D8L transmission pump issue

Okay so little history, customer called and asked me to come rebuild the trans, this is his 3rd trans to go out in 9 years or less. (So I'm already thinking there is a underlying problem that has caused so many failures)

Everything was working before I pulled the trans but they said it would slip when it got warm and that they finally just ran it in second gear for the last 2 months until it was trashed as well.

So I pulled the trans, did a complete rebuild (new seals, bearings, plates, disc and pistons, as well as any other parts that appeared burnt) put it all back together, resealed the valve (found that the priority valve had been installed backwards by previous mechanic, my initial thought was that this has been the issue all along causing the failures since the customer said they have always just swapped the valve from one to the next, no rebuild) put it all back together and in the machine, fill it with 45 gallons of Cat Transmission Oil SAE 30 ( I found out they previously used hydraulic oil every other time, another sign to me that could have caused premature failure)

Start the machine and let it idle for a few minutes then go to shift, nothing, not a thing. So I pull the axles and make sure all looks okay, center two axles are spinning, reinstalled outer axles. Check for pump Pressure and find none, removed rear filters and hey are just as dry as when I unwrapped them from the factory and installed. Remove scavange screen on pump and cleaned it, lots of trash from previous failure, I forgot to clean it before reinstalling trans. Reassembled and still nothing, no pump pressure, no oil to filters, no oil to priority valve, no oil pressure at pump etc.

So I'm wondering, if the pump was working with the 10w hydraulic oil, is it possible it was weak anyways (assisting to the premature failures) and that now it will not suck up the 30w oil due to being worn out ?

Anything else I should/can check before throwing a pump on the girl? I know my customers initial reaction is going to be that it was working before I pulled it....I can't help but think I've overlooked something simple, but I spent an extra 6 hrs there today (mostly on my own time), check and rechecking things.

TIA!
Slim

Dunno mate.

I know nothing of modernish machines but it would be unthinkable to me that after three failures they are still running the same pump . . . I wouldn't imagine it would be a big ticket item.

Cheers.

From my point of view, and I may have missed somethings here, to rebuild a transmission in anything with out removing and inspecting the pump is not right.

Also any cooler and lines to and from it should be removed and cleaned, actually I would lean heavy on replacing the cooler unless system was extremely spotless inside.

Then every valve body should be stripped down and each and every spool inspected very close. One fleck of crud in an orifice will stop a machine dead. I had a 988 that had stopped moving once and the cause was a fleck of crud about the size of the period at the end of this sentence in one spool's orifice.

Some people, mostly certain bosses, think that just because these machines work in nasty and dirty conditions a little dirt will not hurt them but that is the farthest thing from the truth!

EDIT!!
Just make sure you took off all the caps and plugs you put on those lines to keep dirt out!!

Pump ended up being air locked, thank you for the replies.

Now I'm checking pressures, torque inlet is within spec, priority valve is within spec, but not getting any pressure to transmission lubrication....? It says 3 psi at low idle 26 psi at high is spec, so it's not much, but I'm not getting anything.

Check your reliefs if not hung open or if the priority valve is not slammed shut.

Sounds like you better pull that pump anyway and replace the cooler.
Lube pressure is created by excess flow from the pump that has gone through the torque converter and then the cooler.
What is your torque outlet pressure?

Oh! forgot one thing I try to ask for first off on just about every request for ideas. What is the S/N? Might not be any changes in systems but never can tell! Some people like some bosses I had thought that say all Cat 988's were the same machine. And it seems like on what most people call "dozers" there are more things different than the same even on a machine built the same year!

kshansen said:

Oh! forgot one thing I try to ask for first off on just about every request for ideas. What is the S/N? Might not be any changes in systems but never can tell! Some people like some bosses I had thought that say all Cat 988's were the same machine. And it seems like on what most people call "dozers" there are more things different than the same even on a machine built the same year!

Click to expand...

53Y S/N prefix

Mobiltech said:

Sounds like you better pull that pump anyway and replace the cooler.
Lube pressure is created by excess flow from the pump that has gone through the torque converter and then the cooler.
What is your torque outlet pressure?

Click to expand...

Thank you, I couldn't find in systems operation where it said where it got lube oil from.

Out of curiosity why replace the cooler instead of cleaning and inspection?

Thanks

Slim Tech said:

Thank you, I couldn't find in systems operation where it said where it got lube oil from.

Out of curiosity why replace the cooler instead of cleaning and inspection?

Thanks

Click to expand...

Generally this is because contamination gets caught between the internal tubes and there is no guarantee that all the foreign mater has been removed after cleaning . After working for a while it can work loose and start circulating back in to the system and cause problems
Mark