Allis-Chalmers HD7G Crawler Loses Power/Regains Problem

westicles · Mar 26, 2020

I have an Allis-Chalmers HD7G crawler loader. Ran and operated great last fall. Then parked for the wet winter.

Runs and operates as should for about 2-5 minutes.

Then loses all transmission hydraulic pressure/transmission lube pressure. Then after sitting for about 30 minutes, will run and operate fine for another 5 minutes, before sitting still. Engine Runs perfectly, Bucket and arm pistons work great. Just won't move forward/reverse or turn.

Last spring when I bought it, I went through all the rotten hoses, all the filters, and screens, and did a full oil change on the transmission, and engine.

I dont believe its a torque converter issue because I think the torque converter picks up and provides pressure for the bucket hydraulics which are working fine.

There is a return screen that I can check, but that requires draining 24 gallons of transmission/wet brake oil (not 303), I've already replaced 2 out of the 3 hydraulic filters today. The screen is the lowest thing on the machine, the filters are the highest.

Anyone an old heavy equipment mechanic with some wisdom that might save me a $400 oil change with something they remember on these machines?

No oil leaks that I can see, Fluid at top level, starts out full pressure on gauges, and acts like its getting vapor locked or the fluid isn't returning to the sump after 3 minutes or so. If i let it sit five mins it wont do anything, 30 or more is required.

Nordic · Mar 26, 2020

What type and viscosity of oil did you put in the transmission when you did the oil change? I assume it operated normally before the oil change? It sounds like the oil could be
foaming and with air in the oil you won't get proper pressure. It is possible that if the oil you put in is very thin and the clearances in the transmission pump is marginal you will
have a loss of pressure when the oil gets hot. Had that exact problem with a John Deere bulldozer in the past. Because the pump was very worn it would overheat in short order
and then the pressure dropped out. It did well while cold like yours.

westicles · Mar 27, 2020

I put ISO 32 in it, the book calls for 10W....all the charts say it is equivalent but with these old machines, it may just not like the other oil, I can pretty much deal with anything as long as its not the torque converter.

It operated fine most of the summer after the oil change, but i didn't put more than 10 hours on it. The oil foam thing didn't cross my mind! It could be cavitating somewhere or sucking air making bubbles even if its not the type of oil.

Nordic · Mar 28, 2020

ISO 32 is very thin oil especially once it heats up. It should, however, be anti-foam in nature. I once talked to a guy that had a transmission failure in an Allis-Chalmers loader in
the sixties. The manual said to have 10W oil in it but once the repair was done the dealer told him to use 20W oil. After a number of years all was still well, no more trans.
failures. In most of my Cat power shift transmissions I have used 30W oil and that's what their manuals call for. The thicker oil gives better pressure and reduces metal to metal
wear protection. I don't know if on your machine you are able to drain the converter, if not you're still retaining a high percentage of the old oil in the system. I really don't think
you have a problem with the converter.

Jim.ME · Mar 29, 2020

You might ask over at Unofficial Allis site on the Construction Equipment forum. There is at least one member there running a 7G. He has just been doing some work to it and might have some pointers for you. Just another source to check.

westicles · Apr 2, 2020

I put AW32 back in it, same as iso32, or equiv of 10w like the book said.
I've been advised by some others, that in these particular models, if you go up to 20w, the transmissions can start to self engage for some reason.

After cleaning both screens, (one was clean, with 1/4" mesh) the other one (really, really, really hard to get to) was caked over solid.
The machine runs as it should for about 20 minutes, brakes and turns better than the whole time I've owned it.
After the 20 min mark, now the trans lube pressure drops out, but high pressure to the clutches stays in the green (before it was both gauges after 5 mins)

I can shut the machine off for 30 seconds, enough for the oil to drain back to the reservoir, and its ready to go another 20min. So that leads me to believe its not a temperature-related problem, or foaming, because 30 seconds isn't enough to defoam or cool off.

I'm looking through this 3 inch thick book trying to find if there's a restrictor valve or a check valve somewhere stuck open or closed, there's an oil cooler, and I don't believe it has a thermostat, but that could explain maybe a couple gallons of fluid suddenly dropping out of the system.

Nordic · Apr 2, 2020

Sounds like it could be something simple alright, like a stuck valve as you say. I guess you loose traction to both tracks? On some of my Cat track loaders the valve(s) for supplying oil pressure to release the steering clutches could malfunction. It would happen to one side at a time when it would. Generally it was a $2 spring that was broken inside the valve, simple fix and a cheap fix but had the appearance of being serious. Sorry I don't know enough about the 7G's to give more help. Take Jim up on his suggestion as there are some real experts around.

westicles · Apr 2, 2020

Been in contact with that guy already, if his handle is Dmiller. He certainly helped me find some screens and such that the book didn't even
mention. The machine is definitely improved since working with him, but still cant use it for more than 20 minutes without fear of running the
clutches dry.

westicles · Apr 2, 2020

Thanks for all your help and comments anyway, many minds are better than one, hoping to come across someone who has dealt with this problem before.

All the experienced mechanics where I live either worked on stuff before the 70's or after the 1980's. I took Jims advice as soon as he posted, and have been in contact with the guy he mentioned. Hes helped me a lot but no solution yet.

The problem with this machine and those like it, is that the brakes, clutches, transmission, and torque converter all use the same oil reservoir and pump pressure.

So a problem with any of the above could cause problems in all the rest.

It loses lube pressure first, the lines that pump for all the bearings and gears, then a minute or two later it gets low on high pressure...what engages the clutches and brakes and final drives and turning.

So i can be pushing a treetop across the hilly field, and as soon as i see the lube pressure start to creep down, i know i better drop the tree and get on flat ground because I'm about to lose brakes too. I cut the engine, and i can hear all the oil drain back to the sump. After about 1 minute, i can crank it up again and go 20 more minutes now. It was 5 before i changed the screens and filters.

Nordic · Apr 4, 2020

Please let us know what has been wrong if and when you solve the problem. I will just say one more thing. I had one Cat track loader where the transmission would not work properly and while checking the fluid level before start up it was at the full mark. Checking the trans. fluid with engine running it was way too low. Once I added oil to the full level mark while the engine was running everything worked perfect. Just be sure you check the oil level on yours with engine running... just sounds like the problem is a lack of oil. All the best to you while you problem solve!

Tigerhaze · Apr 22, 2020

(quoted from post at 13:36:52 04/02/20)
After cleaning both screens, (one was clean, with 1/4" mesh) the other one (really, really, really hard to get to) was caked over solid.
a couple gallons of fluid suddenly dropping out of the system.

Hi Westicles- I was wondering if you could answer a few questions about this as am going through the same issue with a 7G-B:

-From reading other posts and this one I know there is a canister filter top left and a suction screen bottom right. However wasn't clear where the other screen or extra filter were at.
-When you say the one screen was really hard, I am assuming that is the bottom right one but what in particular is hard- is it clearance to get the hose loose or another issue like a pan blocking it?

Thanks in advance.

westicles · May 22, 2020

(quoted from post at 15:10:40 04/22/20)

(quoted from post at 13:36:52 04/02/20)
After cleaning both screens, (one was clean, with 1/4" mesh) the other one (really, really, really hard to get to) was caked over solid.
a couple gallons of fluid suddenly dropping out of the system.

Click to expand...

Hi Westicles- I was wondering if you could answer a few questions about this as am going through the same issue with a 7G-B:

-From reading other posts and this one I know there is a canister filter top left and a suction screen bottom right. However wasn't clear where the other screen or extra filter were at.
-When you say the one screen was really hard, I am assuming that is the bottom right one but what in particular is hard- is it clearance to get the hose loose or another issue like a pan blocking it?

Thanks in advance.

I've attached 2 pictures and a rough paint diagram. One from above and one below with the skid plate and floor panel removed. The blue circle is the pain in the butt filter.

There is a plate with 2-3 hoses coming off of it underneath the cannister filter that is by your left foot under the floor panel.

You'll have to take the cannister filter off and the skid plate off to get all of it out, and probably stand on your head.

Theres one more tiny screen on some models between the torque converter and the transmission, some don't have it. Mine was clean and hadn't been checked since it was installed I'm guessing. 1/4 inch mesh screen doesn't catch much. Mine was within a foot of the torque converter on a small hose going toward the rear of the machine. Its about the size of a toilet paper roll.

Hope this helps.

westicles · May 22, 2020

The bottom right screen is really easy to get to. The top one is difficult because its just barely out of arms reach from the top or the bottom and a lot of hoses had to come off for me to get wrench clearance.

HoneyBadger11681 · Apr 7, 2022

Did you ever figure out the problem with your loader I have the same problem going on with the same 7G loader I have any updates would be appreciated thanks

HoneyBadger11681 · Apr 7, 2022

Did you ever figure out the problem with your loader I have the same problem going on with the same 7G loader I have any updates would be appreciated thanks

westicles · Apr 13, 2022

Not yet. Its been on the back burner. Got a divorce since I last posted. Ive got a brake band going out, Im starting to think its full of trash and needs a big flus out and new bands.

HoneyBadger11681 · May 30, 2022

(quoted from post at 02:48:49 04/13/22) Not yet. Its been on the back burner. Got a divorce since I last posted. Ive got a brake band going out, Im starting to think its full of trash and needs a big flus out and new bands.

Thanks for the update Ill let you know if I figure anything out on mine its got to be something simple

HoneyBadger11681 · Oct 15, 2022

i know you said you put your hd7g on the back burner but i was wondering if you ever got a chance to mess with it and figure anything out i have been working on mine and have got it to run longer but keep loosing pressure in the right brake

hector gemme · Oct 15, 2022

(reply to post at 07:18:44 10

/15/22)

Did you clean the big streaner in front transmission and part number is 70623268

HoneyBadger11681 · Oct 16, 2022

yes i did clean it and changed fluid i ve gotten it to run longer but can t get any pressure in the right brake maybe it s the brake bands? i dk where to get them yet or if that s it

hector gemme · Oct 16, 2022

Did you try to adjusted this right brake band and when you change oil did you flusch the housing and lines

HoneyBadger11681 · Oct 30, 2022

(quoted from post at 13:32:15 10/16/22) Did you try to adjusted this right brake band and when you change oil did you flusch the housing and lines

yes i have a broken brake band the bottom middle band been looking a round trying to find on been a struggle

hector gemme · Oct 31, 2022

I should have a good used brake band for HD7G

HoneyBadger11681 · Oct 31, 2022

(quoted from post at 06:57:19 10/31/22) I should have a good used brake band for HD7G

would you be willing to sell
it is it the bottom band if you do t want to sell it i understand just let me know plz and thanks

This post was edited by HoneyBadger11681 on 11/01/2022 at 07:07 am.

hector gemme · Nov 1, 2022

I am sorry am still have only one and it is the rear section and the other am still have are for HD7 GB and they not fitt on 7G.

Allis-Chalmers HD7G Crawler Loses Power/Regains Problem

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