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Cruise Control Problem

ddick9366

Member
Joined
Feb 25, 2007
Messages
24
Location
Greenwood, IN
Corvette
'94 Polo Green Convertible
The cruise control, although functional, does not maintain constant speed, even on the most level straightaways, and regularly decreases speed gradually, and only resumes correct speed setting by feathering the accelerator.

Is this a simple crusie control unit replacement? Or is there anything else that can be done to isolate and solve the problem?

Thanks in advance for your advice.
 

jmccloud

Well-known member
Joined
Feb 1, 2003
Messages
694
Location
Maryland
Corvette
1995 White Coupe LT-1 Auto
Cruise control problem

Your symptoms are often due to poor vacuum. Check the reservoir (spherical ball) and hoses. There are many any posts that describe failure of the double 't' on the passenger side of the intake (under the plastic fuel rail cover) or failure of the check valve (cone topped cylinder - about 1/2" dia) in the same location.

It could also be a bad servo. Use the 'search' tab above to locate cruise control threads. There are lots of them on how to troubleshoot the diaphram etc. Often repairs are easy without the need to replace the unit.

Good luck
 

G Winter

Well-known member
Joined
Aug 21, 2004
Messages
2,177
Location
NW Iowa
Corvette
1990 red convert 6 sp
First thing to check is all vacuum lines for leaks. That is the most common cause.


Glenn
:w
 

93*Corvette

Well-known member
Joined
Jan 20, 2006
Messages
66
Location
Colorado
Corvette
1993 White Coupe
The cruise control, although functional, does not maintain constant speed, even on the most level straightaways, and regularly decreases speed gradually, and only resumes correct speed setting by feathering the accelerator.

Is this a simple crusie control unit replacement? Or is there anything else that can be done to isolate and solve the problem?

Thanks in advance for your advice.
My bet is this valve under your fuel rail. They always go bad.... (This is a pic showing my 1993 with it's part number)

2j1rj11.jpg
 
H

HD2HVETTE

Guest
Mt 93 had the same symptoms-If you can find a post by lone73 dated 8-23-06 he describes a repair procedure that fixed my problem. 5 minute fix,easy to follow instructions.
 

John Robinson

Gone but not forgotten
Joined
May 3, 2005
Messages
1,555
Location
Muncie, Indiana
Corvette
1993 Polo Green Coupe
Mt 93 had the same symptoms-If you can find a post by lone73 dated 8-23-06 he describes a repair procedure that fixed my problem. 5 minute fix,easy to follow instructions.
Been there tryed that. If only it had been that easy;help
 

ddick9366

Member
Joined
Feb 25, 2007
Messages
24
Location
Greenwood, IN
Corvette
'94 Polo Green Convertible
Cruise Control Problems

Thanks so much for the great and timely feedback. Anxious to try and trouble shoot my problem. With so many good suggestions, I'm fairly confident about finding a resolution. Anxious for the weekend for some time off to "spend some time with my hobby" and try out some of the suggestions.
 

SPANISHVETTS

Well-known member
Joined
Nov 5, 2002
Messages
1,143
Location
El puerto de Santa Maria Spain
Corvette
before 60,63&82 now 90 383 Stroker & 91 ZR1-Sold
The other common source of failure is the vacuum switch on the brake pedal.

The switch is made of crappy nylon and with time will slip back in its adjusting nuts allowing the vacuum to bleed off.

Replacing a crappy part with a new crappy part does not really appeal to me so I used a heavy duty plastic tie-wrap to hold the switch in place. This cure has been working for 4 years on Thunder and was working great on the ZR1 when I sold it last month.

Side question
:madWhy do GM vacuum hoses and weather stripping rot after only a few years. I have a 30++ year old Toyota LandCruiser that has never-ever been garaged. Every piece of rubber and every hose that came on the truck are still serviceable. Thunder never sleeps outside and never stands in the Summer Sun. I have had to replaced every piece of rubber and all of the hoses on the car - some more than once.
 

ddick9366

Member
Joined
Feb 25, 2007
Messages
24
Location
Greenwood, IN
Corvette
'94 Polo Green Convertible
I removed the electronic activator from the servo unit and checked/cleaned the connections as Lone73 suggested on the post dated 8-23-06.

I also removed and inspected the vacuum junction block located under the passenger's side fuel cover as 93*Corvette suggested and noticed it appeared as if it might have a heat generated weakness/failure around the circumference joint seam.

I applied a coat of acrylic seal adhesive on the exterior circumference and let it setup a few hours and dry enough to reinstall.

I haven't toroughly checked the vacuum system throughout, but I'm going to wait now until the weather turns better and take it out on the interstate to see if the cruise control acts any different after these minor maintenance checks.

I have not yet checked the vacuum switch on the brake pedal as suggested by SpanishVette, primarily because I haven't located it yet!

But I had a great Saturday of relaxing and tinkering with the Vette! Thanks for all the above guys who through their encouragement and counsel contributed to my enjoyment.
 

SPANISHVETTS

Well-known member
Joined
Nov 5, 2002
Messages
1,143
Location
El puerto de Santa Maria Spain
Corvette
before 60,63&82 now 90 383 Stroker & 91 ZR1-Sold
There is an easy way to check the vacuum switch without pulling the underside of the dash apart.

Pull the vacuum hose off of the Cruise Servo and suck on it.(or use a vacuum pump & gauge if you are squeamish about what you suck on)

The hose should hold a vacuum. If it does not then you can move down to the Tee that branches to the brake switch and intake manifold and preform the same check on each side of the tee.
 

93*Corvette

Well-known member
Joined
Jan 20, 2006
Messages
66
Location
Colorado
Corvette
1993 White Coupe
I removed the electronic activator from the servo unit and checked/cleaned the connections as Lone73 suggested on the post dated 8-23-06.
I can't find this thread... Can you post the link... (I did a seach and nothing comes up)
 

XLR8

Gone but not forgotten
Joined
Jun 23, 2004
Messages
6,662
Location
Mississippi Gulf Coast
Corvette
2003 AE Convertible, 1998 LCRM Convertible

ddick9366

Member
Joined
Feb 25, 2007
Messages
24
Location
Greenwood, IN
Corvette
'94 Polo Green Convertible
After trying all the previous posting suggestions with no improvement, I purchased a servo control unit from eBay and replaced the original with the replacement, presto, problem solved, holds speed steady now, and cruises great, just like I expected/wanted!
 

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