I'm looking for info on common areas to check on a non-functioning cruise control on my '84 Firebird.

Down in the front left side of the engine compartment is a large vac diaphragm linked to the carb throttle. When I hook up my hand vacuum pump to either of the 2 ports and try to activate it I get no movement like it's not even trying to creat a vacuum movement.

But before I condemn the vac unit could it be ported or vented in some special way? Is there a 'trick' to testing these I'm not aware of? Why two hoses? (one to vent?) Are these common to fail and can they be rebuilt or just replaced? (probably expensive)

 

OK sir... here is how everything works.


These work much like an egr, on a pulse system.
There are two little electronic valves on the system, one to let in vacuum, obe to vent. By varying the pulse frequency and duration, the vacuum can be controled

When i got my car, the vacuum valves were frozen with corrosion, because they are made of metal, with very tight tolerances. To check, remove the diapram from the car. The diaphram is actually two pieces.... the diaphram itself, and the electronic portion, which actually just pulls off once you remove the shroud. The valves consist of a metal needle that is surrounded by a coil. When activated, the metal needle moves, and allows vacuum in.
Make sure all the compnents are free. If you want to check, you can trip the valves with a small current, (i used 9v). If everything is free to move, and the coils correctly actuate the valve needle, i would look elsewhere.

 

Thanks! Exactly the confirmation I was looking for. I'll be taking the diaphragm out of the front of the engine bay soon and checking all that.

 

It is pretty cool once you see everything, and how it works. I took mine apart quite a while ago, so I am a bit hazy on the specifics, but that is the general idea.

 

It's fixed!

indeed toehead. I took mine apart tonight. Found the same thing. I'm thinkin my inlet vacuum needle might be messed up(it was missing 0.o). I wasnt sure if the valves got a pulse signal or resisted signal so i didnt want to test them and blow them to crap with a full 12 volts.

The only inlet power i could find to the system was 7.9v. Im thinkin that actually just a test curcuit to see that the right unit is plugged in. Can anyone confirm what the two middle plugs are actually doing? They seem to run to a big resistance coil or the like. Does that actually prevent actuation of the diaphragm by holding the rod?