I have '74 164E with a BW35 auto trans. It seems to drive ok in the forward gears and also reverse. The car doesn't roll backwards going up a hill in drive at idle. The only problem is when I take a 90 degree turn the engine revs go way up because the bw35 is slipping. I have not changed the type F in as of yet
since it's fairly new to my stable. Hope it's nothing to $$$$
What do you think it might be ?

If the slipping only happens on a 90 degree turn, could your problem br in the rear end?

Are right and left turns the same?

Does this happen when cold and warm?

A mass change of fluid can cause issues - many do a slow fluid change, drop the fluid in the pan and top up - this is what the 740 service calls for every 30K ( I am not sure of the 30K figure)

Speed the slow change by changing the pan fluid every oil change.

For fire trucks with automatic transmissions -

1 - Drive several miles to warm up transmission fluid
2 - put transmission into every gear for 15 seconds or so
3 - check fluid level at 1000 RPM in neutral

The transmission fluid appearance looks good , a light pink in color , not any burnt smells.
The level on the dipstick shows slightly overfilled while shut-off and cold.
I will try the warmed up as you suggested, then check the level.
I wonder if the previous owner had placed Dextron in the BW35 instead of the Type F trans fluid ?

Finally changed the trans. fluid and they found the screen quite dirty....also the level was a little low cold or hot according to the trans. repair shop.
I felt it was a tad over-filled in either case.

The 164 seems to working just fine now.

Thanks for all your input in this matter.

Low on fluid. Don’t do a flush. Drop the pan, drain, change filter and add till full on the dipstick. After this drain and fill another 2-3 times .

Best to check the level first.
1.) Car on level ground in Park.
2.) Engine Running.
3.) Engine and transmission warm.
4.) Car has been driven at least a short ways (ie: not just started and allowed to warm while standing).
5.) Pull dipstick, wipe, re-insert and check the level.
You might have to do 5 several times to confirm the reading.