Grabbing brakes

[Goliath P. Jones]

Finally got the m939 running and driving again, then today when we made a run to the recycling center the truck started a new issue. When slowing down while braking the truck feels like it has the rear brakes are coming on strong and grabbing. It only does it the last part of braking when coming to a complete stop. The only thing that has changed is putting in the new protective control box. The box is for both the m809 and the m939, my truck is an a.b.s setup truck, is there a possibility that the box is not working with the a.b.s system correctly and causing the grabby brakes? Or is it likely just a problem with something else in the brakes? Thanks.

 

[Goliath P. Jones]

I noticed the abs dash light that's normally on is not lit up anymore. The brakes are grabbing really hard at the last part of braking. Any thoughts on how to correct this or get the pcb box to communicate with the system? Any thoughts or advice would be greatly appreciated. Thanks.

 

[juanprado]

The abs has a mini ato fuse in the wiring harness under the dash towards the middle if i recall correctly.

 

[Goliath P. Jones]

The abs has a mini ato fuse in the wiring harness under the dash towards the middle if i recall correctly.

Thank you very much for the info, I will definitely check it out. I am sure there is a strong possibility that I could have blown the fuse when I shut it down incorrectly. At least I am hoping that is the case.

 

[simp5782]

Do you hear the 2 clicks when you turn the battery switch on? Light should be on then after the clicks should go out and stay out unless there is a fault.

Grabbing brakes is from non use . Drive around and take a temp gun then check to see which one Is actually Grabbing more than the other. Then we can follow the road to diagnosis and correction if needed

 

[Goliath P. Jones]

Do you hear the 2 clicks when you turn the battery switch on? Light should be on then after the clicks should go out and stay out unless there is a fault.

Grabbing brakes is from non use . Drive around and take a temp gun then check to see which one Is actually Grabbing more than the other. Then we can follow the road to diagnosis and correction if needed

I am not hearing the two clicks at all. But will definitely check again to verify that. The truck has only sat for a couple weeks of no use, and I have been driving it several times before letting it sit for those two weeks. I'll get a temp gun today and get a reading on it to see which one is grabbing more. It grabs so strong that it feels like it is slamming the drum. I think it's coming from the back axle set. But will definitely check to be positive. Before when I was driving it, the abs light was always on, I guess that should have been an indicator that something was not right. It does come on for a couple seconds during start up now, goes out. That's when the grabbing issues began. I'll check the fuse as well of course. Thanks for the suggestions on the temp check, I'll get that done and report back with the results.

 

[juanprado]

ABS working or not should not cause the grabbing/pulling. Wes is right of another issue possibly that the brake shoes can rust to the drum with little use. Very common on the tractors since they are totally exposed most of the time.

 

[Floridianson]

ABS working or not should not cause the grabbing/pulling. Wes is right of another issue possibly that the brake shoes can rust to the drum with little use. Very common on the tractors since they are totally exposed most of the time.

Do not know if that's his problem. I do know I have to keep all the cans caged on my 1088 or they will lock up / rust up tight. Have to break out the BFH to roll.

 

[Elk1111]

What is your air pressure doing while this is happening?

 

[Goliath P. Jones]

What is your air pressure doing while this is happening?

Air pressure always remains fairly consistent at around 110-120psi. I think I may have misdiagnosed what is going on. As today I drove it a few miles down the road to get a load of dirt and when I pulled into the quarry yard I wasn't even touching the brake pedal and noticed when the transmission downshifted it hit hard again. I'm beginning to think that maybe the transmission is out of adjustment or something. Going to have to do a fluid and filter change on it anyway as the last time it was serviced was before it went to demilitarized auction 5 years ago and 5k miles ago. I have to hit the manual to see how to adjust it if possible. Though it still feels like its hitting or locking up at the back end of the truck. Maybe it's the transmission downshift sending it down the driveline.

 

[simp5782]

They all hard shift 2 to 1. It's the modulator cable itself. Ccc cable in Houma, LA makes a replacement that cures this.

Switching to atf helps some but it's still pretty noticeable.

 

[Goliath P. Jones]

They all hard shift 2 to 1. It's the modulator cable itself. Ccc cable in Houma, LA makes a replacement that cures this.

Switching to atf helps some but it's still pretty noticeable.

Okay, I'll use ATF in it when I do the change. And go with a Ccc cable, but when I say it hard shifts, it's more like a slam. I have thought that something is going to break or split, as it's just started to happen unlike any downshift before. I half expected to see the tailshaft come off the transmission or drop a driveline.

 

[simp5782]

It's typical you can adjust your cable but it won't help It's the spring at the cable base

 

[Goliath P. Jones]

It's typical you can adjust your cable but it won't help It's the spring at the cable base

So then getting the replacement Ccc cable should help? Or would I need to replace the spring at the cable base as well if that's possible.

 

[Goliath P. Jones]

Checking out Ccc cable, I see they have modulator cables for the mt600, I'm thinking that the transmission I have is the mt654? As I'm not familiar with allison transmissions. I can be completely wrong, does the mt600 cable modulator they have work on mine or do I need one specific to the mt654? I tried to call them, but they are on a different time zone so I missed them. I will give them a call early a.m tomorrow and get what I need ordered up.

 

[Goliath P. Jones]

Well after some looking at the cables, linkages, and whatnot I didn't see anything that stood out as obvious to being the culprit for the hard downshift into first. I even had the local Peterson cat heavy equipment repair shop take a look at it. They said with the exception of maybe one of the solenoids needing adjustment and that only being a possibility that all looked right. So I drove it home and it kept the hard downshift going. After I loaded it up to the hilt with multiple 1500 pound stumps and debris driving through thick deep muddy pastures it started downshifting smoothly again into first. A few heavily loaded road trips to the yard debris place and so far so good continued. Not sure what changed. But I'll keep my fingers crossed and hope it was just an air bubble or something debris wise in the fluid that passed.

 

[Floridianson]

I am still running engine oil in my 915 and it takes more than a few miles for my Trans. gage to even come up to below normal. It will go to normal when trailer is hooked up or running in town stop and go. You said you loaded it down an it seemed to shift better maybe you got the oil up to correct temps if you are still running engine oil. I also give it a 20 -30 min. warm up before I leave on a hot day first start. Yea switching to ATF would help.

 

[Goliath P. Jones]

I am still running engine oil in my 915 and it takes more than a few miles for my Trans. gage to even come up to below normal. It will go to normal when trailer is hooked up or running in town stop and go. You said you loaded it down an it seemed to shift better maybe you got the oil up to correct temps if you are still running engine oil. I also give it a 20 -30 min. warm up before I leave on a hot day first start. Yea switching to ATF would help.

With the time it takes to build up the air brake pressure, move the other rigs out of the way to get it pulled out of its parking spot it usually gets a warm up period before I take it out. I had asked the crew at Peterson cat to do a filter and fluid change on the transmission but they just checked the fluid, said it looked really clean and didn't bother to change it. Even though they said that they recommend it being changed from 10 weight to atf fluid. Which kind of made me mad that they didn't bother to do what I had asked of them. But I guess I can always do it myself, usually the only way to get it done right anyway is to do it oneself.