Wrangling a clue.
4r70e/4r75w tranny play - page II
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Last updated Aug 20, 2020
This is a nine dollar vacuum modulator. It's function -
regulating transmission line pressure relative to engine
load - is straightforward, the execution simple, and it's
done it's job exceedingly well for over half of a century
with nothing more than a little diaphragm in a can thingy.

Unfortunately, this particular one is attached to an
entirely different vehicle and has been deemed obsolete
by some group of engineering ass hats. Better in
modern times to replace it entirely with a thousand dollar
computer it seems.

Of our four signal lines commanding the transmission,
the line pressure control solenoid signal (which replaces
this old school modulator technology) is arguably the
most important, because getting it wrong very well may
burn up our tranny posthaste.

From Ford literature I've gleaned that the line pressure
default (if the transmission harness were to be severed),
is full pressure, and pressure modulation is achieved by
a pulse width modulated solenoid working against a
spring in the valve body.

Dandy, but what is the pulse width duty cycle, and under
what circumstances?
Well - If we can smell it, then we can spoof it.

Here looking at the LPS signal from the ECU.

A ghetto rig with an ancient B&K O'scope, a car battery,
a UPS working as an inverter, a handycam, and a ten
mile mixed load drive - narrating throttle, speed, tach,
and estimated load for later review to gather datum and
hopefully plot a useful graph.

In hindsight, I should have installed and had a manifold
vacuum gauge in view, and will likely repeat this
experiment with such in the near future.

I'm confident however that we're not going to require 32
bit ARM precision to drive this puppy - the eventual
solution likely just monitoring and following the MAP
sensor.

Indeed, at a glance, the duty cycle roughly correlated to
what I'd expect to see.
The pair of shift solenoids are far more straightforward,
commanded either on or off at 100 percent continuous
duty cycle.

So although it may appear arse backwards at a glance,
I've decided to tackle gear selection and display before
the torque converter and pressure control circuits.

This more to do with weekly project time constraints
rather an any technical reason, but as well as allowing
me to determine if, or how unhappy the ECU will be with
the shift solenoids severed from automatic control.