M931a2. Brakes lock with abs

[SETOYOTA]

This is just a FYI I have been meaning to post. It goes without saying you need to always be watchful while operating this type of equipment and be prepared for anything and even though all seems fine it may not be.


while traveling to the ssga14 rally I had a car pull out in front of me . Even though my truck has abs when the brakes were applied the truck began to slide with the rear tandems locked. The fronts were still turning and I was able to steer into the slide. Stopped the truck with no contact/accident.


The truck even shut down which all of this is what the abs is supposed to rectify in the 939 trucks.

i have no fault light on for the abs.

i guess my point is always be careful when rolling down the road.

I think I need to trouble shoot my abs. Any ideas.

I was pulling a m105 without the brakes hooked up.

 

[juanprado]

put an haldex info center on the abs connector port and check for codes. Does the light come on and turn off when starting the truck?

 

[SETOYOTA]

I will have to see if the light comes on when starting the truck. I don't think it does but not certain. Is it supposed too?

Where do you get a haldex info center? What is that?

 

[juanprado]

The light should come on for a few seconds when starting then cycle off. My guess is you have codes stored but the light is burnt out possibly.

http://www.haldex.com/en/North-Amer...ock-Braking-System-ABS1/Handheld-Info-Center/

The connector is the smaller one next to the ice? military diagnostic port on the passenger side under dash.

The info center appear on epay from time to time. A hd truck shop might be able to also pull the codes.

 

[Tow4]

My plan of action is to stay on the brakes until the truck stops if I lock up the brakes and the truck is doing anything other than going straight down the road. Personal experience has shown me that the vehicle goes in the direction of travel on a level surface when sliding even if the vehicle starts rotating in the slide (turns and sloped surfaces introduce other forces). Once you let off the brakes and the wheels start turning again, the vehicle is going in the direction it is pointed. If you have steered into the turn, you will have a possibly violent turn in that direction. If the engine is stalled, you may not be able to correct the oversteer.

 

[Suprman]

The info center diag tool is m939 specific as far as I know. The civ model looks the same but has a different connector. Guys want big bucks for them on the bay. I have had to swap the abs ecu in several trucks.

 

[98G]

My plan of action is to stay on the brakes until the truck stops if I lock up the brakes and the truck is doing anything other than going straight down the road. Personal experience has shown me that the vehicle goes in the direction of travel on a level surface when sliding even if the vehicle starts rotating in the slide (turns and sloped surfaces introduce other forces). Once you let off the brakes and the wheels start turning again, the vehicle is going in the direction it is pointed. If you have steered into the turn, you will have a possibly violent turn in that direction. If the engine is stalled, you may not be able to correct the oversteer.

Thing is, if you lock the brakes the torque converter can't release fast enough to prevent you from killing the engine. At that point you lose the power steering and the steering wheel is (frequently tly) jerked from your hand, resulting in high likelihood of a rollover.

 

[Doug Roberts]

If your ABS system operates , you have to stay on the brakes. If you are pumping the brakes the ABS will fail

 

[wheelspinner]

Check the ground point for the ABS/PCB under the hood. The system is grounded through the stud and spacers, some of which I have seen had paint on three mating surfaces, forcing the ground to float. Look at the PCB and you will see the grounds. Pull and clean.scrape all the four surfaces.

 

[Tow4]

Thing is, if you lock the brakes the torque converter can't release fast enough to prevent you from killing the engine. At that point you lose the power steering and the steering wheel is (frequently tly) jerked from your hand, resulting in high likelihood of a rollover.

I believe that is what I said. In an emergency situation, I doubt you can tell if the engine has stalled until it's too late. If you release the brakes and the wheels start turning, it will go in the direction it is pointing, and if the steering wheels are turned, it may make a violent turn (yes, resulting in a rollover). The idea of staying on the brakes if they lock up, is to try and prevent the out of control turn if the truck isn't pointed straight.

With that said, all situations are different and YMMV.

 

[Suprman]

The moral of the story is drive slow and carefully.

 

[74M35A2]

If your ABS system operates , you have to stay on the brakes. If you are pumping the brakes the ABS will fail

It won't fail, it simply won't stop the vehicle as effectively.

 

[mikey]

I also own an M931A2. You said the ABS worked on the front but not the rear. I believe that's due to the lack of weight / traction you have in the rear. The back of these trucks is so light that I sometimes need to engage my front axle just to get around my yard. Sometimes I laugh at the sight of the rear wheels spinning in the mirror on slightly wet grass.

Mikey

 

[SETOYOTA]

Keeping the front wheels straight and letting off the brakes is fine if your not headed into oncoming traffic.

i was maybe going 30mph at best..

 

[Jason O]

Keep in mind the m939 trucks are NOT ABS on the steer axle. They are equipped with a ratio valve which reduces applied air pressure to the steers, unless you really mash the pedal.

Another quick and dirty way to check your ABS -
Turn the engine switch from OFF to ON and listen for two sets of relay clicks. That is the sound of the ABS system testing the relay valves.

Next, build air in the system, hold the service brake pedal down, and cycle the switch again. You should hear each ABS relay valve bleed off 2 short bursts of air during the self test.

 

[M35A2-AZ]

I think the ABS starts working around 30mph.

 

[Hard Head]

I had a conversation with the man that developed the ABS for the 939 series. The issue is mainly that the trucks braking system is not designed for an empty truck. It works great when you have tons of weight on the truck. So ABS was added to compensate for too light of a vehicle. Several people were killed on secondary roads due to this issue. The system has 2 sensors that detect rotation of the tires and control the air to prevent the wheels from locking up. You can get the relays working right, the lights going on and off right and still have sensor issues. So every now and then it is good to just check them out in a controlled environment. He also told me that if your abs light comes on do not drive in 5th gear. The torque converter locks in and will stall the truck. Pucker power will result.