AlpineRAM

The 47RE valvebody does not lock up in 1st and 2nd gear- no hydraulic pathway. - but with the lockup commanded if you accelerate until normal 3rd gear speed you will go into 3rd locked up and can do a lot of damage. (At least stalling your engine, the trans can no longer downshift and unlock in that situation)

texasprd

Any updates?

APD

No. Im Stumped.
I called Suncoast again and talked to the guy who builds the valve bodies.
He confirmed what I have already said: if it's locking in 3rd, then the TCC clutch solenoid and the lockup hydraulic pathway are functioning properly. TCC lockup is functionally the same whether in 3rd or 4th.

He said try swapping the TCC solenoid (even though he agreed it is working)
When I told him I was turning 2100 RPMs in 4th gear at 55mph, he said that is normal for these trucks (not true at all, it was working fine a few months ago). It usually drops to 1600-1700 when 4th locks up at 48 mph.
If 2100 was normal at 55mph, then at 70mph it would be screaming at around 2700, redline is 3000.

Im beginning to think that the electronics are functioning properly, and maybe the clutch is slipping just a little in 4th? is that possible? Perhaps this a fault in the OD unit itself. OD clutch slippage or OD piston seal damage? Maybe everything is functioning properly electrically and hydraulically, but just not enough pressure to hold tightly in OD?
I dont know the inner workings of auto transmissions as well I would like to, please let me know if any of these ideas seem plausible or incorrect.

texasprd

Your RPMs sound rather high compared to mine - what size tires and gears are you running? I'm running stock 245/75-R16 tires and stock 3.54 gears. At lockup in OD at about 48 MPH, my RPMs drop to a little over 1250 (1300? 1325?), according to the tach. At 70mph, I'm pulling about 2000 RPM per the tach (not sure what the scan tool would show, but definitely not 700 RPM difference at that speed).

I'd say slippage sounds like a definite possibility - both in the trans and TC. I'd say your next step is to put a pressure gauge on it. You said you have the FSM - pressures and test port locations should be in there. You may also want to find an ATSG manual.

AlpineRAM

OK, the function of the OD unit is as follows- no pressure applied- direct drive, pressure applies, spring compressed, OD clutches gripping, OD is active.

A simple way to see if the OD clutches are engaged while the TC is locked( by lockup switch) is to vary the throttle from maintaining 55 mph to coast- if the OD is engaged the rpm will drop to close to idle, regardless of the vehicle speed, if the OD clutches are not engaged the vehicle speed will be strictly proportional to the vehicle speed- this is a function of the overrunning clutch in the OD unit.
A blown out OD piston seal will lead to a general pressure drop in the transmission, and to a lot of your clutch material in the pan and the filter.

I would suggest getting a set of pressure gauges to determine what is happening in the hydraulic part of your transmission.

APD

Exactly.
This is how my truck ran before my OD problems.
Stock tires and gears on my truck.

I did all the pressure tests in the FSM a few months ago, and everything was in normal range. I have not tested since the loss of OD.
My scanner says that Governor pressure is 96 PSI while driving, which is normal IIRC.

texasprd

Any update on this?

APD

Pressure test revealed no pressure in the overdrive unit. Overdrive unit has been rebuilt and all is back to normal.

texasprd

Thanks for the update! Someone on another forum is having an issue much like you had, so I was wondering what you found - if anything. Glad you got it resolved!