Hi,

I see you are still struggling with getting into the age of machines from your application of operating Velcro.
You know it may have had instructions, if you used them to fasten things onto something else like a wall or desktop. So I still think there is hope for you! (:-)

Since you told me you have an automatic there should be only one switch located below the dash on the brake pedal.
When the cruise system is installed on an automatic transmission there needs to be a wiring change made as noted on the bottom of page 22B of that PDF.
Check to see if that was done as something might hooked together.
Move one circuit over to only the brake pedal/vacuum switch.
It needs to Blue with a red stripe.
The green and red stripe wire is not used and is left disconnected.
I would even put a piece of tape on it.

The name on the box under there, seems to me or my memory, is the company Volvo used stateside by our dealers shops.
In the early years of 240s even the air conditioning was added on here as the Swedes were not big fans of it in their cars! They have mountains snow scattered in Europe. Safety was needed for some large animals and the car was sized inside for their people.
America was just getting into downsizing with the fuel embargo of 1974. The Ford Granada was as close as it got and a Volvo was still priced higher, as an import, without A/C.
But they had to get it anyway or nothing sold. So, dealer installation got it going. Cruise control was even later! Cruise control is not for mountains and narrow European roads.

We need to find out why that Servomechanism operates below 35 mph.
You should be able to leave it on all the time until you “set it” above 35 mph.
Of course, they recommend that you only turn it on with the turn signal stalk, when needed.

I’m suspicious about what is going on electrically,
A valve has to get power to close and modulate vacuum on the “throttle cable servo” below.

On page 23 is this what you have on the car?
Make sure you have the black wire from box “A” grounded as shown. I know the chassis is grounded but check for a wire coming out of either harness since there is two connectors.
There are no other electrical wires except at the brain controller box “A” and then up to that firewall housing unit “E” ... correct?

Maybe the valve in the servo controller is plugged up dirt and “not venting” IN some air and passing all the vacuum down below! (?)

I’m confused as you as to when and what the test procedure is testing. I think you are as well, especially if its malfunctioning. Hiss and no hiss from what storage place?

According to the PDF, the firewall controller sends the vacuum down to the foot servo, period.
The switches down there break the vacuum to it, for safety and backup reasons.

So it’s that can and firewall controller that works with the brain box under the dash.
Along with this, the switch down there, pauses the brain box with a power to ground circuit opening, until resume is used!

I hope you are getting this reasoning to followed this around, in a step by step manner, to trace why it’s working, while sitting there, still in Park!

You asked about the ECU or ECM being connected.
The brain box needs to see a Speed signal from a speedometer and or a RPM signal from the engine.
ECU’s are very common today, but on early aftermarket units, a driveshaft magnet provided speed only.
Some cruise controls can monitor the engine RPM and hold that as the cars programmed speed!
Both might even be used in case of a sudden change.
This will shut it down electrically but having both makes it for manuals and automatics capabilities.

That’s about all I can tell you at this point.
Edit,
Just saw the post.
No pictures, but I don’t think it’s easy, as I’m not computer savvy either!

Phil