The vacuum dump switch operation can be tested either with a vacuum pump or by sucking on the end of the hose at the servo. Pull it off and ****, be carefull no crap gets down your throat, I keep my tongue in the way when doing something like that. You could also remove it from the brake pedal and **** on the end of it while pushing and releasing the plunger. There is also the vacuum hose to the tap on the intakle manafold, make sure it is good and not cracked.
Yah, it's properly called a servo. I would not normally expect the servo motor to be bad, they are pretty robust. On the voltage end, you can test the voltge level to the speed control switch. If the horn works then voltage is at the input to the switchs.
At the connector to the speed control amplifier under the dash, you can check that you are getting voltage from the steering yoke switches. look for a blue with black hash mark wire. When the speed control on button is pushed you should get about 12V. The other buttons develop differnt levels ( not sure what they are off the top of my head and don't think it is important if the control doesn't engage in the first place).
Like I said in a previous post in this thread, the diagrams I posted are for a 83 Bronco. You can use it as a guide.
Good luck,
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At the connector to the amplifier there should be a white with pink stripe wire. It should have 12v with the key in the run position. Also, 12v should be at the light green wire only when the brake is applied comming from the stop light switch.
Check the ground which is a black wire on the same connector.
Also check the speed sensor, Other than a continuity check, I don't know how you would test it other than changing it. You should be able to get a reading thru it.
If these check good, then first I'd change the amplifier, the last thing I'd change would be the servo motor.
