300SDL

My SDl is showing the same symptom, (heat at idle and low speeds, no heat at highway speeds). This is with a newly-installed monovalve rebuild kit. I've confirmed that the coil-operated monovalve mechanism is operating properly when 12v is applied to it; when not installed, 12v makes the valve jump out of the coil about 1/2 inch or so. Installed in the system, with the monovalve two-prong electrical plug disconnected, the heat behaves the same as with the plug connected. That is, there is heat at idle and low speeds, heat diminishes to no heat as soon as I get on the highway.

Below, possibly under "too much info," are some voltage measurements I took.

I'm not sure the proper way to measure the voltage at the two prongs of the monovalve with it installed and the ignition on; I measured it from each prong to the metal monovalve cover(assuming it was a ground?), as well as across the two prongs.

Across the two prongs:
I get near 12v across the prongs when the system is off or when the temp dial is set to "min". I get 0 volts when the temp dial is moved to "max."

Purple wire to monovalve cover:
I get about nine volts from the purple-wire prong to the monovalve cover at all times with the ignition on, regardless of temp dial position.

Yellow wire to monovalve cover:
From the yellow-wire prong to the monovalve cover I get 0 volts with the system off or with the temp dial set to "min." I get about 9 volts when the temp dial is moved to "max."

Do I still need to test the monovalve on the road with 12 v applied in all conditions?

Does this help with ideas for a solution?

thanks
Bruce S