When bucket raises to 3/4 height, truck engine kicks down to very low idle

[Kgw51gmc]

Lift is an "ETI PALFINGER ETA 33". The electronic control box under truck hood is abandoned, I just run hydraulic pump clutch wire direct to battery to turn pump on (for now).
When I raise bucket roughly 3/4 of the way up the truck goes from a high idle to a strained sounding low idle. As in, panic it may stall (never has), or break something.
I thought the pump was dead headed or pressure building up in excess but I think not. The trucks idle, which is only seen by clipping a wire under the dash, is unaffected by hydraulic pump operation. For whatever reason, truck is always on fast idle.
Important info- the purple wire on back of truck pcm, when snipped, takes the truck to the same low idle (sounds like the 6.5 is working hard during the low idle). Reconnect purple wire and truck returns and starts/runs/stays on fast idle. All this is true whether or not pump is running.

There is no way to guess this on a forum, unless it already happened to you so this is a shot in the dark.

 

[ChoppyChoppy]

Knowing what truck would be helpful. 6.5L, so 92-03 GM.
DB2 or DB4 pump depending on year.

DB2 has a fast idle ability using a pull solenoid. Same one used on Quadrajets.
If that's the pump you have, sounds like they've got it always on. Though it would only come on if you touch the throttle, it's just a hold coil.

DB4 I think has a fast idle as well, not too sure, never worked on one.

Sounds like the bucket has no control on the engine revs. Like no auto idle/fast idle?
If engine stays at fast idle, to me that points at something loading the engine down.

Is ~3/4 up operating something or a binding point?

A pressure gauge would be helpful.

 

[iowagold]

click here for test gear
yea pressure gauge sure would be nice to see if it is loading the pump.
and a clamp current meter for dc. as well as a valve magnetic field detector.

it can also be an issue with an unloader or pressure limit valve sticking.
sometimes on this gear it is the simple things like trash in a valve or an orfice.

track down service manuals for both the truck and the lift with electrical printouts.
think safety interlocks.

 

[PooPro2020]

Look for broken wires, the wire insulation stretches at a certain position of pull and separates the wire core that is brittle.

 

[J D]

I'd start by putting a multimeter/test light on that purple wire & see if the power there dissappears when your issue occurs. To me that sounds like the easiest place to start & if it is that signal cutting out you can trace back from there. Bear in mind it may be a switched earth rather than positive voltage. As PooPro2020 says, I'd be suspecting a wire or connection being strained or shorting on something at a certain elevation. Possibly a contactor or solenoid that fails at a particular angle of orientation. These things can be a PITA, good luck!