Inoperative Cruise Control - 78' GP

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PartsPile

Not-quite-so-new-guy
Mar 2, 2016
15
3
State College, PA
Greetings,

I have a recently de-garaged 78' Grand Prix LJ with an inoperable cruise control. Tried pushing the button on it yesterday (for the first time since getting the car), and nothing happened.

I immediately checked the usual under the hood suspects and everything seems to be in working order.

The cable to the throttle is not bound up, the servo will hold vacuum if you put your finger over the inlet, and the vac lines are not rotted or making poor connections (and one of them looks a lot newer than the rest, leading me to believe the last owner replaced it 5 or 6 years ago)

Ran out of daylight to test the electrical side of it, that's this weekend. In the meantime, I was wondering. Does anyone know if you get some kind of light on the dash when the cruise is on? Is one of those dark sections of that middle gauge above the column (that has the turn signal/hi beam/parking brake warning lights etc) supposed to say "CRUISE" or something? Most of the wiring diagrams I've been using up until now don't even mention the cruise, and I don't have an owners manual that would tell me. Otherwise, I can't tell if pushing that button on the inside is actually doing anything.

I haven't found any blown fuses, but the fuse block under the dash is so old, the lettering has worn off most of them, and I can't tell which controls what anymore. FWIW: I also have no rear defrost, but everything else electrical is in working order.

Anyone have any additional experience servicing/troubleshooting these? Is there a way they seem to like to fail?

Any information would be appreciated.
 

pontiacgp

blank
Mar 31, 2006
29,156
113
Kitchener, Ontario
By the diagram there is no light. I don't know anything about the cruise control but may this diagram will help...

 

PartsPile

Not-quite-so-new-guy
Thread starter
Mar 2, 2016
15
3
State College, PA
The fact it looks to share the radio fuse helps, because I know the radio works (just upgraded it to a CD/MP3 last week) And the only two wires going into it under the hood are the same color.
 

PartsPile

Not-quite-so-new-guy
Thread starter
Mar 2, 2016
15
3
State College, PA
Well, the wiring diagram didn't lie, the two wires in were green and blue, and I have 12 volts there, on/off with the switch (whew, no tearing open the steering column) so that leaves either the unit itself as bad, or, the third vac line off of it to the brake pedal which I haven't tested yet (and looks to have been taped) so hopefully, that's the problem, it's leaking vacuum and sending the cruise unit the "Turn off" signal the instant I hit the button... that'd be nice if that's the issue, vac line is cheaper than the cruise transducer.
 
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