The car has been sitting for 15 years, is there is easy test, the fuse is good. thanks
78 I believe was the change year between 2 different types of systems.
The earlier system had a gearbox assembly inline with the speedometer cable that in the big Eldo's was mounted on the top center of the firewall. It has the 2 speedo cables, a couple electrical connectors and some vacuum lines. This unit had some valving in it that modulated the vacuum signal to the vacuum motor that tugged on the throttle.
The later system did not have that gearbox so the speedo cable just ran from the trans to the back of the cluster in the dash. These had an electronic sensor built into the back of the speedo that sent a signal to an electronic control module. The control module then operated vacuum solenoids under the hood that then modulated the vacuum signal to run the motor.
Let us know which version you have and we can give more specifics.
The early system were not really built to last so age and mileage are not especially kind to them. Both system depend on all the vacuum lines and components being in good shape.
Both have a vacuum 'dump' valve on the brake pedal that is a fail safe to the electrical part to make sure when you step on the brake the vacuum motor on the engine can not still be pulling. This is a fairly large hose that runs through the firewall around the brake booster. Its usually teed into the line that runs to the vacuum motor. Disconnect it there and use a hand vacuum pump and gauge to see if its holding vacuum. You can also use the same thing to check and make sure the motor is working and able to pull on the throttle.
Both have an electrical switch on the brake pedal as the primary control system. I can't remember if it was ever its own switch. Most I recall working on it was a section of the regular brake light switch.
And this is the tool I am speaking of for testing the vacuum stuff. They are cheap these days and handy for all sorts of car projects including brake bleeding. No DIYer should be without something similar. MightyVac is a higher end brand name most parts stores carry if you are looking for something a little better.
My 79 Eldo used the electronic version. It has a big red plastic module
behind the dash and a lot of wiring to tie all the pieces together. I don't
know availability of that module, but I have one hardly used from my 79.
looks like i have the newer version, thanks for the suggestions