&f=3&sid=99d61404d367a8dd4673d12f5abcaf3d#
My Abs light came on and, of course the odometer does not work. Checked the wiring and they are OK. Bought a new sensor with no results. Checked the resistance (Ohms) between terminals in both of them and get readings for an open circuit. I do not believe that that is correct, unless i got a defective sensor from the store. Has any one checked this sensor and can provide me with the proper reading?
Thanks,
Sergio

flash7210 · **Posted:** Tue Sep 19, 2017 2:49 pm

Are you absolutely sure that the wiring and connector is good?
It might be good now, but once bouncing down the road it could lose connection.

The sensor itself is activated by a magnet. No magnet = open circuit.

On mine, I had to replace both the sensor and the harness connector.

Sergio del Castillo · **Posted:** Tue Sep 19, 2017 3:41 pm

Hi flash,
Glad to hear that you made it OK with Irma. My brother in Miami was without power, telephone or internet until Thursday last week. Today he has no telephone (land line) yet, nor internet. We communicate through his place of work internet and he has now mobile phone service. No damage to his house.

Now to the issue.
Last July I had the same problem coming back from Siesta Key, and with the help of my son in law in Charlotte, we rebuilt the wiring removing old spade splices and redoing the splices with new soldered connections, heat shrink insulation, and new sensor connector, as the old one was kind of loose (too much play). I also installed a new sensor, but it did not worked. I installed the old sensor and everything was fine until two weeks ago. before that, every time I had the Jeep on a lift, like for an oil change, the ABS light came on for about the first half a block. I have checked all the connections, and I do get 12+ volts at terminal 1 of the connector as should be. I have replaced several times both sensors with no results. I have used my ohm meter to read across the terminals of the sensor and get open circuit in both cases, the new and the old.

You mention that the sensor is magnetically activated, this means that the proximity of the tone wheel tooth will close the circuit to send the signal. No magnetic field = open circuit, with magnetic field present, closed circuit. If the movable component inside the sensor is stuck in place in the open position, then no signal would be available. Should this be easy to verify outside by passing a steel bar back and forth under the sensor? I am going to try it. Also, I am going to remove the rear seat and access connector C307 to confirm return signal continuity. Will keep you posted.
Thanks again

tjkj2002 · **Posted:** Tue Sep 19, 2017 9:37 pm

There is no moving parts in that rear speed sensor,it's just a magnet.

That sensor is just a AC signal generator.

Are you testing the sensor cross the 2 terminals using the ohm setting? I don't have the exact spec but typically most should have between 5k-10k ohm's of resistance.

Sergio del Castillo · **Posted:** Tue Sep 19, 2017 11:22 pm

Thanks tjkj2002,
Yes I connected the Ohm meter across the two terminals. The following are the results:
Try one: Open circuit, the sensor was set horizontal on the work bench.
Try two: 420 kOhm's. Sensor vertical (normal position) after light tap on the bench surface.
Try Three: 420 kOhm's steady Sensor vertical moving a steel bar under it in the same direction as the tone wheel.
Try four: Changing from 420 to 590 kOhm's moving the sensor vertically up and down about 1/2 inch by hand.

These are from the new sensor. I have not tried the old one as it is in the Jeep.

Question: Should I assume that then sensor is defective as there is indication that there is something loose inside, and the readings are that low?

If the sensor is an AC signal generator that means that the signal back to the ABS module changes as the the tone wheel moves under it, thus changing the strength of the magnetic field, then, I agree, there should be no moving parts inside. It reminds me of the pointless distributors of the 60's and 70's. I will get another new sensor tomorrow.

GordnadoCRD · **Posted:** Wed Sep 20, 2017 1:03 am

I agree something seems wrong with those readings.

The signal would be AC waveform voltage, but the resistance should not change with metal proximity. Temperature fluctuation could change resistance, but not magnetic proximity to iron.

Is there a way to check for resistance to ground? There should be no continuity.

Sergio del Castillo · **Posted:** Wed Sep 20, 2017 9:54 am

Hi GordnadoCRD,
Yes, I can check the sensor resistance to ground by connecting the meter from terminal to the brass ring at the base where the holding bolt secures it to the differential. The rest is plastic, thus isolated.
Thanks for the tip. Also I can put 12 volts directly from the battery (power pack) and check for the voltage fluctuation with metal proximity.

flash7210 · **Posted:** Wed Sep 20, 2017 11:53 am

http://www.autoserviceprofessional.com/article/91902/scoping-out-abs-wheel-speed-sensors-what-could-go-wrong

Sergio del Castillo · **Posted:** Wed Sep 20, 2017 12:50 pm

Thank you flash, Great document in Speed Sensors.

I concluded that my tools are not accurate enough for all the testings. My only comment is that it said, at the end of paragraph 6, that the two wires are "ground and signal" while the Jeep has 12v+ and signal.

I am going to double check the "signal" wire as I know that the 12v+ is OK. If that one checks OK then a new speed sensor will be installed.

Thanks again

WWDiesel · **Posted:** Wed Sep 20, 2017 1:03 pm

If the speed sensor contains a wire wound coil around a small magnet, the ohms or resistance read across the two contacts should be a constant reading between 1k and 2.5k ohms with the sensor lying on a bench with no movement! :shock:

Very low or no ohm reading or very high resistance could indicate a bad sensor, bad connector or connection, or a wiring problem either broken or high resistance in wiring...

This is a very good article on how they function:> http://www.underhoodservice.com/wheel-s ... nd-scopes/

:SOMBRERO:

Sergio del Castillo · **Posted:** Wed Sep 20, 2017 10:43 pm

Thanks WWDiesel,
This article explains why it is a 12+ volts and signal wiring.

Some more testing to do.

Sergio del Castillo · **Posted:** Thu Sep 28, 2017 4:47 pm

Update:
After some detailed investigation the end result was a fault at the Right Front Wheel sensor that my instruments could not detect properly. After some time testing all the ABS system, my friendly technician a the dealership got the right diagnostic using the sensible tools at his disposition.
The overall cost for me was less that me buying an expensive tool that I may use once every two or three years.
Thank to all of you for your comments and advice.

05 Rear Dif. ABS Sensor

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