77-78 Eldorado Cruise Control

[jeffs7]

Cruise inop.
Yellow switch light is on but cruise never engages & switch lite never turns green.

The fuse, relay, wiring and hoses are all present & accounted for.
The 77-78 maintenance book has innumerable electrical checks to find the cause or source of the malfunction.

However, I wondered if there was anything I should be looking at 1st that has been known to fail on these cruise control systems.

[TJ Hopland]

I'm thinking 78 was the change year from the inline speedometer cable gearbox on the firewall to a speed sensor in the speedometer head and a control module.   Which do you have?   If its the gearbox take it off and clean the mounting points especially the one with the ground strap on it.    Does the vacuum stuff check out?

[mgbeda]

I don't know if 78's used the same setup as 76's, with a mechanical device called a "transducer" on the firewall.  If they did, the problem I generally found with that part was that the coil in the transducer would burn out.  This would keep the green light off too since the green light is triggered be the plunger moving into place.  The plunger should move and the green light should go on even if there's no vacuum, so unless you have multiple problems (always possible) I don't think a vacuum leak is your problem.

I know that this transducer was used on various GM cars even into the 80's.  I once saw one under the hood of a front-wheel drive Skylark.  But maybe Cadillac got the improved all-electronic version sooner.

-mB

[cadillactim]

Cadillac went to electronic cruise control in 77. There is a photo "eye" in the speedo head that counts the revolutions of the speedo cable. This eye may be bad. Of course chieck the vacuum solenoid to make sure it is not purging vacuum and not allowing the servo to engage.

Tim