Volvo 940 d24TIC downshifts at high speeds

[ngoma]

Yes you are correct; the "logic" section of the ZF 4HP22L is like a computer, but an analog (mechanical) computer, not digital electric. No "reset" function while running. All shifting logic is controlled by the mechanical motion of the governor, and the labyrinthe pathways of the fluid thru the channels and thru the ball (check) valves, accumulators, and piston valves, working against springs, small orifices, and back pressure-- all fluid mechanics (hydraulic logic system).

Your issue is interesting and if based in the logic internals, maybe beyond the skills of us here on the forum, unless someone has had a similar symptom and was able to solve it.

The ZF HP series was used in a wide variety of vehicles (BMW, Land Rover, Porsche, etc.) and there are still experienced mechanics scattered about who probably can quickly ascertain the problem from your description. The hard part is finding the ZF expert who can do so.

Mine had a problem several years ago, now forgetting exactly what it was, maybe slow or sloppy 2-3 upshift? Dropped the pan, removed the applicable valve body, disassembled/inspected/cleaned, reassembled, and resulted in maybe a little better than before? At least not worse! It was not fun nor pleasurable (fluid drips for weeks) but thought provoking. Forum member v8volvo had his completely fail on the highway, tried R/R valve body with a known good one-- didn't work at all.

See if you can do some research on BMW and Land Rover (much higher population of ZF HP than for Volvo) forums with their versions of the ZF HP, see if any of their reported issues are similar to yours, maybe you will find a solution.

There are excellent Volvo factory service greenbooks dedicated to the ZF4HP22L logic theory, fluid flow, troubleshooting, etc. Depends how far into it you want to go.

IMPORTANT: Don't overlook what v8volvo suggested to inspect and report back the routing of the kickdown cable.