955K transmission, converter problems

One day machine was fine, next day I had slow ground speed and low pushing power. Still moves in all gears in both directions. Checked trans oil and it was at full mark. Pulled filter, screen and magnets. Cut open filter and found nothing abnormal. Magnets had some very small fines and screen was clean. Checked pressures at speed clutch, at spec 355 psi. Directional clutch 300 psi, in spec. Torque converter inlet 15 psi, bad, should be 135. I pulled pressure control valve body and Selector valve body. All pressure relief valves and spools looked good and moved freely. Does this point to a bad converter? More test needed? Thanks for your help. Muzz.

Hi Muzz,
the 135 PSI for T/C Inlet pressure is a bench test only--it is a maximum safety pressure relief valve that should only ever come into play with very cold oil, max. Rpm., Stalled out condition and likely never to be achieved in service.
We tested the outlet RV at the Dealer by completely blocking the flow thru the T/C under strict test conditions for a second or two to ensure the T/C outlet RV did work as designed.
From memory as a rule of thumb if I recall correctly the operational T/C Inlet oil pressure is generally around 15 to 20 PSI above the T/C outlet pressure.

A check and report back of the T/C inlet and Outlet pressure and Lube pressure could point us to such cracked parts leaking causing low pressure in all 3 circuits.
Trying to think if there is a Thermostatic by pass valve in the T/C cooler line and if it could cause an issue--I think it simply shunts T/C Outlet oil flow away from the cooler when the oil temp is low, so would likely not be an issue.

From memory there is a spline in the T/C to do with the Stator which could wear away but if that occurs the engine will only lug very heavily and not drive well.

Sometimes if the operators platform gets impacted it can change the effective linkage adjustments to the Trans. control valves from the lever in the operators compartment gate and hold the Trans. valves in a partly engaged position for each Speed and or Direction this causes lower clutch pressures and so lack of drive due to slipping clutches.
Same impact damage could effect the adjustment of pedal seer linkage adjustments causing dragging steer brakes and/or slipping steer clutches--ditto if it is not pedal steer.

Another area of parasitic load could be a hydraulic circuit issue--infra red heat gun may show hot spots because of leakage--Hyd. pump outlet line high pressure due to restriction showing a high pressure reading with all valves in hold could show an issue with control valves not being centered--we checked all this on the unit I speak of and found nothing of issue.

Lower than expected T/C Inlet pressure could be due to a cracked Impeller or main case in the T/C--saw this when machines were not correctly warmed up before putting to hard work--admittedly these units were Compactors that had sunk into the ground overnight--the mud had dried in the wheel cleats etc. and scraper bars leading to high initial loads to get moving at the start of each shift.
We put pressure gauges on these and told the operators not to move off until a certain max pressure when stalled was seen--cannot recall now what it was.

Will think on it some more.

It has just come to mind that we had 955K or L that would be sluggish in drive we did all Trans. and T/C tests--all OK but engine stall speed was marginally down--Dyno. tested the engine, overhauled the injection pump and governor -- all at spec. including fuel usage rate.
Tried another T/C--no change-- checked for parasitic loads like rock guards on a machine working in mud with dried mud acting like a brake on the rollers etc.--none evident--binding in undercarriage using a Infrared heat gun for hot spots/components--none evident.
Client sold machine as it was costing too much to go further so we never found the cause that was thought to be a one off--was disappointed to not get a chance to find the cause of the issue and so rectify the issue--I hate to not know what was wrong.

Cheers,
Eddie B.

Check the torque converter outlet pressure. Should be 40-52 psi @ high idle, 3rd speed forward, brakes on and torque converter stalled.

I checked the converter out pressure and the trans lube pressure, both at 0. I pulled the torque converter drain plug and did a stall at high idle. Oil was POURING out. Looks like I will be replacing a torque converter.