I just did this splice fix and it worked perfectly on my 85. Posting in case someone else has my symptoms.....
My tach worked through about half of the throttle range. Where it operated it was fine, but through about half the pedal travel it would drop to idle speed and a loud clicking would come from the rack position sensor on the injection pump.
The clicking was loud enough to be heard in the car and loud enough to make folks on the sidewalk look as I drove by.
The EGR connector was full of water from a new windshield leak I now know I have and must have been shorting across to the rack position sensor.

To the previous poster, I am not sure the transmission has much electrical control beyond the kickdown switch behind the throttle pedal. I would suspect something going on in that circuit.
I think the kickdown switch circuit uses signal from the tach so if it is stuck closed or otherwise malfunctioning, it would cause the transmission to hold first gear like that because it thinks you are holding the throttle to the floor. Without tach signal its not working at all and shifting normally.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660