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Old 05-25-2010, 04:29 AM   #1
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Default ABS light on. How to check rear ABS sensor?

I'm getting the one ABS light on, and I've heard a few guys have had the rear diff ABS sensor as the problem. So, I did a check on all 3 of my ABS sensors with an ohm meter. I did this by seperating the connector to the sensor and reading the resitance of the sensor by itself. I'm getting 650 ohms on each of the front wheel speed sensors. Then I checked the rear diff ABS sensor, and I'm getting open circuit. ie infinite resistance. I tried disconnecting a front sensor, starting the truck and I see that in addition to the ABS light, the Brake dummy light also comes on. This happens for both front wheel sensors. For this reason, I think I've ruled out either of the front sensors as the problem. I forgot to unplug the rear sensor and see if anything changes. Does anyone have any more ideas? I did check my brake fluid and fuses already.
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Old 05-25-2010, 10:36 AM   #2
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Excellent troubleshooting! Refreshing to see someone take the initiative to troubleshoot this way first before asking someone to provide a step by step proceedure first.

I don't have my paper repair manual copies at hand to help you continue, but perhaps Gatorb8 will assist with his electronic copies he has on hand.
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Old 05-25-2010, 10:17 PM   #3
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Well just did a test. Unhooked the rear wheel speed sensor and measured it again today with my Fluke digital multimeter (which I trust more than the mastercraft one I used yesterday). I think it was 13.8 Mega ohms. I should re-measure front ones with this meter becuase I don't think 650 is normal.

Anyways, I confirmed that whether the rear sensor is hooked or unhooked, the ABS light remains on (engine running) and the Brake dummy light is not illuminated. As you may have read in my first post, by disconnecting either of the front wheel speed sensors, the dash ALSO illuminates the Brake dummy light.

I think what I need someone to do is crawl under their truck unclip the rear wheel speed sensor, and start their engine and see if this causes the "ABS" light and ONLY the "ABS" light to illuminate. If it does, this is probably enough evidence for me to warrant replacing the rear sensor. I suppose I could try swap with someone with a similar truck....

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Old 05-26-2010, 05:33 AM   #4
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ok, get this, I did lots of reading online, and I have come across info saying that my rear sensor should be between 1200 and 2000 ohms. So I substituted a few resistors to see if the intial start up check of the ABS would pass and then not trigger the ABS fault light. I tried resistance values of 1200 and 1500 and just on a hunch, to match the front one, 660 ohms. In all 3 cases, restart the engine and observed the same ABS light on. hmmmm any thoughts anyone?

Think someone could measure the resistance of their working rear diff speed sensor?
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Old 05-26-2010, 09:17 AM   #5
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Get the DTC pulled for free at autozone if you don't have a scanner. There are DTCs for each front and the rear sensors (C1230 is the rear code).

I found a different pinpoint test than the one I was referencing in your earlier thread, and this one has your answer. All three sensors should show between 3.8 and 5.2 MegOhms.

FYI, the part number for the rear sensor on my 06 is 6L3Z9E731A, rockauto.com sells the Motorcraft (OEM) replacement for $42.
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Old 05-26-2010, 02:18 PM   #6
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Gatorb8, thank you very much for that info. Unfortunately I'm in canada and there are no AutoZone's in Winnipeg, Manitoba. I was hoping to not have to pay $80 to get it hooked to a machine. I also came across the 13.8M measurement information in my 05/06 service information program, and I am a little baffled why i measured 650 ohms on both my front ones. that doesn't make any sense, I'm gonna blame it on my cheap ditigal meter and double check with my fluke.... thanks gatorb8.
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Old 05-26-2010, 02:30 PM   #7
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Check your fuse #14 (if i remember correctly). Somehow i blew this one causing the parking brake light and ABS lights to come on, and it took me about three days to figure out what the heck was wrong.
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Old 05-26-2010, 02:34 PM   #8
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thanks. I'll double check!! I do not have the brake light on. just the ABS.
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Old 05-26-2010, 02:41 PM   #9
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Quote:
Originally Posted by bloodvette View Post
Gatorb8, thank you very much for that info. Unfortunately I'm in canada and there are no AutoZone's in Winnipeg, Manitoba. I was hoping to not have to pay $80 to get it hooked to a machine. I also came across the 13.8M measurement information in my 05/06 service information program, and I am a little baffled why i measured 650 ohms on both my front ones. that doesn't make any sense, I'm gonna blame it on my cheap ditigal meter and double check with my fluke.... thanks gatorb8.
IIRC, when OBDII first came out on the Bronco's those guys figured out how to read the codes by jumping the pins in the OBDII connector. I may be thinking OBDI, but it may be worth looking into to save the $80 charge
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Old 05-26-2010, 03:49 PM   #10
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OBDI... you must be as old as me! I too remember jumping two points on the connector, then counting the "check engine light" flashes to setermine the code.

Then again, I remember hand chranking the Model A I had to start it.

I really did own a model A....did not buy it new, just had one for a few years.
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Old 05-26-2010, 03:49 PM
 
 
 
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