lots of brown flakes in oil?
not a good sign, sounds like damage in the clutch packs, excessive wear. Did you find any metal particles too? they usually stick to the filter or if the drain plug has a magnet on it they are all stuck to it, you may find some floating around in the oil. Remove the filter and cut it open and see whats in it.

as for the pressures there are many to check
all pressures are checked at operating temperature (50C) at 2000 rpms.

Port1 (pump pressure) 965 to 1170kPa (140 to 170psi)
Port2 (Fwd Clutch Pressure) 724 to 860kPa (105 to 125psi)
Port3 (Rev Clutch Pressure) Same as forward (not more variance than 70kPa (10psi) between the two)
Port4 (torque converter) 724 to 827kPa (105 to 120psi)
Port5 (oil cooler pressure) 105 to 175kPa (15 to 25psi)

Test port locations:
Port1 is located behind the filter next to the dipstick on top of the transmission.
Port 2 is located in front of the control valve on top of the transmission in front of where solenoids are mounted.
Port 3 is located on top front right of transmission (2 identical plugs, one in front of the other) its the one behind
Port 4 is located back where the port 1 plug is but to the right of the oil filter almost center.
Port 5 is located up where port 3 is in front of port 3 in very corner.

You'll need an allen wrench (or allen socket) to get these plugs out and probably need some metric adapters to adapt your pressure guages. Go to a hydrauic hose and fitting supply shop and they will probably have them.

Ok I drained the oil and found lots of little brown flakes in the oil.
And still drives forward.
What pressure minimum should be in the transmission?
I got almost 9 bar when cold.

Regards Steve

Daft question but what colour is the transmission oil?? Is it light brown or red. These trucks run on jd40 spec oil ( tractran 9) not standard aft (dexron ) I've had clutch packs break up and jam causing it to try to drive all the time when the wrong oil is used

I suppose once you rule out anything electrical then it very well could be a problem in the clutch packs (forward pack)
The only way you'll know for sure is to take pressure readings to confirm if it is a mechanical failure or not.

About a year or so ago i had a cat truck with this exact symptom and in the end it did turn out to be in the clutch packs. A reman tranny and it was back to normal.

A quick way to prove it is a bad forward clutch is to drain the oil, start it, and if it still goes forward, time to remove trans. I've seen them break a plate and cause the clutch to seize or drag badly but the oil looks fine.

It's the large on the coloum type.
The oil is clear
No burnt smell.
Will do some checking tomorrow.
Thanks guys.

Regards Steve

Kevin is right fwd clutch pack either toast or lining come off the plates and jammed it up. What oil is in the transmission??

forward clutch pack will be toast !! pull the dipstick and have a sniff for burning oil

ok directional lever is on steering column
is it the mechanical type lever that operates 4 switches inside or is it the thumb lever style with the magnetic switch?

with this xm series i'm not sure if they incorporated the smaller thumb switch in that truck that early on, it does not show in the manual i'm looking at so i will assume not, so lets go with the mechanical lever assy that operates the switches.

you should remove the steering column cover and check those switches (should be 4 there) and see if one of them is stuck or adjusted wrong. From the sounds of it the truck is staying enguaged in forward. If you find no problems with the switches then your problem is most likely in the control valve solenoids not releasing (sticking).

The forward and reverse lever is on the steering column

Hi
Sn H177B02963V

got a serial # for it?

from the general description it just sounds like it needs calibrating or maybe a new pedal sensor or both.