Next up -- ABS problems

[wyle.e.kyote]

Have about 2000 miles on the rig now. Got it home last week; washed it and put it away.

This morning when I started it up, the ABS light came on and stayed on. I managed to figure out where the blink code switch is under the LH luggage bay. But -- it makes no difference. The light doesn't blink regardless of its held down 1second, 3, of half a minute. The light in the cab and the light on the ABS ECU frame both stay on.

Also. the ABS has disappeared from the inventory my ProlinkIQ takes. Prior to today, it would show the DDEC, Allison and the Wabco. Now its only showing the DDEC and the Allison.

All the fuses seem to be good, so tomorrow morning I'll start chasing wires to see if its getting the right voltages into all the pins. (The Meritor Wabco book suggest that a solid ON means "Bad wiring") the steps are "1. troubleshoot wiring"

When it was visible to the Prolink, the following inventory was taken from it

Brakes, Power Unit - WABCO - Vehicle Info
Software Revision: D15 Serial Number: 002416 Manufacture Date: 22/1998 Part Number: 8840044200 Configuration: 4S/4M
ECU Type: Unknown
Make: Meritor WABCO Brakes (Air)





Besides chasing wires, any other suggestions?

[JIM KELLER]

Doug, Can't help on this one but sure do enjoy your sense of humor.

[wyle.e.kyote]

Got to have a sense of humor, getting mad never helps and at least this way its entertaining.

Oh well, breaks over, back to yanking on wires and listening for the whump sound.

[wyle.e.kyote]

Whelp.

Pulled, prodded, cursed, threatened with a hammer and water .. nadda .. I can drop power to the ECU and get the light to go out so its not as simple as a short to ground across the light. Nothing else seems to bother it and the trouble shooting guide is fall off after step 1.

The Wabco is getting power in the right places, and ground in the right places. The J-bus appears to have a signal (at least theres stuff I can see on my o-scope that looks like a signal) lacking the tools I can't decode it. At this point I have to presume the ECU has gone bad, though I find that difficult to believe that it waited 12 years to just crap out in my driveway. The part number on the case is 4460044200 but a Google search of that turns up several Chinese vendors presumably selling knock offs. Anyone have a captain midnight decoder ring and can turn that into a part number that can actually be found?


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[phorner]

For what it's worth, a couple of years ago, I parked the bus and all was well. Next time I started it, the ABS light came on.... and stayed on.

In fact, it stayed on until I got it to Prevost in Jacksonville. Turned out to simply be a bad ABS sensor at the left, front wheel position, which they handily replaced.

Turned out to be an easy fix.

[wyle.e.kyote]

Theoretically at least, you can find whats wrong with it by using the blink codes .. when its working that. Right now no amount of toggling the blink-switch makes the blink-light go out, it steadfastly refuses to blink. That and the Wabco disappearing entirely from inventory makes me lean to the gonna-be-over-priced-I-just-know-it wabco abs ecu.

[wyle.e.kyote]

Oh I meant to add this for the record

http://www.abstroubleshooting.com/ab...-abs-lamp.html

Doesn't help me, but hopefully it will help others.

[Coach Pro LLC]

My 2 cents:

In all likelihood it is one of the rear ABS sensors, and not the ECU or wiring. These are somewhat common to fail.

Your coach will have a Rockwell drive axle. The sensor location / mount is right at the cut-off point, but I am betting your ABS sensors are mounted internal to the axle. This is a bummer since later mounts are "piggy-backed" externally and far easier to replace.

The sensor is about the diameter of a nickle and reads the "ticks" on an interanal disc at the two inside drive hubs. That sends a pulse to the ECU which calculates wheel speed and in hard braking situations flutters or pumps the brakes rather than locking them up.

To get at the sensors a section of the drive train will have to be removed to expose the differential housing. (assuming yours are internal which I think they are). A tech can than ohm check the sensor(s) and determine which one has failed. At this point you will face the classic "since we've gone this far you should replace them both" discussion with the shop. I would agree with that and if it is the rear sensor you probably should do them both while you can get at them, the cost of the "extra" sensor being relatively small when compared to the labor to get at it.....

Notes:

Once the Wabco ABS is reset for any reason, the light will come on until the coach reches 5 miles per hour. After 5 MPH the light will / should go out for good. If not, there is still a problem.

When the light is on, the entire ABS system is disabled. This is not directly related to service braking, but you have no anti-lock brakes and it should be fixed ASAP.

If it does turn out to be a sensor, do not let the shop talk you into a sensor AND an ECU. The odds of both failing simultanioulsy are huge. Have them do the sensor(s) first and then test drive it yourself. I'm not saying anything, but be mindful of an unnecessary up-sell.....

So, my guess is one of the two rear ABS sensors. A good shop should have a firm diagnosis on this in an hour or so, after gaining access and testing the sensors.

Safe Travels,

Pat Sprenger
Coach Pro LLC

[wyle.e.kyote]

Hey pat.

I didn't Ohm the sensors as they are inductive sensors. I can't find a document to confirm, but I suspect they are hall effect devices and the metal from the shaft causes an inductive change. Anyway; I did lift the wheels that were attached to them and give them a spin watching for pulses on my scope. All of them seemed to generate the same basic patterns. I did this from the connector at the wabco itself so it should be good all the way without having to take apart the drive train. It does make me think I should get some connectors and make a handy dandy test your sensors without breaking down the bus kit. Anyway, you are correct that these are all internal and as such the disassembly is beyond my meager ability (not to mention kind of scary in that gotta crawl under the lifted bus sort of way)

At this point I'm going to take it to Prevost over in Fort Worth. I suspect its the ECU, but its worth the shop-labor $ to perhaps learn something if its not.

Also, attached is a way nice guide the Meritor-Wabco people sent me on how this system works. Its a good read in that bus-tech nerd sort of way.

Technical_Brochure_05_reduced 2.pdf

[Sticks]

After 4 years of ownership of our 2005 H3-45 Liberty, the ABS warning light mysteriously appeared about 3 times. Usually when wet. Later in the day after stopping ( for what ever) with the ignition turned off and then back on the light goes out and everything OK. Since it is so sporadic and infrequent I haven't bothered to delve into it . The Liberty people have indicated that this is not too uncommon but , yes, when the warning light is on you don't have ABS brakes.