A/T - THM Late or No Upshifts
TECHNICAL BULLETIN # 179DATE: July 93
TRANSMISSION:
125
200, 200-4R,
325, 325-4L
440-T4
700-R4
SUBJECT: LATE/NO UPSHIFTS
APPLICATION: GM
125, 200, 200-4R, 325, 325-4L, 440-T4, 700-R4
LATE/NO UPSHIFTS
A late or no upshift condition on GM Transmissions (with T.V. Cables) can be caused by a hydraulically bound throttle valve. Although the throttle valve may not be stuck when you check it during a rebuild, it can stick while in use. You may have seen this as a cable that has tension with the engine off but is loose with the engine running.
Here's what happens...
In the example you can see that T.V. pressure works on the end of the throttle valve (area "B") and area "A" of the T.V. plunger.
Because area "B" is larger than area "A", the throttle valve and plunger will always want to move to the low pressure position. When oil leaks occur at the throttle valve (through normal wear and tear) the force on area "A" may exceed that of area "B". The valves now want to move to the high pressure position. Enlarging orifice "C" (T.V. balance) will ensure there is enough oil to the work area "B" and feed the leak.
During the rebuild you should make it a practice to enlarge the "T.V. balance" hole in the separator plate to .046" - .055".
To find the T.V. balance hole, locate the cavity that feeds the end of the throttle valve (area "B"). The figure below has an example of this.
Note
Some Plates have more than one hole for T.V., make sure you only enlarge the one that feeds Area "B".