aletheian
As a follow up to my first post here are a few more items to consider:
Vaccum lines - Did your father check all of them??? There is a vaccum source to the servo unit, probably tapped off a line to the purge cannister and the brake release vaccum switch. The line to the brake switch will start at the servo and run through the fire wall to the brake. The brake vaccum switch is mounted just above the brake pedal under the dash.
The servo unit is a electrical to vaccum transducer. Is if functioning correctly? If the internal diaphram is bad (won't hold a vaccum) the curise control will not function. You can test this by putting a vaccum tester on the vaccum connection for the servo and pulling a 14" vaccum. If it drops below 9" in one minute, the servo is bad and will need to be replaced.
You can test the brake switch by putting a vaccum on the servo and pressing the brake pedal. The vaccum should be released at this point. Then release the brake pedal and see if you can pull a vaccum on the servo again, if not the brake vaccum switch may be bad.
Note that if you have a manual transmission you may also have a clutchh vaccum switch. The vaccum line to this will be tapped off the line to the brake vaccum switch.
Electrically you have the switch on the column, a crusie control control module mounted someplace in your dash, a second control module mounted at or in the servo, an electrical brake switch and if you have a manual transmission a electrical clutch switch.
You can test the column switch by disconnecting the wire connections at the curise control module( probably a connector), and using a test light, see if the light comes on when that particular switch position is closed. (you will need the ignition switch in acc or on for this to work.) Then reconnect the connector for the wires to the switch. Then go to the servo and disconnect the connector and test for voltage there, using the various switch positions. Don't forget to also test the electrial brake and clutch switchs. You will have to take a look at the schematic to see which wires should have voltage at the servo. Sorry but I don't have a test procedure for this part. If you can find a 85 Service Manual for your car, it will have the test procedure. Or maybe someone else will be able to post some suggestions.
One other item to check is the crusie control cable connection to the throttle cable, at the carb, TPI or TBI. Again if this connection is bad the system will not function.
You shuould also take a look at this thread. However disregard all my references to a TRANSDUCER located next to the servo unit. It appears that the transducer was replaced in the 84 and up models with the cruise control module. The photos wil give you some ideas on where to look for the servo unit and the vaccum line to the brake and clucth, if you don't know where they are.
http://www.thirdgen.org/techboard/te...wont-stay.html (Cruise Control Won't stay Steady on 84 Trans AM)
Good luck and let us know if you figure out your problem.
Dave
