Cat 943 drive problem

[Aussie Gold]

G'day from Australia to the "Old Country",

I have recently purchased a Cat 943 with about 10200 hrs on it - or so the meter tells me. It's a hydrostat drive system. I use it for pushing dirt in the West Australian goldfields. I've added only 100 hours to it since I bought it. It has now developed a problem that becomes apparent only after 2 hours or so of operation. It was first apparent in reverse but is now obvious in forward. I only just managed to get it out from the bush onto a low-loader the other day to get it back to the nearest town.

Even back when I bought him and first used him, I was never able to run the engine speed lever in the “rabbit” fast position – in the detent position, without the machine slowing a lot. If I put the engine speed lever into the detent, which is where it should be when operating, the machine would barely move. I’ve always had to run at about 3/4s of the way to detent. The previous owner said that where he always ran it as well. At first I thought it was me not knowing how to drive it, but it seems that the problem may have always been there.
Whilst I strongly suspect the Hydrostat drive pump (motor), here are the symptoms and my observations. I’ve had a bit to do with oils and hydro pumps many years ago – but I’m no expert.
As above, reverse speed drops to near stop after a few hours of working. Forward speed now drops in a similar manner - now, but didn't at first.

I’d expect some very slight drop in speed if the oil was hot due a the slight drop in oil viscosity at higher temperatures. But nothing like this problem. The problem manifests itself after the trans oil temperature hits about 60 to 65C. This takes about one to two hours of driving but less time when pushing hard.

Hydrostat oil tank level is within the correct range
The “forward – park-reverse” linkage appears to be correctly adjusted.
No adjustment to the above linkage has ever been attempted by me. I just removed the clevis pins and checked the lever movement versus the selector arm at the hydrostat selector (under the bonnet)
The hydrostat oil is “bubbly” when checked with the engine at idle speed.
Engine speed and power are both OK. The revs are there, the power is there but the power to the the tracks isn’t. It just drops away as the day goes on.
I don’t know where the hydrostat filter sits in the hydro drive line – suction filter or discharge filter?? The oil would be colder and thus more viscous in the mornings so it would have greater difficulty getting through that filter if it was dirty. BUT – the drive is OK with cold oil. So I doubt it’s the filter.
The oil looks a bit bubbly or foamy, could be the oil is breaking down when hot?
The oil’s viscosity is breaking down when hot??
My first suspect is “tired” hydro drive pump – oil is hot, pump pistons or swash plate are worn, slippage through the pump? If it is the hydro drive, I expect that to be massive dollars and I’m not convinced the rest of the machine is worth such a major job. It’s a workshop job from what I can see and I couldn’t fit a new drive myself as it’s far too big a job for the bush.

Any ideas for a simple fix or is this terminal?? Thanks

[Thermoguard]

Hi Les,
Hey, I'm new to this stuff too but just a thought: Foaming oil to me suggests air being entrained in the oil. Could it be sucking air into the system from a leak on the suction side of the pump? Are there any external pipes or hoses between the oil reservoir and the pump? If all this stuff in inside the transmission cases, it would be a lot harder to check. Ian

[Aussie Gold]

Ian,

Thanks for the response. Today, I had a look at the Caterpillar Hydrostatic drive documentation. These hydro drives are plenty complicated. In response to your suggestions, the hydro drives are a bit like an auto tranny in a car in that they are self-contained with a "wet sump" configuration. There are no suction hoses etc.

The more I read of Caterpillar's troubleshooting guide - all 38 pages of it and Cat's "Principles of operation and repair", all 96 paees of it, the more I realise that even a competent (ex) mechanic such as myself hasn't got a snowball's chance in hell of diagnosing this fault. There are simply too many interlaced and interreacting factors that can cause a problem. Without a workshop environment and the right diagnostic equipment, it's a job for Cat trained workshop experts.

I'll change the transmission filter and if that doesn't work it'll have to go to Westrac in Kalgoorlie for diagnostics.

Thanks all the same.

Les

[jcb4cx]

welcome to the forum , TCtractors may be able to give you some info

he is on here from time to time

cheers graham


_________________
graham in hook

[shovel man]

Hi Aussie Gold,
have you fixed your 943 transmission problem yet ?

shovel man.

[SRB]

I think i remember something about leakage on some internal pipes, if you pull the lid off i think there is a couple of pipes in there, (think they are something to do with underspeed override? - really scraping back of the old grey matter now) anyway, with the lid off, there should be the linkage visable also, just check nothing untoward with it, your problem sounds like air or a leakage problem, if there is no puddles under the machine must be internal leakage.....

Happy hunting!

SB