C3-1978 Speedo/Cruise Control

[Barry #41479]

I had an earlier post about a bounce in my speedometer needle. I received several great pieces of advice on lubing my cable and/or replacing the cruise control transducer. I drove it about 500 miles round trip this weekend to a chapter judging meet in Knoxville, (2nd Flight first time judged for a driver with 77K miles so very pleased), and the needle kept the same mild bounce and would occaisionally swing a bit more wildly.

On my return after the meet I decided to try the cruise control which worked fine about 6 months ago. I was going about 80 and when I engaged the cruise and it immediately started accelerating. I touched the brake to disengage and waited a couple of minutes and tried agian. It accelerated again on it's own and after I disengaged with the brake the speedometer needle bounced a couple of times and then fell to 0 and stopped working altogether. The odometer and trip odometer both quit working as well.

It sounds like it is the cruise control transducer at fault but I thought it best to get a second, third, etc...opinions before I source one. Also, any recommendations the best place to get a rebuilt transducer? I have seen them listed from $88 to $109.

[Mike Cobine]

Re: C3-1978 Speedo/Cruise Control

The transducer was a mechanical nightmare that broke in almost everything I have owned with one.

When you replace it, it isn't a bad idea to replace the cables at the same time, at least the drive cable from the transmission to the transducer since you do not know what kind of strain it has put on the cable.

I tried rebuilding a couple and never had much luck. Of course, it might have helped to have a manual on them rather than funble around inside one. There isn't much there, a could of rotating disks, and some switches. I imagine if someone rebuilds one, then it is as good as the next guy's.

[Alan D.]

Re: C3-1978 Speedo/Cruise Control

Hi Barry,
I own a 77 Corvette and have had similar problems with the cruise control. First of all 2 new cables(1 from the transmission to transducer and 1 from the transducer to the speedometer)are required. Be aware that if the cable centers are too long there will be a binding and speedometer bounce will happen.
The cruise control on a 1977 transducer has a vacuum bleed valve on it. This must be set so that the cruise unit will engage and disengage. This will have to be set by trial and error. If this is not set correctly the unit will not engage or once engage cause the vehicle to continue accelerating after setting.
Also, look in your 1978 service manual for information.