2nd Gen Non-Engine/Transmission RWAL Brakes on 96 2500 - ABS light woes

[Rogan]

Ok, here's the nickel tour of what I'm trying to fix. .



ABS/Brake lights come on, but ONLY AFTER the truck has been sitting stationary (stop sign, traffic light, after startup, etc. ) for about 20 or so seconds.



Scenario 1:

Start the truck in the driveway. . Sit and idle for about 1/2 minute, ABS & BRAKE warning lights illuminate. They stay on until the ignition is cycled.



Scenario 2:

Start the truck, and start moving before the light comes on, and it's good to go. ... Until you stop at a light or whatever, for 20-30 seconds. . Then it comes on. Again, it'll stay on until you cycle the ignition.



Scenario 3:

Start the truck, immediately get it moving. Only stop/go at stop signs, with no delays, make it to the interstate, drive 2+hrs, NO ABS/BRAKE warinig lights. . Hit a few stop & go's with only a few seconds of stationary time at each, lights STAY OFF. Stop at next light for 20-30 seconds. They come on.





NO check-engine lights, no codes pulled, nada. Brake fluid is fresh and full. All connectors reseated.



I am at a loss as to why this is happening. And not being any more knowedgable with brakes and ABS, aside from changing pads/shoes, etc. , I have no idea where to start.



Does anyone have any idea what/where to check next?

What is the little 'solenoid-looking thing on the driver's inner fenderwell, with a vacuum line connected, and a 3-pronged harness?



I've tested the light coming on at different stationary idle speeds (thinking it might be vacuum-related) of 650, 1000, 1500 rpm, and it still does it. All other vacuum operations seem fine (cruise control, etc. )



I REALLY need to get this fixed, as state inspection has to take place this month, and it'll fail like it is. .



Regards,

 

[HHhuntitall]

There is a vacuum sensor on the driver side fender. It could have gone bad, but most likely, and the most common that I see, is the ABS sensor mounted in the top of the rear axle is bad. It reads the ring gear on the rear axle to sense when it locks up. The sensor is around $15, if I remember right, and the Borg Warner part number is ABS101. The magnet that reads the teeth on the ring gear rusts over time, somehow. It's mounted in the very top of the differential housing, and uses an O-ring seal, held in by a metal plate and one bolt. I think the 4x4 models have a metal shield bolted down with the sensor. Occasionally, I see the ABS modules go bad under the master cylinder, but they don't appear to be prone to go bad.

 

[Rogan]

Thank you. I'll check the sensor tomorrow.



Do you know the function of the vac sensor? Does it only check for vacuum signal?

 

[HHhuntitall]

I believe it only checks the vacuum. It only has a two wire connector, and can easily be located if you don't have hydraboost brake system, as it runs directly from the vacuum brake booster to the driver fender where it goes to the sensor. I can't remember where it's located on the hydraboost systems, I beleive in the same place.

 

[Rogan]

Ok, thanks, again!



I'll be pulling the sensor out of the rear diff this evening, and report back my findings.

 

[Rogan]

My buddy brought his Snap-On scan tool by, this morning, to scan the ECU for the ABS codes.

Yeah, the ECM doesn't doesn't store readable codes, like the engine stuff.

To read the codes on 96 and older trucks (and apparently '97 ECMs as well) :

1. turn key to the "on" position.
2. for just a second, ground pin # 13 on the diagnostic plug located on the knee bolster (#13 is a black wire, it's the 4th from right bottom row).
watch your ABS/Brake light for codes. Start counting with the first long flash, end with last short flash. There are only 13 codes.


1. Not used.
2. open isolation valve wiring or bad control module.
3. open dump valve wiring or bad contol module.
4. closed rwal switch.
5. not used 4wd. over 16 dump pulses in 2wd.
6. erratic speed sensor reading. (quite common)
7. electronic control module fuse pellet open, isolation output missing, or valve wiring shorted to ground.
8. dump output missing or valve wiring shorted to ground.
9. speed sensor wire resistance/ high reading.
10. speed sensor wire resistance/ low reading.
11. brake switch always on, ABS light comes on over 40mph.
12. not used.
13. electronic control module phase lock loop failure.
14. electronic control module program check failure.
15. electronic control module RAM failure.

 

[HHhuntitall]

I believe the 97s had OBD II, which is the stored engine diagnostic codes. Later made '96s had it, but earlier made '96s didn't, it was the original OBD. Lot of good it does, huh?

 

[Road Dog]

The ignition switch toggle trick and basic code readers only read the ECM. It takes a more expensive scan tool to read other modules in the truck. The very first step is to check the brake fluid level. If it's low (and even around MIN) then the ABS light will turn on. On later model trucks the ABS sensor is also signals the speedo but I'm not sure about 97s.

 

[HHhuntitall]

Dead on on the brake fluid level Dog. I presume that was checked..... and the speedo is run off of an analog field generator on the back of the transmission or t-case just like the 1st gens until 98, when it all goes through the ECM and ABS system.

 

[Rogan]

I replaced the sensor and disconnected the batteries for 30min. Still does it. Fluid level is fine.
I followed my posted procedure above, and got 9 short flashes (wire resistance too high)

Where is the abs module? Can I run temp wires from sensor to wherever they go to (i dont know where)

hh, can u call me? I'll pm u my number.

 

[HHhuntitall]

Rear Wheel Anti Lock brake diagnosis, ABS Light and Brake Light on

This is really for Rogan, but if anyone else finds this helpful, please use it. Rogan has checked the Stop Lamp fuse, Brake fluid level, wiring connections, vacuum and sensor, and replaced the RWAL WSS(at my erronous advice). DTCs were checked, cleared, and checked again, with the only DTC #9 being flashed. That leaves us to RWAL diagnosis Procedures, Test 7. Note that DTC codes 9 and 11 clear with each key cycle.



Rear Wheel Antilock brake diagnosis, DTC 9.

This code usually identifies an open in the RWAL WSS (Rear Wheel AntiLock Wheel Speed Sensor) circuit.

1. Disconnect the CAB(Controller Anti-lock Brake, located behind the dash panel under the ashtray, in front of the 4wd shifter, to the right of the knee panel) 14-way connector.

2. Connnect a voltmeter between cavity 14 and a good ground.

3. Set the scale of the voltmeter to register approximately 2 volts DC.

4. Turn the ignition switch to the RUN position (engine off)

5. Turn the PARK lamps ON.

6. Apply the service brake.

7. Move the shifter lever to REVERSE.

Conclusions:

1. If the voltmeter is registering 0. 05 VDC or greater, then refer to the TSB for proper repair of connectors that may have moisture contamination(I believe this is behind the driver front tire).

2. Open circuit between terminals 13 and 14 and the RWAL WSS.

3. Poor connection in the 14 way connector

4. Poor connection at the RWAL WSS

5. RWAL WSS

6. CAB

The RWAL WSS should have a resistance between 1000 and 2500 ohms.



No pictures available, as I don't have any trucks with the dashes out right now on the yard.



Thanks for the weekly puzzle, Rogan!

 

[Rogan]

Thanks, H! I greatly appreciate all the time you spent on the phone with me. And yes, it is quite a puzzle, but I think it's more like a monthly puzzle, given it's obscurity.

I'll try to run the diagnostics you've posted, tomorrow. It is quite weird, given how it trips the light(s). .

guess I'll be 'punching that hole' in the dash HAHA. I'll see if I can access it another way, and take pictures as I go (so long as I remember to keep taking pics. )

 

[HHhuntitall]

Yeah, I forgot to post that it appears to be accessible from the knee/kick panel under the steering wheel, sorry. I was in a hurry to get some stuff done before dark. You're more than welcome for the help. I'm sorry I had you buy some parts you didn't end up needing..... hopefully the voltmeter will tell you what you need to know. Getting back to the TSB on wire corrosion, you might pull that connector apart behind the driver tire. If the positive was corroded and spliced around, some of the others might be, too. If that's the case, it would explain the problem. Once you start moving, the RWAL WSS generates enough current to get through the corrosion and make contact. When sitting still, it can't, so it triggers the fault code..... just a thought, maybe before you tear the dash apart. The white wire is the negative, and if it can't get a ground, or is only grounding with a little more voltage, it would explain the problem. But I've been wrong more than once today..... and on the same subject, dangit!! Let me know, either way... .



God bless you and yours,

 

[Rogan]

well, due to the rain, I never got to do the diags on it.

On a positive side, I posted the truck for sale, and it sold fast! I think I asked too little for it LOL

HH thank you very, very much for your help and brainstorming with me! I'll be down there in November, so we'll have to find something cold to drink while I'm there; my treat.

 

[HHhuntitall]

Well, I'm all up for a cold, adult, beverage. Call ahead, let me know, so I can be around. I've got to guide a Mule Deer hunt the first of November in and around La Plata, NM. Guide some hunts to get a hunt. I'm really liking the idea of some time off away from the ranch and shop!!



God bless,

Casey

 

[Rogan]

I'll be there week of and week after thanksgiving week

 

[KRStone]

Revival!



What was it? How did this turn out?



KRS

 

[HHhuntitall]

Which part? The ABS light or the hunt? The truck was sold on page 1 before he found out what was wrong with it.....

 

[KRStone]

Awwwwwww carp. Same problems as mine.....

 

[HHhuntitall]

Wellllllll, did you go through the diagnosis listed on page one?