RonR80

Hi guys ,I've searched for a cruise control repair or vaccum diagram but so far nothing , if someone could point me to an article or a repair forum I'd appriciate it , I have an 80 "my own " and a 81 to repair also, thanks a million , Ron

Sean82

http://www.iotech.no/corvette/refere...-control-works

http://www.docrebuild.com/dr-r-web/PLUG-WIRE.PDF

Duke94

There is a trouble shooting section in the 79' GM Service Manual I have. What does yours do that it's not suppose to do? I can scan the page from the manual if you need it.

Gary

7T1vette

There are only a few parts in the system. Speedo cable "IN"...speedo cable "OUT"; control head; vacuum diaphragm servo on the throttle; brake disconnect vacuum switch; and the push button switch to operate it. The servo must be replaced if bad...but that is rare. The control head can be disassembled, cleaned and [most likely] reused. I don't know where you can buy "parts" for the control head; several places sell the entire control head, including Rik's Corvette Parts of Morganton, N.C.

RonR80

Thanks for the quick replys , It still engages and hold speed but seems like I get a bump and then it jumps a few miles an hour faster, and keeps repeating this till I'm going way too fast , If you touch the brake it disengages fine , I have the Gm 1980 manual but ,what I need is a vacuum diagram , ,Ron
UPDATE , sorry I read the responses after I wrote , I do have a vac, diagram thanks to Sean 82, I'll poke around and keep you posted! Thanks again Ron , BEERS all around!

7T1vette

Since it functions [at all], the vacuum lines and connections are most likely fine. Your problem sounds like the 'friction wheel' inside the control head is worn/dirty and needs a good cleaning. Check the internet for instructions for doing so ("GM cruise control 1977"). It's a pretty easy job if you are careful to take digital photos/notes on how it comes apart...so that you can re-assemble it properly. Also clean up any passages/orifices so that there is no debris in them.

RonR80

Good idea I 'll take apart and keep you updated , thanks , Ron

RobertHuggins

1981 Corvette Cruise control operations, re-conditioning, and trivia.

I saw a post that said to google search "GM Cruise Control 1977" to find rebuild information. I never could find a web site with the information I was looking for to describe the operation and rebuild of the friction wheels. Thanks to the web site at http://www.iotech.no/corvette/refere...-control-works for the operational aspects of the cruise control. There are excellent pictures on that web site and there is even an animation showing the operation of the rubber clutch which is gripped by the horse shoe shaped clutch spring.

Here are my notes from my rebuild.

External marking: AC CRUISE MASTER

Three Spade Connector:
HOLD - Vertical spade connector
ENGAGE - Vertical spade connector
LIGHT - Horizontal spade connector

"A" above a larger vacuum connection
"B" above a smaller vacuum connection - from Resume solenoid
"C" near the brass adjustable vacuum connection that bleeds vacuum to control the speed

Two mounting bolts, one on each side, horizontal mounting.

Metal "snap-on" bottom with four vent holes. One hole in each corner with air filter material visible through each hole.

The center "lower" speedometer cable connector is the smaller one, which connects to the transmission.

The top "upper" speedometer cable connector is the larger one, which connects to the speedometer.

Both speedometer connectors have 18 threads per inch, the diameters are much different.

The smaller connector has a hub that sticks out farther, and has a brass washer external to the connector that the lower speedometer cable goes through.

The smaller "lower" speedometer cable connector is the "center" one as far as the cruise control mechanism is concerned.

Inside the cruise control, there is a simple 1:1 gear that connects the two connectors.

So it would be possible to replace the lower and upper speedometer cables with one long speedometer cable and no calibrations would be different. The cruise control would simply be removed from the system.

So it would be possible to replace the cruise control with a "cruise control eliminator" that adapted the two speedometer cables and no calibrations would be different. The cruise control would simply be removed from the system.

The Computer Command Control = CCC is not involved with the cruise control.

The cruise control's inputs are:
Speedometer cable running through it
Engine vacuum
Electrical connections to ground, on/off, resume, set (engage)
The cruise control's outputs are:
Variable vacuum to the vacuum servo hooked to the carb throttle

Notice that there is nothing going to the CCC related to the cruise control.

If you could look at the speedometer connector on your transmission, the hub would spin clockwise as you are driving down the street.

This means the center connector hub of the cruise control will be spinning counter clockwise.

This means the bigger "upper" connector hub of the cruise control will be spinning clockwise.

This means the hub on the back of the speedometer in the instrument panel will be spinning counter clockwise.

If you use an electric drill with a square bit to test your speedometer, put the drill in reverse.

If you use an electric drill with a square bit to test your cruise control using the center smaller connector, put the drill in reverse.

One simple test of the cruise control before you ever take it apart, is to test continuity across the engage and hold connectors. At rest there should be an open circuit. This prevents the cruise control from turning on under 35 MPH. Use an electric drill to spin the speedometer cable, and at a certain "medium" speed of the electric drill (around 500 RPM), there will be continuity between the engage and hold connectors. Slow the drill down below 500 RPM and there should be no continuity between the engage and hold connectors. If your cruise control transducer fails this simple test, it could be dirty contact points between the engage and hold connectors. Use sandpaper or a thin key blank file to clean the corrosion off these contacts. Be careful not to bend the springy wire!


Internally, there is a friction wheel, a solenoid, a switch, a fixed orifice, and a variable orifice.

The friction wheel is a brass to steel "rub". The steel wheel or round plate is connected to the center, smaller speedometer connector that goes to the transmission.

The brass wheel or round plate is connected to the cruise control internals. The brass round plate cannot spin much, because it is connected to a spiral torsion spring. The brass round plate can only spin slightly one way or the other depending on the deflection of the spiral torsion spring. This deflection is variable according to the speed of the car causing a variable amount of friction between the two plates.

When the car is going less than 35 MPH, the deflection of the spiral torsion spring is so slight that the switch mechanism between the engage and hold connectors is still being held open.

Over 35 MPH, the switch mechanism between the engage and hold connectors closes and it would be possible to start cruising.

The cruse transducer solenoid is designed in such a way that it takes a certain amount of current to engage it, and a certain lower amount of current to maintain it.

There is 40 OHM resistance wire in the "ON" circuit of the cruise control. There is no resistance in the "ENGAGE" circuit of the cruise control.

Let's say you are going 60 MPH. The brass friction wheel is causing a certain deflection of the spiral torsion spring. If you were to turn the cruise control "ON" and press the "ENGAGE" circuit, the solenoid will activate and lock the rubber clutch inside the horse shoe shaped clutch spring in the cruise control mechanism. Any future speed reduction, like going up a hill, will cause less friction between the steel and brass friction wheels and allow the spiral torsion spring to cause the rubber clutch to spin slightly, this causes the variable orifice to close and bleed less vacuum and therefore send more vacuum through the fixed orifice to the vacuum servo which causes the carb throttle to open. When the car speeds up, the friction in the friction wheels will increase, causing the torsion spring to load up more, causing the rubber clutch to twist more and open the variable orifice up, bleed more vacuum, and therefore send less vacuum to the vacuum servo that is pulling the carb plates open.

If you hit the brakes, a solenoid mounted above the cruise control transducer stops vacuum from reaching the transducer and servo. If you hit resume, this solenoid allows air to pass through the cruise control again and the trapped rubber clutch is still controlling the car at the previous speed.

Weaknesses in the system:

The friction wheel is a mechanical wearing mechanism subject to dirt, grease, and wear. The variable of this dirt and wear must be within the range of the variable orifice. There is an adjustment for this on the cruise control. Loosen the two screws and the position of the spiral torsion spring with regard to the brass friction plate can be adjusted about 15 degrees. This will impact the 30 to 35 MPH cutoff switch as well as the variable orifice.

Vacuum leaks. If your corvette already has vacuum problems with the headlights, your cruise control might not work properly. The PCV valve, brake booster, distributor vacuum advance, climate controls, etc. all work off the vacuum. The brake pedal and headlight switches are vacuum switches also, a leak anywhere can cause a lack of vacuum.

Vacuum clogs: The variable vacuum orifice can clog. The fixed vacuum orifice can clog. The vacuum lines can clog. Excessive heat can melt vacuum lines. The routing clamps can compress vacuum lines shut.

Things I do not understand:

Mystery 1: The steel friction wheel has a round magnet wheel under it. It looks like a 4 claw steel wheel clamped onto the back of the magnet and a 3 claw steel wheel clamped onto the front of the magnet. Maybe the brass friction wheel is just brass coated and it has a slight attraction to the magnet under the steel friction wheels? This is all part of the design? GM wanted more friction at high speeds and less friction at low speeds. Maybe a linear correlation between speed and friction? The magnet helps with this? It gets the tension of the friction wheel correct without loading it with a compression spring?

Mystery 2: I've seen misinformation about the two wire and three wire cruise control transducers for a 1981 Corvette. In fact the 1981 Corvette SHOP MANUAL on page 9-3 shows a unit much different than what is on my 1981 corvette. The 1981 SHOP MANUAL is showing a 2 wire transducer with the resume solenoid mounted differently. My solenoid is mounted on top of the transducer with just one bolt. The 1981 SHOP MANUAL shows the resume solenoid mounted on the front of the transducer with two bolts into holes not molded into my transducer's housing. That third wire is labeled "LIGHT". What is that used for? Was that ever used for an instrument panel light for a C3 Corvette?

Mystery 3: The transducer is not a simple vacuum valve, allowing vacuum to flow more when it needs the car to go faster and allowing vacuum to flow less when the car need to go slower. If you look at the vacuum lines, there are three vacuum hoses going to the transducer. Two of them are "T"ed in with the brake switch vacuum line and the vacuum servo vacuum line. One of the two enters the transducer through a fixed orifice. The other enters the transducer through the variable orifice. Why was this fixed orifice design needed? It seems to me that the variable vacuum orifice could have been used to control the vacuum instead of to bleed vacuum? Maybe the bleed design is more robust?

Mystery 4: The 1981 SHOP MANUAL talks about a bead chain on page 9-8? My Vacuum servo is connected to the carb throttle lever with a metal rod with a big slot in it to allow the throttle to advance when the cruise control is off. Does anyone have a 1981 corvette with a bead chain connecting the vacuum servo and the carb throttle? I'm sure Bubba has been here with the previous owner because there has been a carb change; but did Bubba redesign the cruise vacuum servo linkage? If he did, he did an excellent job!

Rebuilding:

If you have an electrical problem, you can exchange your transducer with the Corvette parts suppliers for about $150 and get a new one.
Maybe the 35 MPH switch is bent or broken
Maybe the transducer solenoid is shorted or open.
Maybe the resume solenoid is shorted or open.

If your electronics are OK, maybe you just need a mechanical rebuild?
Use brake clean or mineral spirits to clean the trash from the gears and friction plates.
Use compressed air to clean the orifice path ways.
Use a Q-Tip to clean any hard to reach areas that you do not want to get solvent on.
Use sandpaper or a key blank file to clean the 35 MPH contact points.
The rubber clutch needs to be clean and free from oils.
The friction plates need to be clean and free from oils.
The horse shoe shaped clutch spring needs to be clean and free from oils.
The only place I put a bit of oil was the needle tips of the clutch shaft.
The friction plates will have circular grooves in them, the copper colored one will be worse. Use sandpaper to clean these up some, being careful not to damage the needle tip. It is more important to get them clean than to get them perfectly smooth. You will probably notice from the circular grooves in them that they were not perfectly smooth to start with.
Test and inspect your vacuum lines, replace anything suspect.
The vacuum servo can be connected to a Mityvac hand held vacuum pump to prove that it works.

Adjustments:
If you are frustrated by cruising to 70 MPH and then turning on the cruise control only to find that it cruises at 66 MPH, use a 7/16" wrench to loosen the lock nut on the adjustable vacuum connector which is the other end of the variable orifice. Use a 3/8" wrench to turn the tube one full turn outward (bleed less vacuum). Remember to snug up the lock nut with the 7/16" wrench and go test it!

If you are frustrated by cruising to 70 MPH and then turning on the cruise control only to find that it cruises at 74 MPH, use a 7/16" wrench to loosen the lock nut on the adjustable vacuum connector which is the other end of the variable orifice. Use a 3/8" wrench to turn the tube one full turn inward (bleed more vacuum). Remember to snug up the lock nut with the 7/16" wrench and go test it!

I'm looking for feedback on these instructions. Please point out where I am wrong and offer advice.

Lakeside49

Respectfully, I am certainly not as smart or as experienced as Mr. Huggins per his above detailed post, but here is how I fixed the speed control on my `82 last weekend, which will help you if you're a relative novice as me. Its almost always the transducer per my technician. The servo that is next to it and pulls the throttle cable, as told by the transducer when it is working, is supposedly fairly reliable. I bought my transducer at eBay for under $100 (reman'd) with an $11 refundable core. I would have paid more now, knowing how nice it is to have cruise control back, but its entirely your call if its worth it for everything to work in your Vette as a lot of good GM engineers worked hard to perfect, and was originally paid for by our predecessors...before they had kids and had to temporarily put aside their dream car. I thought so, but my regular shop quoted $350 for the job, so I saved big doing it myself as a very amateur hobbyist and the guidance instructions that came with it were straightforward. One online article said the C3 speedo design is a "primitive" design vs. today's, but it is a minor engineering marvel, and cool looking apparatus, inside and out if you have any appreciation for that sort of thing.

So, cutting to the chase, getting it out is a bona-fide bear. Maybe, give up now if you have extra-huge hands or extra-small patience. Reinstalling it is not quite as challenging because you've already moved the hoses/cables out of your way with tie-cables or tape, awaiting re-plugging in, and remember how to position your wrench and socket. And, because you know you're on a roll, clean new parts, etc. Do it in the natural daylight of your driveway - way too many shadows even with a bright shop light (I started but gave up the night before in my garage).

OK, first, remove the washer bottle top tube which is designed to separate from the washer bottle with one bolt then by careful twisting it off. Spray the joint with penetrating oil or silicone to minimize the separation effort(AutoZone sells a washer bottle repair kit if you get overzealous). Then, be sure to put a strip of electrical tape cushion on the sharp fender nut where that bolt attached to protect your hands in those tight quarters- or, put it off and have a few small bandaids handy like I did. Then, remove the hoses on the transducer, then the speedo cables - label their location with masking tape/marker - you'll be very glad you did. While you are at it, it is important to remove the speedo cables (I tugged them out with needle nose pliers) and heavily lube them with lithium or other heavier grease. Don't worry - they feed back in easily while twisting. Put a gob of lubricant in your palm and feed it back in while lubing and twisting the cable. Then, unbolt the two attaching nuts and bolts - the toughest experience of my own C3 hobby experience to date. You'll certainly gain practice getting a wrench on the nut and a socket on the bolt due to interference and cramped spaces - but, use a longer socket extension and most importantly, a flexible joint (bought at Lowes, along with 2 new nuts and bolts) at the socket (put between the socket and the socket extension rod), coming in at as steep of an angle that you can, and be patient. Wear your glasses if you need them to save time. At first, when you finally get a bite on the bolt/socket, you'll sometimes measure turns in single clicks when you find that perfect angle.. and, that you'll have to re-find a few times. After you are victorious there, the rest is more or less down hill. Next, unbolt the `Resume' solenoid bracket, if applicable (`81-`82)' while its still down in the fender nest. Its too large, or I didn't figure out the right Rubick's Cube angle despite trying - again, I'm new to this stuff. I would recommend clear coating the nice new transducer and silver- painting the old `resume' solenoid before you install it to give it a new, longer lasting OEM look if that's a goal. As a word of encouragement, it was a bit easier to reinstall the transducer than taking it out thanks to the (2) new acorn nuts and bolts that Lowes had in their slide-out `stainless steel fasteners' tray in the nuts & bolts isle, and hoses/cables being pulled back cleanly out of the way - at least a 3-4 minute task in itself.
The thrill of the cruise working about 2 hours and 3 band-aids later on a smooth-road test drive was a highlight of my entire day. My wife accused me of "beaming" when I came back from my test drive where the speedo worked, exactly as stated in the `82 Vette owner's manual. Good luck, be patient, and maybe spend a touch of that significant $ you saved by taking your co-pilot/wife out to celebrate... sitting on an outdoor dining deck, - looking out at your beauty parked in the vicinity of your hand picked open table - watching the next generation of C3 admirers and hobbyists staring and sometimes pointing at it as they walk by. Good luck and would be interested in knowing how it goes with yours! Dave

More at: http://forums.corvetteforum.com/c3-t...ransducer.html

Maxx76

Interesting reading indeed.
Mine was blocked, the cable from transmission was broken.
I opened the transducer, cleaned it, but now the are some hard points when turning either cable connector.
I read there is no need for any oil in the mechanism. Then how can I have the gears turn smoothly with no undue resistance ? Are the mechanism and gears worned out or is there a way to save about 100-150 bucks to fix this ?

Thanks.

Lakeside49

It is beyond my familiarity with the cruise control system to offer guidance at the same level of understanding as you seem to have. I actually opened the old transducer out of curiosity and noticed that it is definitely a marvel of electro-vacuum-mechanical engineering - but it looked too complicated for me to sort out and fix whatever had failed. We think we're pretty smart today - and we are. But, our engineering forebears were just as smart, or maybe better since they worked without the design technology that we have today - look at the insides of a transducer if you have doubts. Anyway, it was worth the cost of the reman replacement to me. Most of the time I have no need to use it, but when I want it, I want it to work - I used it several times this weekend during long rural-road stretches on a long (9 hrs. on lovely back-roads, 300 miles with lots of small town shop stops along the way) Labor Day weekend get-away cruise with my wife to a favorite bed & breakfast on Lake Huron (Lexington, MI) - I was glad to have it. And, BTW, you don't take it for granted after you've lost it for some time - you smile every time you kick it in and it effortlessly takes over.
Dave

Morgazm916

can you please scan this for me? there is no cruise control with mine but there is the rod to set it.
Thanks
Morgan

Serial Steve

This is gold. Thanks!!

Blondie - C3

I'm in canton also with my 81. My speedometer stopped working and before that the cruise would race and slow and race and slow. Did yours do that too? My wife and I have been to b&B in Lexington, at inn the garden and also a night to remember. Both nice places.

Lakeside49

Hey -
thanks for the B/B tips. And, you might try The Captains Quarters (early 1900's Mich. Governor's summer home) the next time you're in Lexington. What a wonderful annual trip we have taking the back roads in the C3 I can give you the general route is you'd like.

Regarding your question, my speed never fluctuated and the speedo never broke but that is a classic transducer failure. The internal mechanisms tighten up and it breaks the speedo cable. The cable (be sure to lube the one that did' break - it pulls out and easily slides back into its housing) and transducer aren't that expensive, but if you do it yourself be aware that it is a tight space. Be sure to clip or replace the vacuum hoses while you're in there - a connection leak could explain the fluctuation. If you don't want to do it, Bobby (Bobby's Corvettes) on Mich. Ave. in Ypsilanti could do it.

Hope me to see you around - we wave!

Blondie - C3

Thanks for the lead on Bobby,s. I have been using bill brown on Lilly for basic stuff. We would like the route you take to Lexington. Always looking for day trips. We wave also. Hoping for an early spring

See you around. Keep in touch.

Blondie - C3

Bobby's corvette worked out great. He knew his stuff and was very reasonable. Now that's it's summer, my wife and I need to plan a trip to the captains quarters. You offered a back road route from canton area. When you have time, please send me your route. Thanks.

theandies

What's cruise control. Never heard of anything like that on a '71. Is it like those mythical cup holders I keep hearing about?????

Lakeside49

I built a map that will get you started. Click on the below link. Feel free to modify it, but its where you want to be. We take a little different route every year, but for us the following is our goal:
Get to the north and east of the bloomfield area. The twisty roads around the lakes are nice, but we prefer the more open rural roadsThink seriously about getting a Windjammer as a gift for your wife. It cuts down on the wind and makes a nice sunshade at certain times of the later day. But, the hex key to tighten the two bolts was annoying. So, I cut off the head of the bolts and screwed each head one of those small black, theaded ***** that you can buy at Northside True Value in Westland. They are in a drawer in the back area of the store. But, it really is a nice accessory.Pack light, but include a light car cover to keep pine tree stuff off your car in the driveway of the Captains Quarters (former Governers Mansion). The owners are fantastic and will point you where to go. We park the car, cover it and walk everywhere. The winery/brewery is right by your B&B but there are good restaurants, too. We are from Colorado and green chile burritos at the nearby bar/restaurant are as good as anything in CO or NM because the cook is from Colo. His wife was from Lexington and they had to come back. Plan to take at least a short drive north on the Lake Huron shore on one of your days. The scenery is pretty and its like your car was made for that road. Wish you guys good weather! Tell the folks at the B&B that Dave & Janel said hello!


https://goo.gl/maps/YDbfGaHtGFN2

Philgran

When my car is cold the speedometer needle vibrates back and forth wildly and makes a metallic rubbing sound. After awhil, it seems to calm down and registers what seems to me to be the correct speed. I have a 1979 C3 with Cruise control (which dies not work). When I bought the car, I noticed that the speedo cable was hanging down beneath the frame and had my mechanic replace it. There was no noise or needle gyrations before that. One of the guys in my club feels it’s the transducer. Any way I can check that? Thank you for your time and help. Phil