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Old 08-01-2014, 02:26 PM   #1
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My ABS Light Story:

My ABS Light Story
The victim was a 2005 Winnebago Voyage 38J W24 Workhorse Chassis
On my way home from a long trip across the country, I was on a small country road and it was raining. I was going around 20 mph and I missed my turn and hit the brakes kind of hard and felt the ABS engage for the first time in the two years that I have owned this coach.

A few minutes later, I noticed my digital dash telling me BRAKE FAILURE CHECK ABS. Sort of freaked me out at first but everything still seemed to work. The brakes acted fine. I was only 100 miles from home so I did not bother to investigate.

Then the troubleshooting began. I unplugged the pressure differential switch fist. Light was still on. Then on to the wheel sensors, pulled all 4 out, cleaned them, pushed them all the way back in till they touched the tone ring per repair manual. Also unplugged wheel sensors and checked ohms, all four were in spec.

Now Iím thinking ABS ECM first I had to find the crazy thing. That took a while. Mine is located in front of the steering box. Now some units may have a Red push button that you can push and hold for about 4 seconds and a code will start to flash. I was not so lucky so I had to cut into the wire harness and locate the green wire coming from the black connector that is plugged into the ECU and splice a wire into it then hold that wire to ground. I got it to flash but nothing that made any sense per the repair manual. Then I thought maybe the wheel signal is not making it to the ECM. So I unplugged the green connector and tested each wheel sensor at the connecter for ohms, all were good. Then I put my volt meter to the AC scale and had my wonderful wife spin each wheel confirming voltage was being generated. A little over one volt confirmed they were working.

Ok so now itís time to hook it up to a computer. I am lucky to have a workhorse dealer nearby. It takes special Meritor Wabco software to read the ABS ECM it plugs into the round connector with a gray cap next to the OBDII connector. But in my case, we got no data stream. So time for a new ECM, ordered one Installed it plugged everything back up and the BRAKE FAILURE LIGHT IS STILL ON!!!

Now Iím starting to get pissed off at this thing. After two days reading and nearly going blind looking and tracing wire schematics I went back to step #1 and unplugged the pressure differential switch and low and behold, the light went out. Now Iím really pissed. Did I just replace an ECU for nothing?

So I put the old ECM back on just to make sure, and thank God the light came on. So now at least I know for sure the ECM was bad. Now back to the pressure differential switch. This is the switch that is on the cast iron part of the master cylinder. Behind this switch is a spring loaded shaft that monitors the pressure from the front to rear brakes, it should always be the same. On both sides, this should not be unplugged, it is there as a safety feature. If you hit the brakes and blow a brake line or fitting, the light will come on letting you know that something is wrong.

So now I have removed the switch and notice that I canít see the groove, it should look like an hour glass as you look into the hole, so now I know the shaft is pushed all the way to one side. Time for a new master cylinder, these are very common I have seen them rebuilt for $250.00
After installing the new brake master assembly, everything is now back to normal. No more ABS failure or brake light.

The final tally
ABS ECU $1163.92
Brake Master $350.63
Got This Problem Solved: PRICELESS
Lucky for me I have Good Sam Extended Service Plan and a local WorkHorse Dealer So my out of pocket was only $250.00 even though I did most of the trouble shooting in order to speed up the process.
I have posted this story so it may help someone else in the future.
PS Meritor Wabco has great service manuals at their web site.
If you need help troubleshooting yours give me a call 606-561-4983
Kevin Caudill
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Old 08-01-2014, 07:22 PM   #2
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Quote:
Originally Posted by ktoutoor View Post
My ABS Light Story
The victim was a 2005 Winnebago Voyage 38J W24 Workhorse Chassis
On my way home from a long trip across the country, I was on a small country road and it was raining. I was going around 20 mph and I missed my turn and hit the brakes kind of hard and felt the ABS engage for the first time in the two years that I have owned this coach.

A few minutes later, I noticed my digital dash telling me BRAKE FAILURE CHECK ABS. Sort of freaked me out at first but everything still seemed to work. The brakes acted fine. I was only 100 miles from home so I did not bother to investigate.

Then the troubleshooting began. I unplugged the pressure differential switch fist. Light was still on. Then on to the wheel sensors, pulled all 4 out, cleaned them, pushed them all the way back in till they touched the tone ring per repair manual. Also unplugged wheel sensors and checked ohms, all four were in spec.

Now I’m thinking ABS ECM first I had to find the crazy thing. That took a while. Mine is located in front of the steering box. Now some units may have a Red push button that you can push and hold for about 4 seconds and a code will start to flash. I was not so lucky so I had to cut into the wire harness and locate the green wire coming from the black connector that is plugged into the ECU and splice a wire into it then hold that wire to ground. I got it to flash but nothing that made any sense per the repair manual. Then I thought maybe the wheel signal is not making it to the ECM. So I unplugged the green connector and tested each wheel sensor at the connecter for ohms, all were good. Then I put my volt meter to the AC scale and had my wonderful wife spin each wheel confirming voltage was being generated. A little over one volt confirmed they were working.

Ok so now it’s time to hook it up to a computer. I am lucky to have a workhorse dealer nearby. It takes special Meritor Wabco software to read the ABS ECM it plugs into the round connector with a gray cap next to the OBDII connector. But in my case, we got no data stream. So time for a new ECM, ordered one Installed it plugged everything back up and the BRAKE FAILURE LIGHT IS STILL ON!!!

Now I’m starting to get pissed off at this thing. After two days reading and nearly going blind looking and tracing wire schematics I went back to step #1 and unplugged the pressure differential switch and low and behold, the light went out. Now I’m really pissed. Did I just replace an ECU for nothing?

So I put the old ECM back on just to make sure, and thank God the light came on. So now at least I know for sure the ECM was bad. Now back to the pressure differential switch. This is the switch that is on the cast iron part of the master cylinder. Behind this switch is a spring loaded shaft that monitors the pressure from the front to rear brakes, it should always be the same. On both sides, this should not be unplugged, it is there as a safety feature. If you hit the brakes and blow a brake line or fitting, the light will come on letting you know that something is wrong.

So now I have removed the switch and notice that I can’t see the groove, it should look like an hour glass as you look into the hole, so now I know the shaft is pushed all the way to one side. Time for a new master cylinder, these are very common I have seen them rebuilt for $250.00
After installing the new brake master assembly, everything is now back to normal. No more ABS failure or brake light.

The final tally
ABS ECU $1163.92
Brake Master $350.63
Got This Problem Solved: PRICELESS
Lucky for me I have Good Sam Extended Service Plan and a local WorkHorse Dealer So my out of pocket was only $250.00 even though I did most of the trouble shooting in order to speed up the process.
I have posted this story so it may help someone else in the future.
PS Meritor Wabco has great service manuals at their web site.
If you need help troubleshooting yours give me a call 606-561-4983
Kevin Caudill

THANK YOU for a very informative post. Hopefully I'll not need to do all you did to identify / resolve the indicator light warning. BUT, I'm a little confused, so please clarify which repair actually fixed the problem. It seems unlikely that BOTH the ECU and Master Cylinder failed at the same time, apparently simply because you used the brakes too aggressively for the conditions. THANKS
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Old 08-01-2014, 07:37 PM   #3
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My experience on the Ford and FL chassis I worked on was that you can reset the differential valve switch by pushing the "hour glass" plunger back to center. To do that you may have to remove a fitting; or, on some there is a rubber bellows type cap which can be pushed on to recenter. The "hour glass" pin completes the circuit to ground.
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Old 08-02-2014, 01:23 AM   #4
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Edgray
Yes it seems that both failed at the same time, that's what made it such a pain to troubleshoot. And to make sure I did not miss something I reinstalled the bad ECU just so I could confirm this.
I talked to a major bosh hydro max master brake re-builder and he said that RV's are more common for this pressure differential shaft to stick to one side and not go back to center.
Also in talking with the techs at Meritor Wabco this ABS System does NOT go through any type of self test, it only activates when there is a wheel slip. So when I hit the brakes hard on a wet road it may have been years since it had been activated, so I assume that's when something inside the ECU blew and at the same time the pressure differential shaft slammed to one side and would not come back to center. That's just my luck.....But I got to say I did enjoy the challenge of figuring this out. Although I would not want to do this every day.
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Old 08-02-2014, 01:52 AM   #5
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Parts replaced
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Old 08-02-2014, 09:48 AM   #6
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Quote:
Originally Posted by ktoutoor View Post
Edgray
............
I talked to a major bosh hydro max master brake re-builder and he said that RV's are more common for this pressure differential shaft to stick to one side and not go back to center.
Also in talking with the techs at Meritor Wabco this ABS System does NOT go through any type of self test, it only activates when there is a wheel slip. So when I hit the brakes hard on a wet road it may have been years since it had been activated, so I assume that's when something inside the ECU blew and at the same time the pressure differential shaft slammed to one side and would not come back to center. That's just my luck.....But I got to say I did enjoy the challenge of figuring this out. Although I would not want to do this every day.
Did the rebuilder offer any reason that RV MC are "more common" to have the spool stick to support his claim???

The reason I ask is : that type of comment was generally dismissed as unbelievable by many owners when we first started dealing with the infamous Bosch caliper fiasco. Lack of regular use was later determined to be a "contributing factor", but some remain unconvinced and still don't follow the recommended PM suggestion to replace the brake fluid every two years, or sooner.
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Old 12-26-2014, 08:01 PM   #7
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I am having a similar problem. Just after starting a 1000 mile trip the Brake Alarm light on the dash came on, along with the warning buzzer. The first thing I noted on opening the hood.was that the ABS Differential pressure Switch had mysteriously broken completely off the Master Cylinder, leaving the threaded part in the MC housing. I replaced that but the alarm condition persisted. A tech suggested that I could temporarily clear the alarm by grounding the wire going to the switch but that did not work. Now they WH Service Center is telling me I need to replace the MC but I have doubts because grounding the switch lead did nothing. Would appreciate any thoughts from you guys.
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Old 12-27-2014, 09:13 PM   #8
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this is Kevin call my cell any time I will try my best to help you.
606-875-0125 most nights i stay up late
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Old 12-28-2014, 01:44 AM   #9
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Thanks Kevin.

The tech at the WH Service Center advised replacing the Master Cylinder because he thought the piston was stuck all the way to one side. I might have gone for it in desperation but they did not have one in stock. After subsequent trouble-shooting on my own I question that diagnosis because I Have verified that the differential switch plunger is grounded which I believe indicates that the piston is centered. I am trying to come up with another fix while waiting for people to get back to work Monday. I am hoping to find a way to rule out (or in) the $$$ ECU.

I will try to call tomorrow (Sunday).
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Old 12-28-2014, 03:26 AM   #10
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Interesting the differential sensor should not light the ABS light, it should light the BRAKE light. (different warning lamp). Perhaps they did something different.

ABS on a Workhorse is a pain in the sensors.

NOT a workhorse but a Dodge Neon... (2001).. Last Christmas Eve (12/24/2014) I was picking up some meds,, No problem there but on the way to the pharmacy some idiot hung a left in front of me,, I spiked the brakes, laying a couple of nice black strips on the road (NO ABS on this car but still stopped straight) Missed him.. Well, about 30 miles later as I park the car at another store (Groceries) I notice excessive pedal travel.. On to get gas, Home to put groceries away and make a phone call,, The BRAKE system warning light (Differential sensor tripped) came on just as I pulled into the repair shop (Talk about timing) One new wheel cylinder and brake bleed later.. light off.

Moral: your foot is the best sensor.
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Old 12-28-2014, 07:59 AM   #11
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The alarm that came on initially was indeed the Brake. That was when I found the dangling switch. The condition that exists now after replacing the switch is that both the Brake and the ABS lights are on. If I disconnect the switch only the Brake light is on. If I ground the switch the both lights are on. That tells me the switch is seeing ground through the MC, which means that the differential piston is centered

Very confused.
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Old 12-28-2014, 11:02 AM   #12
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This may or may not help you, but it was a TSB put out by Workhorse as a result of those lamps being illuminated with audible buzzer after completion of the brake caliper recall.

1- With key in the On position and the engine off, verify that the Brake and ABS tell-tale lamps are illuminated.

2- Remove the electrical connector from the brake reservoir fluid level switch. If the Brake tell-tale lamp turned off, proceed to step 4. If the Brake tell-tale lamp is still illuminated, proceed to step 3.

3- Remove the electrical connector from the pressure differential switch. If the Brake tell-tale lamp turned off, proceed to step 6. If the Brake tell-tale lamp is still illuminated, go back to the beginning and repeat this service procedure and verify that DTC 86 is the only code set and all steps have been properly followed.

4- Remove the brake reservoir caps, and using a long wire tie or flexible plastic rod that can reach to the bottom and center of the reservoir, lightly depress and release the float
with the fluid level sensor connected. Verify that the Brake indicator tell-tale lamp in the dash turned off. Clear any brake related codes using the Tech2 and perform another test drive to verify brake performance. If the Brake tell-tale lamp illuminates or brake codes are generated, proceed to step 5.

5- Remove the master cylinder from the maxi-booster following the service procedures and cautions under Section 5 (Brakes) of the Workhorse service manual. Flush the
master cylinder reservoir and verify that the fluid level float moves freely before reinstalling. Verify that the brake lamp has turned off. Clear any brake related codes using the Tech2 and perform another test drive to verify brake performance.

6- Verify that you have properly bled the brake system and removed all of the air. Remove the pressure differential switch connector and the pressure differential switch from the master cylinder body. Note: This chamber is not filled with brake
fluid. With the switch removed, locate the notch in the pressure differential rod that runs through the center of the master cylinder body. The notch should be centered in the switch opening and in-line with the pressure differential switch actuator. If the notch is not centered in the opening, and while the brake pedal is being applied and released, use a pair of needle noise pliers to gently reposition the rod so that the notch is centered as described above. Reinstall the pressure differential
switch and attach the electrical connector. Verify that the Brake tell-tale lamp has turned off. Clear any brake related the codes using the Tech2 and perform another test drive to verify brake performance.
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Old 12-28-2014, 12:26 PM   #13
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This may or may not help you, but it was a TSB put out by Workhorse as a result of those lamps being illuminated with audible buzzer after completion of the brake caliper recall.
Thanks for the pricedure. I followed it and verified that the lamps turned off after removing the fluid level and differential switches but was unable to complete Step 6 because the Differential Switch port is too small to see determine the piston position or to get a pair of needle-nose pliers in to move the shaft as directed. Net result is that I do not glean any new information.

Do you agree that the fact of the Differential Pressure Switch seeing a Ground indicated that the piston is centered? Given what I have reported would your diagnosis point to a bad Master Cylinder, a defective ABS ECU, or something else? What would your next step be?
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Old 12-30-2014, 07:17 AM   #14
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Mills Supply carries the master cylinders, switches, hydomax brake booster and the backup motors. Here is a link to Mills Supply Workhorse Brake Parts
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