OEM Cruise Control Acting Strangely

[charles mullally]

My original OEM cruise control system was working well, but recently my cruise control won't engage (or light the green dashboard tell-tale lamp), or
stay engaged unless I slightly lift the brake pedal with my toe. Sounded like an out-of-adjustment vacuum release valve, right? So, I readjusted the
valve to further depress the spring loaded plunger - no change in operation. I removed the vacuum release valve and bench tested it with my hand
vacuum pump and found a very gradual leakdown. I pulled the rubber seat from the valve and found minute age related cracks in the seat area. I then
installed a new valve (bench tested OK), thinking my problems were solved but no change, still required a toe lift on brake pedal for proper cruise
control operation - even after readjustments. As a test, I plugged the vacuum hose and tested for cruise control - behaves the same, still requiring
that the brake pedal be lifted slightly for cruise control engagement or operation, drops out as soon as I move my foot from under the pedal. Brake
lights OK and brakes work normally with proper free play and no dragging. What am I missing?
--
Chuck and Lureen Mullally
West Michigan
1975 Glenbrook

 

[hal stclair]

Must be the brake/cruise switch. The brake light switch has an extra set of contacts which open when the pedal is depressed that disengages the
cruise. Sounds like that side of the switch has an issue.
Hal
--
1977 Royale 101348,

1977 Royale 101586, Diesel powered,

1974 Eagle Bus 45',w/slideout

 

[charles mullally]

Probably right Hal. The wiring schematic is not very clear on that, shows the brake light switch without a second set of contacts and the cruise
control switch is shown without an indication that it is piggy backed with the stop light switch. I have readjusted the brake switch for more travel
and I expect that will fix the problem. Thank you.
--
Chuck and Lureen Mullally
West Michigan
1975 Glenbrook

 

[Jim Hupy]

The cruise control was an optional feature and is barely covered in the
manuals. There is a separate GM manual that covers just the cruise controls
in all GM products of that era. It has in depth details on how to service
and adjust the factory unit. I don't know the numbers of the manual, and
it is long ago out of print. I still have one safely put away (where I
can't remember) along with my alternator manual and HEI manuals that I got
when I attended GM Service school. Ain't getting old fun?
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> Probably right Hal. The wiring schematic is not very clear on that, shows
> the brake light switch without a second set of contacts and the cruise
> control switch is shown without an indication that it is piggy backed with
> the stop light switch. I have readjusted the brake switch for more travel
> and I expect that will fix the problem. Thank you.
> --
> Chuck and Lureen Mullally
> West Michigan
> 1975 Glenbrook
>
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[john r. lebetski]

Did you set them both by doing the automatic ratcheting procedure? Slide them for more brake pedal interference then pull up on brake pedal one time
to set.
--
John Lebetski
Woodstock, IL
77 Eleganza II
Source America First

 

[a.]

> The cruise control was an optional feature and is barely covered in the manuals. There is a separate GM manual that covers just the cruise
> controls in all GM products of that era. It has in depth details on how to service and adjust the factory unit. I don't know the numbers of the
> manual, and it is long ago out of print. I still have one safely put away (where I can't remember) along with my alternator manual and HEI manuals
> that I got when I attended GM Service school. Ain't getting old fun?
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403
The MM for 1974 says the cruise control switch on the brake pedal is a separate switch from the brake light switch.

"stop light switch from top of brake pedal. If equipped with cruise control remove the switch next to the stop light switch (figure 30)."

As far as "barely covered in the manuals", the Cruise Control section begins on page 12-31 of "Chassis Electrical" and ends on page 12-40 with
explanations, diagrams, photos, and charts that probably rivals what is in the separate GM manual.

How much difference is there in the Cruise Control system among the different model years?
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Highest price does not guarantee highest quality."

 

[Jim Hupy]

The electrical switch is supposed to disengage the electrical part of the
cruise control the instant any pressure is placed on the pedal. There is
also a vacuum switch that cancels the vacuum supply to the big diaphragm
bellows to release the throttle cable. Sort of a belt and suspenders
approach to cancelling the cruise control. The switches are sensitive to
adjust. Sounds like yours is just a bit too close to the pedal arm.
Jim Hupy
On Sep 24, 2016 12:24 PM, "John R. Lebetski"
wrote:
>
> Did you set them both by doing the automatic ratcheting procedure? Slide
them for more brake pedal interference then pull up on brake pedal one time
> to set.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
> Source America First
>
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> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org