M931 a1 Rear Brakes Locking Up

[Artisan]

#1- Don't listen to me or anyone else, hire a heavy truck pro.

Make a GLADHAND X Your Shop Air Hose adaptor.
Chock All Wheels!
Make sure E Brake is on.
Connect adaptor to the DS front gladhand making sure it's valve is OPEN.
Make sure rear DS and PS trailer feed gladhand valves are OFF.
Fill truck w/ air then SHUT OFF DS front glad hand VALVE.
LISTEN FOR AIR LEAKS.
Release P Brake and listen again.

See any big pressure drops in just minutes?
if so FIND ALL LEAKS and repair.

Before you drive on road do a complete and comprehensive C.O.L.A.S. test
or park it till it is fixed right.

Every try THESE guys?

The air tank sensors have a short ground wire from the lower
sensor to the frame, was it a #55 wire? Pull it and make SURE
you are getting a good ground connection, reassembling w/ proper
internal/external star washer.

www.idahomotorpool.com has Failsafe Warning Control Modules for $100 I think.

I had multiple problems, I finally started replacing all parts, both the tank
pressure sensor switches and cleaned the ground and new FWCM and
finally all the lights worked as advertised. I checked for continunity on
the wire from the Pressure switch sensors to the FWCM too. I also
poked at the pin connections to make sure they were all touching there mates,
be careful.

With low or no air the red warning lights will come on at start up every time.

Make sure ALL THE AIR VALVES UNDER THE TRUCK ARE OPEN!

See note #1


EDIT; Johnny brake is 100% NOT APPLIED RIGHT?

 

[The HUlk]

Built air pressure on system to above 120 psi, alarm shuts off at 60 psi as should, dummy light also shuts off. Also troubleshot air system per technical manual with no issues present. Parking brake also shuts off when disengaged.

This sounds like correct functionality. When air is good and parking brake is released there is no buzzer or lamps. Correct?

The wire is pin A on the failsafe connector which is the engine ignition circuit. Theres no dummy lamp associated with this buzzer.

When you disconnect the PIN A wire you are removing power to the failsafe box which kills the buzzer as well as all other functions. This is correct functionality.

 

[Artisan]

I should have generalized better.

Do listen to huLK and others, just CYA stuff.

 

[Hollndr]

I believe i have proper function of air and parking... dummy lights go out when should.

Your very correct Hulk, didnt have time to fully check but yes my "a" wire powers the whole module, just went out and tried, no lights or buzzers at all. Back to drawing board. Prolly wont have more time for a while but will keep posted.

Thanks again.

 

[The HUlk]

I believe i have proper function of air and parking... dummy lights go out when should.

Your very correct Hulk, didnt have time to fully check but yes my "a" wire powers the whole module, just went out and tried, no lights or buzzers at all. Back to drawing board. Prolly wont have more time for a while but will keep posted.

Thanks again.

No problem. No buzzer or lights sounds like no power on PIN A or no ground on PIN H. Those are what give the failsafe box it's power. If you would like to speak on the phone when you are ready to troubleshoot again send me a PM.

I believe i have proper function of air and parking... dummy lights go out when should.

Does the buzzer still sound when the air low and parking brake lamps go out?

 

[Hollndr]

Well there was no lights (or sound) because i had cut pin A which unknowingly shut off all power to it as you suggested. It works when pin A is reconnected just alarm does not shut off ( no dummy lights and all dummy lights work properly)... so i would assume ground is good or the module wouldnt function similar to not having any power supplied to it.

Thanks, appreciate it.
Take er easy guys

 

[The HUlk]

It works when pin A is reconnected just alarm does not shut off ( no dummy lights and all dummy lights work properly)

This post is a reminder for myself to rule out the engine over temp sensor connected to PIN D, which was eliminated in most trucks, when you are able to get back at it.

 

[Hollndr]

That was actually my next wire snip had i not gotten fooled by the power source.

Any idea where the temp sensor was located? Or what this wire goes to? My temp guages all have been normal operating temps. I always rely more on guages than dummy alarms.

 

[The HUlk]

That was actually my next wire snip had i not gotten fooled by the power source.

Any idea where the temp sensor was located? Or what this wire goes to?

PIN D goes to an over temp sensor that is seperate from the temp gauge circuit. I believe the over temp sensor is located on the top side of the motor near the surge tank. It can be identified by one of the two wires showing 36 on the tag. This wire goes to PIN D, and the other wire is a ground. If disconnecting this sensor(wire 36) and putting ground on wire 36 stops the buzzer then the sensor is bad(stuck open circuit). There used to be a high temp lamp near the air and brake lamps. It may have been eliminated while the high temp sensor may not have been.

 

[The HUlk]

Did you solve the riddle hollndr?

 

[wheelspinner]

The sticking brakes (original issue) is most likely from lack of use. The wedge "polishes" itself by being used. When not used often the edges can get rough and hang up when applied. This is why whacking it with a sledge releases.

 

[Hollndr]

Did you solve the riddle hollndr?

Not yet... have a good feeling its the temp sensor as you suggested. Time was a little short (whats new) so ordering a new one to swap in and hope it takes care of issue. Will post what i find out, if not it doesnt hurt to have a spare...

 

[Hollndr]

The sticking brakes (original issue) is most likely from lack of use. The wedge "polishes" itself by being used. When not used often the edges can get rough and hang up when applied. This is why whacking it with a sledge releases.

Unfortunatly that didnt work, that was the first thing i tried... well a big splitting maul anyways, maybe i need to start carrying a bfh onboard. Im sure the system was in need of a good lube which is what i did and so far think it solved that issue. Fingers crossed. Thanks

 

[Hollndr]

No luck with the new temp switch itself, still getting an audible alarm. Need to check the wire itself yet.

So far this is what i have:

Failsafe module pins:
A - engine ignition circuit (power to box, assume if bad it wouldnt work)
B - blackout override input ( havent tested)
C - not used
D - engine temp switch ( installed new, no change. Need to test wire yet)
E - engine overtemp light ( dont think my truck has one, need to look if theres a wire in pin and where to)
F - air pressure switches (tested per manual, checked out good)
G - low air warning light ( works)
H - ground ( good or box wouldnt alarm)
I - parking brake switch ( works)
J - parking brake warning light (works)

New failsafe module was installed also.

Hoping its the temp wire to switch otherwise im down to blackout override or engine overtemp light. See what weather allows me this weekend to look at it.

 

[patracy]

M939 series sticking rear brakes

So once again my M931/SEMTT had a locked up rear brake. This time it wasn't a simple backup and it unlocked. I had to get onto it with a sledgehammer. But it got me to thinking. I *think* most of the problems with locking up rear brakes happen with the super singled tractors correct? This due to the fact that the drums are exposed to rain easily. So I'm thinking about making a simple "rain shield" for the rear drums. Just a piece of sheetmetal with two tabs sticking out bent at a 90*, then curve the larger area to match the profile of the drum. I'm thinking some short self tapping screws could be used to mount it to the drums. Anyone think this might be an idea worth pursuing? I'll mock up something this afternoon with cardboard to show what I'm thinking.

 

[wheelspinner]

It actually happens to both varieties. Especially the tractors but not exclusively. Most of the time it is actually the wedge that gets set up and hitting the drum frees it free.

 

[patracy]

Here's my idea in paper. And mocked up on the truck. Probably make it longer of course, more tabs, and cover at least 1/3 of the top or more.

 

[Jeepsinker]

That isn't a bad idea at all. Some grease in the proper place would help too, but tearing each wheel end down just to apply a little bit of grease is off putting at best.

 

[Mike929]

I haven't had an issue on my m931a2. How long does your truck sit between uses? Curious if I've just been lucky, or if I move my truck more.

Mine sits in the open and we had a lot of rain over last fall, but our summers are very dry.

 

[patracy]

Honestly it hasn't moved in some time. (Dead battery, flat rear) Just got it moving again. I'd say easily 4 months or more.