Bobbing a 5 ton with ABS

[Russ344]

Hello, can a 5 ton with ABS be bobbed? I know the truck can be bobbed, but can the brake system be minus an axle and still be made to work? I've been told it's not possible. I want a bobbed 5 ton but need the auto transmission due to health reasons. Thanks!

 

[Derrickl112]

i have a m925 without ABS for sale if your interested.

 

[chess]

i was wondering the same when i bob my m923 i plan to keep the ctis and abs...

 

[tobyS]

5 ton bob by removing front axle

Look at a brake schematic when google-ing air hydraulic brakes. Looks like simply closing off the lines that feed the specific axle can simply be plugged, with no resulting failure to the remainder of the system. Although I have NOT done it and confirmed the opinion. Re bobbing a 5 ton chess, I'm looking at the suspension and believe a great addition can be air bags and installing a fourth (or more) member. My purpose is to remove some weight and stay under 26k cdl, whilst having a good platform for a knuckle-boom. Also want to come off the front of the engine for a hydraulic pump. Ever seen such crazy ideas carried out and discussed where a new 5 ton owner can join the conversation?

 

[rchalmers3]

I am familiar with automotive ABS, but not the system in the M939 series. Having said that,

I propose you look into pairing together the rear axle sensor lines onto the remaining rear axle sensor. That would be the easiest solution!

There are signal generators that can be purchased to replace the missing axle signal if you cannot devise a simpler solution.

Let us know what you come up with.

Rick

 

[tobyS]

Rick, I know what you mean, but that may be a short circuit, when and if an axle abs must act and it is electrically tied into the second sensor, which would not be indicating a locked condition. At this point I do not know for sure and one of the many experts on this forum may... but it appears each axle has a sensor and removal of one entirely, would not affect the performance of the one remaining. I imagine someone can correct me, but that's how mine appears, having received it on Wednesday and quickly going through some brake and trailer research.

 

[jonesal]

There is only one pair of axle sensors on the 900 series... making your conversion that much easier. Check out the ABS section of the TMs. Chapter 4 in the -23 series. Also check out the ABS WMO instructions. That will get you heading in the right direction. I recommend you test your ABS prior to any mods and then compare after the bob. Comparing those two, proof in the pudding concept, will be one method of troubleshooting. From the looks of it, you will get an ECM error if you delete one of the dump valves. Two options that quickly come to mind, you could dummy it in place with the appropriate plugs in the air lines. Or you could find the resistance values of the three dump valve wires and design how to trick the ECM into thinking its there. Not very difficult to do. That way your ABS light will work. And the ECM scanner tool will work too. The second method would need proofing as the powerup sequence of the dump valves may have variable resistance values when the ECM self checks. Making option one a potential better option.

That all being said, I would never modify any brake system on something as big as a 5 ton. That stuff makes me nervous...

Good luck.

Al Jones
'84 M923

 

[rchalmers3]

E-gads, I just thought of the liability being created in this conversation..... modifying brake traction control on a big truck...... something to consider if you join the road with the civvy world.

Rick

 

[tobyS]

Rick, every bobbed truck has to deal with this. The point is to do it correctly so the remaining system still works properly. My issues are abs and air system, not insurance. My insurer will be the judge for liability coverage. If your point is that an an insurer may not want to insure an altered vehicle, that may be another issue, but my comments are for research prior to making a change, which every bobbed vehicle must make. You might want another topic concerning insuring bobbed trucks. Having this conversation "creates" no liability, acting irresponsibly without proper and full information does. And personally, running around on my farm will provide my test and proving grounds, where the "civil" people confined to roadways are not threatened. I still want a proper functioning abs system, even lightening up by one axle.

E-gads, I just thought of the liability being created in this conversation..... modifying brake traction control on a big truck...... something to consider if you join the road with the civvy world.

Rick

 

[Suprman]

I believe there is a haldex guide on the abs system just like dana has for the ctis. I dont think it would be a problem with the ctis the system would not know you were missing an axle as long as you bypassed it properly. The valve body either sends pressure thru one air line to fill all 3 axles or sends a constant 12psi to hold open the wheel valves and allow the tires to dump air thru the quick release valves. The 3 axles are daisy chained and the 2 tires on each axle are connected thru that axles quick release valve.

 

[porkysplace]

Look at a brake schematic when google-ing air hydraulic brakes. Looks like simply closing off the lines that feed the specific axle can simply be plugged, with no resulting failure to the remainder of the system. Although I have NOT done it and confirmed the opinion. Re bobbing a 5 ton chess, I'm looking at the suspension and believe a great addition can be air bags and installing a fourth (or more) member. My purpose is to remove some weight and stay under 26k cdl, whilst having a good platform for a knuckle-boom. Also want to come off the front of the engine for a hydraulic pump. Ever seen such crazy ideas carried out and discussed where a new 5 ton owner can join the conversation?

The 939 series 5 tons are full air brakes, not air over hydraulic

 

[tobyS]

I was reading about air over hydraulic because of a trailer that has hydraulic ...which included a truck schematic that I believed was a five ton ....and the air to hydraulic booster. Perhaps not, I'll look into it further.

 

[Suprman]

It's definitely full airbrake. I don't see how removing an axle will compromise braking though. Other members have bobbed 5 tons there was a bobbed a2 advertised for sale here a while ago it was painted a snow camo pattern. I think the guys name was Jason. If it were me I would ask him what he did about the abs and ctis.

 

[smittyjr355]

I am bobbing a m931a2 right now. It has the abs and ctis. Like stated earlier the 900 series only use one rear axle for abs sensing since both rear axles are "locked" together by the drive shaft. So when bobbing the axle with abs needs to be used. In my case I used the most forward rear axle and will be switching the abs sensors to that axle. There are two abs brake valves that supply air to each side of the axle. So each valve supply air brakes to one side of the truck. So when removing a axle simply put a plug in each valve for the axle removed. Now as for the ctis the rear ctis valve can simply removed. Hope this helps and isn't confusing.

 

[jcappeljr]

I bobbed 4 =939 series 5 tons,I am currently doing one now.I never had any braking problems after removing 1 axle.

 

[tobyS]

John, re "bobbing"...have you ever retained the link suspension and rear axle and put on air bags, rather than going to leaf springs?

 

[350TacoZilla]

I just finished bobbing my 923 down and had same questions with abs but like stated above, you keep the axle that has the sensors, plug both control valves to the sensors in same order they were originally and then plug the ports that went to the other axle. I also had to fiddle around with turning the brake cans on axle to clear springs but with both brakes caged it wasn't a tough job, just lots of hoses to re-route. I also noticed on start up that my abs light comes on then goes out like stock and verified ABS to still work by doing a 40mph full brake stop in field, no stalling and brakes pulsed back and forth on axle till stopped.

I had thought of using the cradle/3 link and just using air bags but you would probably have to add a panhard bar to keep side to side movement down and it would be alot of work to slide the tandem setup forward to be able to use the arms and cradle for the rear-most axle. I'm currently trying to figure out how to add my shocks on the back as they seem to interfere with the air brake cans no matter how I look at mounting them (using stock mounts).

 

[jcappeljr]

I always use 5 ton front springs.