ABS sensor diagnostic - Output voltage too high?

[Black88GTA]

Hi all, I'm trying to tackle an ABS fault on my 92. When I started fixing the car up, I found that the ABS relay had been removed, so I replaced it with the one from my parts car. After I did that, the system showed a fault code 5, which corresponds to the front driver's side wheel (I'm sure why the system was disabled in the first place).

I've spot checked the sensor tips (they look good) and tone rings for cracks / bends, and did not find any - although I did so while viewing with a flashlight behind the rotor. If need be, I'll disassemble for more inspection, but both front rings look good from what I can tell.

I found Trevor's post in a very old thread here, quoted below for reference:

Originally Posted by Trevor

You can test the sensor by connecting a mulimeter to the two leads from it, with the sensor unplugged from the harness.The resistance of the sensor winding should measure between 80,000 and 1.3,000 ohms. Set the meter to measure millivots AC. When you rotate the wheel, you should be able to measure an output of 200 - 300 MV. If the sensor is not correcty positioned, the resistance will be OK, but not the output AC voltage.

The correct gap is :- 0.5 - 1 mm (0.020 - 0,038 ins,

With this in mind, I put a multimeter across the sensor plug first, and got a reading of 102k ohms, right in the middle of the range. The passenger side sensor was almost identical. So far, so good. Then I put an oscilloscope on the sensor with the car running up on stands, to see the output voltage. Video link below (the forum won't embed the Youtube links for some reason):

https://youtu.be/8jROnHYiPxk



Passenger side readings for comparison:
https://youtu.be/Zr24CdHer1A

The waveforms look OK to me (although admittedly I don't know exactly what to look for) - there are minor fluctuations, but it seems to be mostly consistent. The fault code is for the driver's side front, but both driver's side and passenger side behave similarly. However, I'm getting between 800 and 1100mV on both of the ones I checked, which is obviously way outside the 200 - 300mV Trevor cites in the quoted post. I was unable to precisely check the spacing between the sensors and tone rings, but just eyeballing it it looks like the gap is way bigger than it should be. Possible I'm only getting a code for the driver's side front because the computer polls that one before the passenger side, and can only display one fault at a time?

My thoughts are that my sensors are good, but need to be removed and re-shimmed in order to get them closer to the tone rings. Does this seem accurate? Does output voltage from the sensors increase as the sensor is moved further out from the tone ring? Would like to know before I attempt to remove the sensors without damaging them in order to take shims out. Thanks in advance for any help!

[Trevor]

Originally Posted by Black88GTA

Hi all, I'm trying to tackle an ABS fault on my 92. When I started fixing the car up, I found that the ABS relay had been removed, so I replaced it with the one from my parts car. After I did that, the system showed a fault code 5, which corresponds to the front driver's side wheel (I'm sure why the system was disabled in the first place).

I've spot checked the sensor tips (they look good) and tone rings for cracks / bends, and did not find any - although I did so while viewing with a flashlight behind the rotor. If need be, I'll disassemble for more inspection, but both front rings look good from what I can tell.

I found Trevor's post in a very old thread here, quoted below for reference:


With this in mind, I put a multimeter across the sensor plug first, and got a reading of 102k ohms, right in the middle of the range. The passenger side sensor was almost identical. So far, so good. Then I put an oscilloscope on the sensor with the car running up on stands, to see the output voltage. Video link below (the forum won't embed the Youtube links for some reason):

https://youtu.be/8jROnHYiPxk



Passenger side readings for comparison:
https://youtu.be/Zr24CdHer1A

The waveforms look OK to me (although admittedly I don't know exactly what to look for) - there are minor fluctuations, but it seems to be mostly consistent. The fault code is for the driver's side front, but both driver's side and passenger side behave similarly. However, I'm getting between 800 and 1100mV on both of the ones I checked, which is obviously way outside the 200 - 300mV Trevor cites in the quoted post. I was unable to precisely check the spacing between the sensors and tone rings, but just eyeballing it it looks like the gap is way bigger than it should be. Possible I'm only getting a code for the driver's side front because the computer polls that one before the passenger side, and can only display one fault at a time?

My thoughts are that my sensors are good, but need to be removed and re-shimmed in order to get them closer to the tone rings. Does this seem accurate? Does output voltage from the sensors increase as the sensor is moved further out from the tone ring? Would like to know before I attempt to remove the sensors without damaging them in order to take shims out. Thanks in advance for any help!

Discount the voltage reading indicated by the scope as this is a peak i.e. maximum reading at the peak of the sign waves. Whereas a reading taken by a meter should be an RMS figure, i.e. what is an accepted average figure. AC voltage measurement involves complex complications to deep to cover here.

You are absolutely spot on with your method of comparing one side with the other, i.e. good with bad. Stick with this rather than working with specific voltage figures and stay with this method of fault finding when considering the mechanical aspects of sensor spacing etc.

Clearly, you are following a very good nose and will get there. LOL

[Black88GTA]

Originally Posted by Trevor

Discount the voltage reading indicated by the scope as this is a peak i.e. maximum reading at the peak of the sign waves. Whereas a reading taken by a meter should be an RMS figure, i.e. what is an accepted average figure. AC voltage measurement involves complex complications to deep to cover here.

You are absolutely spot on with your method of comparing one side with the other, i.e. good with bad. Stick with this rather than working with specific voltage figures and stay with this method of fault finding when considering the mechanical aspects of sensor spacing etc.

Clearly, you are following a very good nose and will get there. LOL

Thanks Trevor, this is great information. The only multimeter I have here is a Harbor Freight special with a terrible refresh rate, but I will put it on there anyway and see what happens. I was not looking forward to trying to remove ABS sensors without destroying them in the process.

Since the front ABS sensors seem to be acting approximately the same and both seem to be working normally (assuming proper RMS voltage output), I think i will swap out the ABS computer under the passenger seat just to rule that out. The light blinks out codes like it's supposed to, but I've found a few failed electronic bits in this car so far (I think it had a lot of moisture in it while sitting) and it costs me nothing except some time to do so. So far the seatbelt computer, OEM stereo, and climate control module have all crapped out in some fashion (and the aftermarket alarm box in there is suspect) so yet another partially failed interior module would not be without precedent here. Will report back with any findings.

[Trevor]

Originally Posted by Black88GTA

Thanks Trevor, this is great information. The only multimeter I have here is a Harbor Freight special with a terrible refresh rate, but I will put it on there anyway and see what happens. I was not looking forward to trying to remove ABS sensors without destroying them in the process.

Since the front ABS sensors seem to be acting approximately the same and both seem to be working normally (assuming proper RMS voltage output), I think i will swap out the ABS computer under the passenger seat just to rule that out. The light blinks out codes like it's supposed to, but I've found a few failed electronic bits in this car so far (I think it had a lot of moisture in it while sitting) and it costs me nothing except some time to do so. So far the seatbelt computer, OEM stereo, and climate control module have all crapped out in some fashion (and the aftermarket alarm box in there is suspect) so yet another partially failed interior module would not be without precedent here. Will report back with any findings.

Check very carefully for faulty connections and points of corrosion. The sensors deliver a comparatively low voltage and very little current, so that the signal is very easily disrupted. Likewise, look for partial short circuiots caused by dampness or damage.

I am being very serious in stating that in these instances, the nose can prove superior as against instruments. LOL. You confirm that you are tracking well. Stay on course and you will win.