48re won't downshift at a stop.

ok, 200 miles and it ran fine. put the gauge back on the gov pressure port and take it for a ride. see what it reads after the first time it wont go to first, if its not 0... put it in neutral then back to drive not first and report back.

Saw second post, if it doesn't go back to 0 until you go to neutral and take pressure away its the pressure solenoid. Does it have borg solenoid, dnj gm upgrade or stock? new parts do fail.....
 
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Couldn't get any help here tonight for a drive test. Got some coming tomorrow afternoon.
Went ahead and too the truck down to the corner store (about 1 mile) and watched the desired and actual on the tablet, and the real pressure on the mechanical guage as best as I could.. without crashing, lol.

Desired and actual gov pressure, according to the diagnostic tablet, ramped up as you would expect them to, and stayed within 1-2psi of each other.

The mechanical guage however, would not show ANY pressure until just before the 1-2 shift, then it would jump to 10-15psi. After that, it was very eradic, jumping from 10 to 40 to 15 to 80 to 60 etc. With no obvious pattern.

This was under smooth, slow acceleration, never passing 40mph.

Important note. The transmission never failed to downshift at stops on this short trip.
 
The manual is describing what I was talking about with solenoid flooding, a mechanical failure where the solenoid is letting fluid past. (Debris)

On the 47, the screw next to the line pressure screw sets the minimum throttle valve setting, and can be adjusted beyond what the linkage does.

Voltage high means voltage isn't dissipating through the circuit to return. The is usually an open circuit, but can also be a short to high voltage (12V) source.




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biggy238 said:
The manual is describing what I was talking about with solenoid flooding, a mechanical failure where the solenoid is letting fluid past. (Debris)

On the 47, the screw next to the line pressure screw sets the minimum throttle valve setting, and can be adjusted beyond what the linkage does.

Voltage high means voltage isn't dissipating through the circuit to return. The is usually an open circuit, but can also be a short to high voltage (12V) source.




Sent from my iPad using Tapatalk
Click to expand...

Im with this "mechanical failure" not sure how bad the old trans failed, im assuming you still have the converter cooler ( heat exchanger ) and trans cooler did you flush all the lines good? those gm solenoids are better handling the pressure but still don't like debris. Maybe something broke free on you. When your help shows up that mechanical gauge will be key...


The screw biggy is talking about for minimum tv is the long one that looks like it has a washer on the end, drivers side of the valvebody. Even if you have 1/8- 1/4 " of slack in the actual cable this can still be a little tight on the valve, start with the gauge first though...….
 
This trans was a complete new unit. And I deleted the stock trans cooler, and installed a brand new remote cooler at the same time..
It should be pretty dang clean, lol.
 
Buddy just stopped by and drove while I was watching the guages. Tried to take video, but it was too unstable... So I took some stills.
Mechanical guage still didn't register any pressure until just before the 1-2 shift. After that it was erratic. Bouncing randomly between 10-80psi.
Desired and actual on the tablet started out as you would expect, but gained a fair amount (50psi) of separation at higher speed (40+mph)
 
Why can't I post pictures ? Every time I try to upload, it tells me that the security token is missing?
 
And a few more.
 

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Dangit. First 3 still didn't load....
 

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One more test. Doesn't really apply to my symptoms, but here it is anyway.
Trans case connector DISCONNECTED, engine running, trans in DRIVE.

Actual gov pressure on the tablet is 55psi

Actual gov pressure on the guage is 70psi.
 

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Oddly enough, it downshifted just fine on both diagnostic trips. Last night, and today.
 
I've collected a fair amount of data through diagnostic testing... I just don't know what to make of it.
Surely there are guys here with the knowledge ?
 
its shifting correctly because the gov. pressure is correct. im guessing it upshifts and down shifts correctly with the exception of going to first sometimes... I bet if you get it to start in 3rd with the gauge hooked up it reads north of 10psi.....but that's just me
 
I'm gonna leave the mechanical guage hooked up until it acts up again.
Strange thing, it would not downshift, at all, for at least the last 100+ miles... Until I tried to diagnose it, of course
 
Hopefully she keeps on going, im guessing its a work truck and you make a living with it...im sure it will get figured out. Just needs to act up when your looking...….good luck keep us posted
 
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