Topic: 1980 T/A transducer vacuum for cruise control

[b_hill_86]

BTW, I found the bracket part that holds the threaded part of the switch and the dump switch not holding them in good. they were pushed out so i push both switches up against the bracket. not sure if this was throwing off the adjustment. wish there was a way to test to see if that was the problem without having to drive the car

You don't have to drive the car. Unplug the connector thats for the cruise, put the multimeter on the terminals on the switch set to ohms. There should be a reading with the brake pedal not pressed. when you press it your multimeter should read OL meaning the contacts inside were opened (supposed to be like that) If the opposite happens, meaning OL when the pedal is not pressed but you get a  reading when you press the pedal, those are for the brake lights then. Check the other set of terminals.

If, now that you adjusted that switch, you have continuity with the pedal out and a zero reading with the pedal pressed, plug the connector back in and go back to the beginning and retry the steps i had you try with the plug on the transducer and pressing the button with the key on and see what you get.

[b_hill_86]

question, what actually grounds the transducer? is the that little thin copper wire?

Yes. it connects the case to the bracket that bolts to the inner fender. I dont know if someone added it to mine or if it was factory at one point (i think someone added it) but mine has a wire going from that nut where the copper wire is to a bolt that holds the inner fender in. You could do that if you wanted but since during the ground test earlier you said the ground seemed good on the transducer I wouldn't worry about it.

[b_hill_86]

so i put the meter to ohm's. put the meter wires on and just got OL, then switch meter wires and get a reading of 0.00. also put a test light to it and got a good light. thanks

I think (since i'm not familiar with your digital multimeter) OL means open line meaning no continuity. That would be correct if you were testing the two terminals connected to the brake light wiring (orange and white). When you said  "switch meter wires" if you meant you tested the other two terminals where the cruise connector was connected and got a reading of 0.00 with the pedal not pressed that would be right too and that's the important one to get the cruise to work. If that DIDNT happen before you adjusted that bracket, then that's probably what your problem was.

ok, switched meter to V. got a reading of 0.15. also that 3 brown wire that goes into the brake switch comes from the connector that goes to the turn signal lever. that brown wire goes with the blue and green coming from the transducer

As far as that, I think you want to test for voltage on the pink wire and a good ground. Again, the colors are different on your car vs my 77 but voltage should come from that pink wire into the brake switch. When the pedal is not pressed it allows voltage to pass from the pink wire to the other wire(s). I only have two wires at my connector, not the extra brown one you have. I'm wondering if maybe the extra one is for resume type cruise control (which started in 1980 i think). Maybe it was easier for them to install the same harness regardless of what kind you have and just not use the extra wire if you have non-resume.

Either way, test for voltage between the pink wire and a ground. If that works, plug the plug back in on the switch and retry the test with the key on where you switch the plug on the transducer so the "engage" wire is plugged into the "hold"terminal only. Press the set button and see if you get the transducer to clunk.

[Dinero]

transducer clunks ok. good reading on ground. well, will hit the road tomorrow for test after I get  instrument panel and all other stuff back on. I rarely use the cruise but it bothers me that it is not working. I appreciate all your help. I am grateful. Will keep you posted. thank you

[b_hill_86]

Ok, just for the record, I was wrong about resume cruise. That didn't start until 1981. So that extra brown wire isn't for that. I found an 80 wiring diagram that shows both brown wires but its not clear as to where they go. One has to go to the button on the turn signal lever for sure. Where the other one goes, i'm not sure honestly. But I think we can still get you rolling so be sure to update when you can.

[b_hill_86]

transducer clunks ok. good reading on ground. well, will hit the road tomorrow for test after I get  instrument panel and all other stuff back on. I rarely use the cruise but it bothers me that it is not working. I appreciate all your help. I am grateful. Will keep you posted. thank you

Awesome! Must have been that switch not being aligned right. Let me know when you try it out what happens.

[Dinero]

Brian, does the Thumping and clicking mean that the transducer is working like it should? No chance to test yet. Had storms almost every night. Streets still have water on them. Hopefully monday. thanks

[b_hill_86]

Not 100% no. It’s possible it could clunk and something else might not work right. Does your speedometer work though? If so, and it’s clunking now I’d say there’s a pretty good chance it can work. In the meantime there a couple more tests you could do to make sure the vacuum portion is working correctly. You’d have to run the engine to do so unless you have a vacuum pump.

[Dinero]

OK, I do have a vacuum pump

[b_hill_86]

One thing you could do is unplug the two hoses that come off that 4 way “T”. One should go to the servo (diaphragm) and the other goes to through the firewall to the vacuum dump valve on the brake pedal. Hook up your pump to the one that goes to the brake pedal and make sure it pulls a vacuum without leaking off. If not you may need to adjust the valve or replace it. If it holds a vacuum without leaking press your pedal and make sure it releases the vacuum.

The next thing you can do is attach the pump to the hose that goes to the servo and pull a vacuum on it. Make sure it pulls on the throttle without binding and holds a vacuum without leaking. If so you’re good so far.

At that point, again assuming your speedo works and isn’t really bouncy or anything, you could take it for a drive and test it. Remember there is a low speed mechanism built in so you should be above 30mph at least for cruise to work. Maybe faster. That’s slightly adjustable if necessary. Could try above 40 to be safe.

If you try it out and it doesn’t seem to work don’t give up. There’s another thing that can be adjusted.

[Dinero]

oK, diaphragm hose holding. linkage moves ok and diaphragm is been sucked good. Vacuum holds.  Now the hose going to brake pedal is not holding at all. Waiting on wife to pull vacuum on it while I get underneath to check for leak or hear it. Will try a  smaller hose and hook up straight to it to see if the main hose is bad. If it holds ok with the short hosem then hose might be bad. If vacuum does not hold with either hose, is the vacuum dump thingy bad? Stand by.

[Dinero]

ok, put different hose straight to vacuum dump valve and it does not hold vacuum. But would a bad vacuum dump valve stop the  cruise from working? thanks

[Dinero]

update,,made sure that both brake switch and vacuum dump plungers were flush against the brake pedal bracket. after that did vacuum test again on  valve and it is holding. Once i step on brake the vacuum releases. 
Can't figure out why the switches are been pushed out that is causing the plungers not to stay all the way in and flush with the brake bracket. I just push both of them in and  they go flush again. OK so now that I have good vacuum, I am ready for a test. Will keep you posted. Thank you Brian

[b_hill_86]

ok, put different hose straight to vacuum dump valve and it does not hold vacuum. But would a bad vacuum dump valve stop the  cruise from working? thanks

Yes. If that doesn’t hold vacuum cruise will not work at all.

No problem though. And sorry I’m not sure why the valve and switch are moving on you so I’m little help there.

[Dinero]

as soon as I get the instrument cluster back in  I will take a test drive. Will keep you posted. Thank you