I've been fighting a abs/brake light issue for awhile now. When I first start the truck all is fine until I drive for about 5 miles or so, apply the brakes and both lights come on. I can clear the lights by restarting the truck and drive another 5-6 miles and the same thing. First brake press the pedal seems to travel slightly further then the second or third, etc. 
Here's what I've done to try to fix it. Replaced the rear diff sensor, replaced brake master cylinder (twice), replaced combination/differential valve, replaced the brake switch under the dash, had the abs module repaired by shop in Austin, TX. I've bench bled the BMC's four or five times now, also rigged up long clear hose from rear and front calipers into the bmc and used the brake pedal to bleed. I can't see any air in this system. 
The front hubs are not that old, maybe six years. 2-3 year old front rotors and pads. Seems odd that it takes driving a few miles down the road before the lights are triggered. If I apply the brakes every half mile or so they won't come on. When the lights are off the ABS system works. When the lights are on it doesn't. Truck brakes straight as a arrow. No pulling or other issues.  I can nail the brakes to activate the ABS and it works as well as this ABS system has ever worked. Stops fairly quick. And smooth. No lockup or excessive pulsing in the pedal. 
I'd appreciate any thoughts on this. I hate driving around with those lights on and no abs. Only other thing I can think to do is replace the two front sensors, rotors, calipers and pads. I sure don't want to do that if I don't have to since they are only 2-3 years old.  Also, FSM has a section that says there is a need for a DBR III tool to run a program where air might be trapped in the module. I've also read there is no need for this. Please help me avoid going to the dealer. They hate my 20 year old truck there. Always want to sell me a new one. TIA

Tone ring is damaged in one of the front hubs. I had them problem and after replacing both of the unit bearings the lights are out. 

Thanks admin for the quick reply. I've had a feeling it was something with the sensor or ring. Guess I'll get some parts ordered. As a old, retired helicopter pilot I can feel my blood pressure rise as those lights come on. Just brings back too many bad memories. Thanks again. 

Chuck

I've done the same thing... Brand new sensors make it 3 sets, ABS rebuilt by Module Masters in Moscow, ID. Then tested every wire 20 times. Reshimed the sensors at least 12 times. Changed the unit bearings and lights when off in 10 feet never came back on. Fixed. The tool that got my answer is the CCD Network tool I could see the right sensor dropping to zero at about 30 MPH. Changed the sensor and still doing it... Change the bearing.. fixed.

Thank you thank you. I had a feeling it was one of two things. Either air in the abs module where the FSM said it was necessary to run a program with a DRB III scanner to clear the air. But this light issue was happening well before I changed anything. Didn't add up. OR a sensor/ring issue that would take some travel down the road to manifest itself. I don't mind spending the cash to fix it right. I'm a believer in ABS when it works right. Even these older trucks. Has saved my bacon more then once. Thanks again, I'll update so this thread has some value going forward for somebody else. 

17 hours ago, Mopar1973Man said:

I've done the same thing... Brand new sensors make it 3 sets, ABS rebuilt by Module Masters in Moscow, ID. Then tested every wire 20 times. Reshimed the sensors at least 12 times. Changed the unit bearings and lights when off in 10 feet never came back on. Fixed. The tool that got my answer is the CCD Network tool I could see the right sensor dropping to zero at about 30 MPH. Changed the sensor and still doing it... Change the bearing.. fixed.

 

Edited November 19, 20177 yr by rotohead
correct spelling

I've greased mine and shortly after I had abs light come and go, every time it gets cold out like now in 30s light doesn't come on. I'm guessing grease is staying solidified and not getting where it shouldn't be.  maybe I greased them to late and bearings are on the way out just buying time now. I did changed sensors with no difference. Seems only when it's cold light stays off.

So I'm back looking for more suggestions. I replaced front hubs, rotors, calipers, pads, sensors and still get abs/brake light after driving a distance of more then 5 miles (I'm guessing). Had a shop scan for codes and do the ABS fsm procedure to bleed the abs system. Get the code 78, foundation brake which could mean anything from what I read. Differential switch as mentioned above has been replaced with new unit. First brake press seems to travel more then normal. If I brake every block or so (like driving through a residential area) no light get on a road and travel a distance (4-6 miles is my quess) and light will come on. Soft pedal. Then with light on pedal feels more normal. Pressure bled so many times and also run tubing from each caliper to BMC and bled with the brake pedal. No air to be found. This is driving me crazy. This differential/combination valve seems a possible issue. How should it be bled? Indicators would make you think there is air in the system but I'm 99% sure that is not the case. It's mentioned about a rear ABS valve. I can't find that anywhere on my truck. I'll try anything at this point. I've at least got a new set of brakes and rotors. NICE!! Thanks for any help. 

You need a live ABS tool and be able to watch the 3 speed sensors in real time. Being able to see real time data from the ABS module is key to finding out whats going on. Like mine was the tone wheel problem. I see you've replaced them. You might contact @Chris O. and ask him about the ABS code he might shed some light on it. 

 

As for bleeding brake system, I've never had to do anything special I use a mayo jar with a piece of vinyl tubing routed into the jar. I do it solo with no partner...

 

Edited November 30, 20177 yr by Mopar1973Man

Thanks, that's exactly what I've got for bleed container. 

What do you think about the combination valve bleeding? Seems to me when you bleed the system the differential valve will be displaced to a side when bleeding a front or rear caliper. Maybe bleed a front AND rear caliper simultaneously? There's supposedly a plug you can buy to temporarily replace the switch to keep the valve centered. I'm just looking for anything. I'm not on tweeter. The excess pedal travel initially seems to be a indicator that is part of the problem. How to bleed combination valve? is there a rear ABS valve in 4 wheel ABS systems? 

Never had to do anything fancy. Even after my system stand dry for days. Just a mayo jar and jump in the seat and pump the pedal slowly. Never needed the ABS bleeder software, never needed to mess with the combination valve. None of that stuff. 

 

Be aware the excess travel is most likely the ABS module running a dumping mode where it dumping pressure to prevent lock up. Even though the ABS light is not on the ABS unit is attempting to resolve a speed issue. This is why I said you need to be able to see all 3 speed sensor in real time. Like my truck was fine on all 3 sensors above 30 MPH drop below 30 MPH the right front axle sensor was dropping to ZERO so if I touched the brakes the truck jump to right and back pumped the pedal for a few second preventing you from stopping then ABS light pops then pedal was normal again. Being the ABS module only has 3 speed sensors you need to be able to watch them in real time then the problem will pop out. 

 

This is why I got a classified ad for used ABS sensors...

https://mopar1973man.com/classifieds/item/13-front-axle-abs-sensors/

Edited November 30, 20177 yr by Mopar1973Man

@Mopar1973Man is this life tool easily available or is it something special. My abs quit coming on now that it's cold out, the only thing I can think of is grease staying solidified and not messing with sensors. I did grease my front bearings awhile back and as soon as I did it i started to get abs light. Just doesn't make sense @TFaoro also greased his and I haven't heard him complaining. Maybe because he used a sensor hole and could only get so much in there, where I put a grease zerk and packed them full, I did get a lot of it out with a vacuum but there must still be a bunch left and somehow it's messing with sensors. 

If there is graphite in the grease it will mess with the front sensors. I had to clean mine out of grease to get it to quit. What happens is the speed sensors can sense the gap any longer and it will create a strange sine wave the computer can't use.

 

Worse case is you get to replace the front bearings again. There is no way to wash that grease out without messing up the bearings. This is one of the reasons I don't like this suggestion. 

Edited November 30, 20177 yr by Mopar1973Man

2 hours ago, Dieselfuture said:

@Mopar1973Man is this life tool easily available or is it something special. My abs quit coming on now that it's cold out, the only thing I can think of is grease staying solidified and not messing with sensors. I did grease my front bearings awhile back and as soon as I did it i started to get abs light. Just doesn't make sense @TFaoro also greased his and I haven't heard him complaining. Maybe because he used a sensor hole and could only get so much in there, where I put a grease zerk and packed them full, I did get a lot of it out with a vacuum but there must still be a bunch left and somehow it's messing with sensors. 

I'd guess it's because you're greasing from the bottom up. That pushes any metal from the bottom up around the sensor. I still grease mine heavily as I have one bearing puking grease. They're still tight as can be though. 

We did one of @trreeds a couple weeks ago and put 42 pumps of grease into it (It was very dry). He's then went to Minnesota and back without an issue. It's all about the metal.

1 minute ago, TFaoro said:

It's all about the metal.

 

Exactly... If there is any metallics in the grease your screwed...

You know, that sounds about right. Last time I greased it i pumped about 20 pumps and it all came out of sensor hole looking a bit dirty, I did it while rotating the hub till it looked clean, then I vacuumed extra out of abs hole. So maybe it's clean grease that stopped my abs from coming on and not cold weather. My bearings are also tight, I guess I'll just flush the grease out more often. Makes sense now, just a coincidence on cold weathermaybe.