jalfredprufrock

Found these line pressures....readings at idle. Trans won't downshift....just shudders. But it hooks up strongly...just a downshift issue. What do those numbers suggest? Thanks for any input!!
P = 74 psi
R = 100 psi
N = 74 psi
OD = 74 psi
3 = 74 psi
2 = 130 psi
1 = 76 psi

B52bombardier1

Those look considerably low to me but I'm no expert. Reverse with those numbers is the big hint for me. That should be a lot higher.

Rick

FranksCustomTrans

The number look fine for just idling .
"I Am assuming truck or gto with 2nd gear start capability" Now WOT pressures would be more useful.

jalfredprufrock

In a 1995 GMC K1500 pickup.
A friend of mine drove it 40 miles on a very low fluid level (bad news!!)...that's when that downshift issue began (perhaps I'm lucky that's all the issue I have!!).
It hooks up solidly...put it in any gear and you can feel it engage. No slippage and it'll get the truck moving no problem. But instead of downshifting, or even accelerating, when I step on the gas, it just falters and 'sort of' shudders. Fluid looks and smells fine. My gauge 'hose' is too short for WOT reading...or maybe I don't know how to do such a reading.

Resigned to pulling and disassembling it...a shame though; it ran so well after I rebuilt it in 2017 (thanks to the direction and guidance of a lot of people on here and the recommended manuals). 20 k miles since the rebuild, and no issues. Someone on here pointed out that transmissions do not like low fluid levels...they are not an engine.

Appreciate the reply.

FranksCustomTrans

Still not real clear on the downshift "issue" or what that means. Simple check for high line is unplug harness from trans then check pressure at about 1500 RPM should see 150 plus PSI depending on build.

jalfredprufrock

Drove it around town today, after warming it up and making sure the fluid was topped up. Engages nicely. Fluid looks and smells fine. Shifts fine, too.
But what I call a 'shudder' is there and worsens the more you drive it. I brought it back b4 I did even more damage. Safe to say something is damaged, I guess.

This shudder feels like something is engaging/disengaging....slipping? It'll idle along just fine, but will take hardly any acceleration. Shifting into 3, 2, or 1 makes no diff. WOT might really break something...maybe.

I haven't unplugged the harness and check the reading at 1500...rain and I'm doing this outside for the next few days.

That 40-mile drive on a low fluid level probably destroyed a few seals...or clutches...pump damage...TC damage...clutch/steel damage? All of the above

Thanks again for any pointers/suggestions/guidance.

jalfredprufrock

Dropped the pan today.
Saw this. Not much...a couple of paper-thin, dime-sized spots...that large spot is where I poked at it. Huge pic.
Fluid looks perfect...no burnt smell.

Problem is it 'shudders' under any acceleration, like a bad misfire. It wouldn't reverse onto the ramps (which it always did, np) and just 'shuddered' when I stepped on the gas.

Thanks for any suggestions!!

Pan contents

FranksCustomTrans

Pan looks good overall expected really with a new build . But still you are where you were before need pressure test harness unplugged . You either have a presure control or pump / leak issue, But since pans off I would replace the PCS and see what happens myself.

jalfredprufrock

Thanks for that. Will give it a go. Takes 2-3 weeks to get any parts shipped here.
Meanwhile, I'll put the pan back on, add fluid and test with harness unplugged.
jl

jalfredprufrock

Here are the pressure readings with harness unplugged and idle at 900-1000 rpm. The readings in brackets are with harness connected.

P = (74) - 220
R = (100) - 260
N = (74) - 225
D = (74) - 230
3 = (74) - 230
2 = (74) - 230
1 = (74) - 230

Again, this trans worked so well, until it was driven 40 miles or so on a low fluid level. Since then it just 'shudders' when you apply any appreciable throttle...as though the engine would misfire on 4 or 5 of the 8 cylinders...can't think of any other way to put it. It wouldn't reverse up the slope of the ramps, just shuddered and shook when I applied throttle...no problem getting up on the ramps before.

FranksCustomTrans

Your reading look good and solid . Replace the PCS

jalfredprufrock

Thanks very much for the reply, Frank.
I'll order up the PCS.
jl

bbond105

Try backing up the ramps with the harness unplugged, if it still shudders as you call it, you may actually have an engine miss fire. Those unplugged pressures look great.

jalfredprufrock

I'll try that! Appreciate the suggestion. No issue driving it with the harness unplugged...just a couple hundred feet or so?
jl

FranksCustomTrans

No its fine to test .