• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

4L80e in Hummer started slipping when warm....

THEFERMANATOR said:
P0730 is an incorrect gear ratio, aka extreme slippage code. My best advice I can give is STOP throwing parts at it, and get a pressure guage on it. Or for a simple test, unplug the main electrical plug from the transmission, and see what it does in limp mode.
Click to expand...
Forget all that drivel from my last post. I was tired. And the day had crept by without hearing from you. I felt alone, abandoned...LOL
Your advice has not fallen on deaf ears!
I dont need to pick up the parts.
Im getting a pressure guage tomorrow if available. But first unplugging the trans to test limp mode. Nice!!!
 
I did pull the transfer case to fix some leaks and put in a new chain while the machine is down. 100 000 miles of pushing 7500 lbs. Cant hurt.
 
Good call on the tcase. I believe 100k should mandate a new chain. Did you do he fork
plastic-bushing-cover-I can’t remember the real name things?
 
"Fork plasticky thingos" 5 bucks bought me the multipack. Just got it on the workbench at 8pm when my wife brought me a G&T to remind me to come in. Ill split it tomorrow as an adventure.
 
As an Amazon Associate we earn from qualifying purchases.
Shift solenoid's read 20 ohms each.
Pan is back on.
Transfer case was interesting. An entire tooth missing on a differential gear.20180331_130503.jpg
 
As well the oil pump pickup was cocked funny and the chain had nearly worn through it. 20180331_131837.jpg
 
And of course the chain was noticably stretched compared to my new one. Found a front axle carrier bearing that was toast too.
 
Tomorrow I test limp mode. What should I do with a pressure guage? My Hummer service manual deals tons with codes but not pressure tests.
 
Should be a test plug on the driver's side right under the shifter linkage. Hook it up and observe readings and report back. You need a reading for every gear. And especially when it's slipping.
 
Thanks! Got a test guage. Nice one. Quick disconnect, numerous adapters, 0-100 and 0-400 gauges. 72" hose that goes right up to the cab.
Let me know if these look complete.
It basically works fine until 95-100 deg when it drops to 2nd and the pressue madly flutters.
20180401_195421.jpg
Let me know if the chart makes sense. Where I specify a range it covers moderate to heavy load(acceleration)
If I need more specific drive perameters at a pressure check let me know.
Ill need an assistant. The warnings with the kit are for real. Watching the pressure while driving and taking notes is deadly....
 
Limp mode is genius by the way. I knew about it but didn't know you could force it by unplugging the harness. What a great way to save a tow truck home.
 
Your pressure in limp mode is good in forward, but a little low in reverse. Max pressure in forward is 171, but max in reverse should go up to 324. Heres the pressure vs current table for a stock 4l80e. You should be seeing 110-150 psi at wide open throttle in every gear forward for holding power. Looks to me like you're losing some pretty serious pressure when not in limp mode.

There is a known issue with the 4l80e and the actuator feed limit valve wearing. When this happens you get low pressure because there isn't enough supply pressure to the valve body controls. But this normally showd up slowly over time, not out of no where.
 

Attachments

  • 0402182317b.jpg
    0402182317b.jpg
    82.5 KB · Views: 6
  • 0402182317a.jpg
    0402182317a.jpg
    82.3 KB · Views: 6
My reverse pressure was just at idle holding the brakes. I didn't load it. I'll redo that test.
 
And why the normal feel to everything up until 95 deg? Viscosity drops that sharply as it passes 100?
 
Pressure in limp mode R is 240 -300 psi. Rapidly fluttering between those 2 points.
 
Sand said:
Pressure in limp mode R is 240 -300 psi. Rapidly fluttering between those 2 points.
Click to expand...
That's not that bad. All indicators point to a pressure control solenoid issue, but you already tried one. The sharp drop off with temp sure sounds like a pressure regulator sticking. Either in the main regulator, or in the valve body for the control side. Unplugging the main plug verifies that the trans can build pressure still, so you're down to a control issue. Your problem is you've probably already toasted several clutches driving it with low line pressure, and it will need to be rebuilt regardless of what you find(once you slip a clutch disc, it will slip easier and easier each time). But if you don't find your root problem, then your rebuild will fail just as quickly.

You're probably going to need one of these kits to fix your valve body as it sure sounds like you're losing control pressure.
http://www.transgo.com/products.php...d=204&countdisplay=4&start=0&addcountview=Yes
 
Last edited:
I like it. Root of issues first. Good news is clutch packs are cheap.
 
You must log in or register to reply here.
Back
Top