Machine pulsating on breaks

[Aussietech]

I have a Caterpillar 972M XE serial AN3
7800hrs
Operator complaint was when he puts his foot on the service brake the whole machine pulsates/surges, in forward and reverse,

No codes, No failed oil samples, no metal in filters. Service brake pressures are in spec,
Transmission charge pressure in spec

The clutch Fill calibration failed, 103 out of range,

Anyone have any idea what to check out next or
Ever heard of something like this

 

[Nige]

Has anyone checked the precharge pressure of both brake accumulators.?

 

[Aussietech]

Yes both are within spec
1950psi

 

[Nige]

Sorry but somebody isn’t testing them the right way. At 21 DegC ambient the nitrogen precharge pressure of the accumulators should be 800psi. Note this is not brake system pressure but the nitrogen gas pressure in the accumulators.

 

[Nige]

Do a quick and dirty test. Machine stopped, park brake on. Start the engine and let it run for a couple of minutes until the charging valve cuts out. Stop the engine then turn the key back ON. Press the brake pedal full stroke and release it. Repeat that process until the low brake pressure alarm sounds. How many pumps of the brake pedal does it take.? It should be three cycles of the pedal as an absolute minimum.

See attached.

 

[Aussietech]

Yes did that test, it was 8 times before the light came on,

Also checked the brake accumulator it's at 1960psi at 15 degrees C
Way to high

 

[Aussietech]

Update
Brake accumulators are 820psi at 10 degrees C

 

[Nige]

Also checked the brake accumulator it's at 1960psi at 15 degrees C
Way to high

I bet the accumulators were not fully discharged before testing to get that reading. Have to press the brake pedal until it goes hard to get a true precharge pressure reading.

Out of interest how many presses of the pedal does it take to put the low brake pressure light on.?

 

[DD57]

Those loaders have a neutralizing feature, when you apply the brakes. Is it possible, the pedal sensor is failing, or out of adjustment, and the operator is feeling the transmission coming in and out of gear? If you can, turn off the pedal neutralizer feature, and try it.

 

[Nige]

Have you checked the operation of the accumulator charging valve.?

Update
Brake accumulators are 820psi at 10 degrees C

That's a bit on the high side. Should be 770psi. Has anyone messed with the charge pressures recently.?

 

[Nige]

Those loaders have a neutralizing feature, when you apply the brakes. Is it possible, the pedal sensor is failing, or out of adjustment, and the operator is feeling the transmission coming in and out of gear? If you can, turn off the pedal neutralizer feature, and try it.

They all have that feature. The worst part is that it can't be disabled and what makes it worse is that there is no right pedal on your machine that operates the brakes without transmission neutralization. Bugger.
Are there any Diagnostic Codes for the left pedal position sensor.?
If you have access to ET then I would suggest doing a recalibration of the left pedal position sensor if there are no Diagnostic Codes present.

Another quick check. Is there any accumulation of material under the back side of the left brake pedal.?

 

[Aussietech]

Hasn't been any codes for anything,

Nothing under the pedal, Yes I have done a pedal calibration, no change

 

[Aussietech]

I had another mechanic with me this morning,

Had him look at the transmission with it was pulsating,
The whole trany is vibrating and pulsating, looks like something on the top of the transmission is pulsating,
Almost like it's clutch slipping,
But no fibres have been in the oil samples or nothing in the filters,
Failed a transmission fill calibration test, saying out of range,
Is there an adjustment for this or is the transmission on the way out

 

[Nige]

How many total hours on the transmission.?
Do your transmission oil analysis results include ISO Cleanliness Codes.? If so what results are you getting.?

Failed a transmission fill calibration test, saying out of range,

Do you happen to have a screen shot of the error message.?

Try running a Datalogger for 10 minutes or so while the machine is working. Look at the parameters for anything out of the ordinary or for numbers that, while genuine, don't seem within the "normal" range. e.g. converter out, trans intermediate speed, and trans output speed don't rise and fall EXACTLY the same way.

 

[Aussietech]

 

[Aussietech]

Transmission is original, 7859hrs

All I got from the office is yellow reading, I will find out more info from the service department

 

[Nige]

Try swapping the solenoid modulating valve for Clutch #1 with any of the others EXCEPT clutch #2 and repeat the transmission calibration test.?
If the problem moves with the solenoid valve you have a simple solution.
If it doesn't the solution may be more expensive.

Transmission is original, 7859hrs

All I got from the office is yellow reading, I will find out more info from the service department

With less than 8k hours I wouldn't expect the transmission to be even close to the end of its life, but I guess a lot depends on how it has been treated.

If you can get a copy of the last few oil analysis results (at least more than one) so much the better.
If you don't want to post them up here you're welcome to send them to me on a PM.

 

[Nige]

There is an accumulator on the transmission to aid in smooth shifting. Has anybody checked the precharge.?

 

[Aussietech]

At step 6 observe the pressure drop after been shut off,
There's no pressure there after shutting off the machine and getting everything up to temp.

 

[Nige]

At step 6 observe the pressure drop after been shut off,
There's no pressure there after shutting off the machine and getting everything up to temp.

That doesn't sound good.

From the Parts Manual drawing it looks like a small bladder accumulator. IIRC they are supplied pre-charged and no test/recharge is possible. Take a look at it. Does it have a charging valve that you can hook up your nitrogen charging kit to.? If it does try putting a whiff of gas into it but don't be surprised if the precharge pressure stays at zero.