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catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 2345
Experience:  Field service technician for CAT 19 years.
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I am not a backhoe mechanic I am an auto tech. my landlord

Customer Question

I am not a backhoe mechanic I am an auto tech. my landlord owns a cat 426 backhoe and he is having a problem, it will not move forward it will move backward and has all 4 speeds. this machine has not been maintained at all there are so many band aids on it .I am told that the electric shift from forward to reverse hasn't worked in years he manually shifts it and it worked that way all along the linkage is hooked up I see what appears to be a 2 pin solenoid that threads in to the top of trans right near linkage I unthreaded it and tested it by applying 12 volts to it it seems to function ok the little pin comes out with 12 volts applied and stays out as long as it has voltage.
Submitted: 2 years ago.
Category: Construction and Road Equipment
Expert:  catmastertech replied 2 years ago.
Hello,I am assuming this is an "A" model since you are saying the shuttle (F-N-R) shift is a linkage to the valve body on the transmission.The electric solenoid is not a speed or direction control. It is only a neutralizer to release the drive in forward or reverse.The four speed lever is just to a synchronized gearbox. The neutralizer buttons are use to aid in shifting under the fly or allowing all power to run hydraulics without the need to move the shuttle shifter to neutral.You do not need the neutralizer to work on the "A" model and most are left unplugged because a short will turn it on and you will have no drive. The most common issue I see is the oil level is not correct. Like in cars, the transmission is checked running. The check mark used for the engine stopped is only close to the operating level and will not be correct when the engine is running.This transmission level must be checked with the oil warm to hot, just like a car. BUT, it does not use ATF fluids. It must use a 30W transmission oil. (CAT TDTO [Transmission-Drive-Train-Oil]) The oil should meet or exceed Allison spec TO-4 and 30W is the recommended weight. If the oil level is correct and the correct type is used, I would next focus on the strainer. This is a suction screen behind a plug on the side of the transmission near the bottom. The oil must be drained to remove the screen.I would remove the screen and check it for contamination. Sometimes vandals will put dirt or other debris in the fill pipe and it will plug the screen causing the pump to have low pressure.Next replace the oil filter and cut open the old filter to see what is inside it for clues to a possible failure. Paper fibers or metal would be a bad sign if it is of a significant amount.If the filter and strainer are clean and the oil level is correct, then next step is to begin pressure testing. If you do not find the issue with these few items, please post back what you did find with the full machine serial number.I will be glad to help as much as I can.Thanks, Donnie
Customer: replied 2 years ago.
There were 2 drain plugs removed both found what looks like the remains of a snap ring on 1 of the plugs but no other metal cut filter apart no metal at all can't see a screen anywhere got specs for you the model is 426 serial#7BC01866 the trans group #6Y-2598 trans serial#68-0678 where is the screen?
Expert:  catmastertech replied 2 years ago.
Thanks for the serial number. I will attach an image of the screen below.I am guessing the oil and level was all correct?The screen is behind a cover, not a plug. Sorry, my mistake.
Expert:  catmastertech replied 2 years ago.
This is looking at the right side of the transmission. The cover is closest to the engine side.
Customer: replied 2 years ago.
Found screen fair amt of metal changed filter filled with new fluid checked it warm goes right into reverse no forward also I unplugged the 3 solenoids didn't make a difference
Expert:  catmastertech replied 2 years ago.
Thanks for the update.Do you have tooling to perform pressure testing?It appears you may find a failed transmission clutch.Will you be planning to remove the transmission and rebuild it?
Customer: replied 2 years ago.
I have an automotive engine oil and trans pressure testing kit and yes will be pulling unit and I may attempt or may send out I have the ability to rebuild automotive Granny's don't know how well that will translate
Expert:  catmastertech replied 2 years ago.
Dealing with the heavy weight and needing a press to work with the clutch springs is the main issue.Pressure testing will require adapters to fit "O-ring Boss" fittings and up to a 500 psi gauge. (I think max pressure is near 300 but, 500 is used for safety)
Expert:  catmastertech replied 2 years ago.
You said you had three solenoids?Where were they located?
Customer: replied 2 years ago.
1 was located on the trans shuttle valve 1 was located rearward of the shuttle valve and 1 was located on left side of machine just above the output to front drive shaft my tester is rated up to 600 psi and I have a 20 ton press
Expert:  catmastertech replied 2 years ago.
Here is a file to help you perform the testing.Verify the shift lever on the valve is moving fully to each stop and not over stroking. This could cause low pressure or no pressure if it is damaged.Check you clutch and pump pressures as shown here.
Customer: replied 2 years ago.
Is it possible that this problem is all in the shuttle valve just curious
Expert:  catmastertech replied 2 years ago.
Yes but, since you found debris in the screen, it is just as likely to have a failed transmission clutch.
Customer: replied 2 years ago.
OK am not at a computer or printer need to go home and print that file
Expert:  catmastertech replied 2 years ago.
Ok. If you need help with any of this, just let me know.
Expert:  catmastertech replied 2 years ago.
Have you completed repairs?
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