Rear ABS not working

ANTHONY ALOI

MEMBER
1992 FORD F-250
5.8L
V8
4WD
AUTOMATIC
118,000 MILES

When stopping hard enough or on a slippery surface, the rear brakes lock up. I have no ABS warning light to indicate a fault. The brake pedal is firm.

I have determined I have a RABS II system, since the control module is located behind the glove box. I have used the remote access connector which is located on the inside of the left inner fender to test the resistance on the speed sensor. I am reading 1320 ohms, which according to the Hayes manual I have should be above 850 and below 2500.

I don't know it this is relative but my 4x4 dash indicator lamp doesn't work. I don't know if it is the lamp or the switch on the transfer case, but I suspect the switch. I only mention this because it is noted in one of the trouble codes in the book.

Thanks for your time.

11 Replies

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi,

Are there any codes related to the ABS? If you remove the sensor from the differential, is the exciter ring clean? I've seen the exciter get clogged preventing the abs from actuating.

Let me know.

Joe

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

MEMBER
58 POSTS

Thanks Joe for your quick reply. As I mentioned in my question, the ABS light come on and goes off when I start the truck. It is not illuminated which I interpreted as the system passed its self test. I did ground the terminal below the dash on the passenger side with the key on in the run position but was not able to generate any code flashes. Also, before I crawl under and pull the diff. Sensor out, I was wondering if, since the speedometer is working wouldn't that indicate the sensor is okay? I have an E40D transmission.
Thanks again for you help.
Tony

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Wednesday, March 24th, 2021 AT 6:06 PM

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi Tony,

I agree with you. The speedometer is actuated from the same sensor. I would think if the RABS module was bad, the light would be on.

Do me a favor. I was trying to see if there were any technical service bulletins that may help. The only thing I was able to find is part relocation info. Interestingly, they mention a few different test ports. See if anything seems relevant to you. See pic 1.

Joe

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

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58 POSTS

I double checked Joe, even the master cylinder fluid level. The only tests ports I see/have are the ones I've described before. According to my Haynes manual, some of your highlighted locations are for earlier models (RABS I). Plus my battery is on the right so I'm assuming the one your notes indicate may be for a diesel. I also wonder why when I ground the terminal behind the glove box, I don't even generate a code 16 (System okay)? Any additional thoughts? I'm at a loss.
I really appreciate your time to help out.
Tony

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Wednesday, March 24th, 2021 AT 6:06 PM

JACOBANDNICKOLAS

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Hi,

I'm not sure why there is a difference in the wiring. I'm looking at a 5.8L V8 gas engine.

Do me a favor and complete the attached diagnostics. The module itself may be bad.

Joe

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

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58 POSTS

Joe, grounding pin 6, I get no reading on ohmmeter.

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Wednesday, March 24th, 2021 AT 6:06 PM

JACOBANDNICKOLAS

MECHANIC
108,179 POSTS

Hi,

That is a problem. Confirm the wire itself which comes from pin 6 is not broken or damaged. If it isn't damaged, it sounds like the module is bad.

Let me know if you find anything.

Joe

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

MEMBER
58 POSTS

Hey Joe,

I get nothing when checking the line (open circuit?). I do get a reading when I connect Pin 6 to another pin at the connector (black/white wire). About 60 ohms and when I turn the key to the on position it drops by about one half. I tried to follow the line first under the dash, but the light blue/red wire (pin 6) seems to go into a module under the dash and it doesn't come out of it. I'm not sure this is the same wire since when I check it for continuity I get nothing and that's only 2 feet of wire from the connector.
The schematic I have shows circuit 535 going direct to the hydraulic valve but I may not have the best info. I do see a light blue/red going through the firewall in a large bunch but I can't seem to follow it in the engine compartment.
Am I doing something wrong?

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

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Joe,

I forgot to ask, why, if a circuit is broken, I don't get any code? All these potential causes seem to have an established code number. It just seems odd.

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Wednesday, March 24th, 2021 AT 6:06 PM

JACOBANDNICKOLAS

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Hi,

Anything is possible. The idea that there is no code doesn't make sense to me either, but there are different conditions that can cause it to happen. I have a feeling the module is the likely problem. I'm basing that on the diagnostics you performed.

I attached all I have on the wiring schematics for the ABS. Take a look through them and let me know if they help.

Joe

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Wednesday, March 24th, 2021 AT 6:06 PM

ANTHONY ALOI

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Thanks Joe. They seem to have more info than mine. I'll go over the system again. One question, is it possible that the hydraulic valve is the problem? I don't see where all aspects of it are part of the system self check specifically the dump valve switch. Is there a way to manually apply power and check its operation? I guess that's two questions.
Thanks again for all you help and have a good holiday.
Tony

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Wednesday, March 24th, 2021 AT 6:06 PM