Cruise Control on '97 Lesabre

[kelshm]

I have a '97 Lesabre and the cruise control will work perfectly sometimes but most often will not engage at all. The Cruise light on the dash doesn't come on unless it is actually working. Generally it will stay engaged until you shut it off but once it quit without shutting it off. Any ideas?

[happydog500]

Sorry to see hear problems with Cruse.

I had cruise control problems on my 97. Not sure why, but I had most all questions answered, except Cruse Control. I searched this and other forums, but they to, when a cruse control question was asked, no answers.

Don't know why that is, just seems to be a thing I noticed on the forums. If you do get it fixed, post back to let others know what it was. It's been a couple years since I got to use cruse.
Chris.

[big white bufflo]

my 92 did the same thing never figured it out but it work know thing to look for the brake switch ,bad light bulb next time that happens try pulling up on brake peddle that might rule out the switch or check it with a meter good luck

[kelshm]

I know that lifting up on the brake pedal doesn't change anything. I have read somewhere, and I forget now where, that the on/off switch often goes bad in these cars. It is also the turn signal lever. I'm thinking you can change it in this car without removing the steering wheel and air bags. I suppose if that's the problem it will have to wait as I just got laid off.

[happydog500]

That is a good thing to check. If the middle tail light (the one in the back window) goes out, the cruse control quits working.

When mine quit, I checked, and the light was out. I was so happy!! It was an easy fix. Until I changed it, and it still didn't work.

It could be the problem for you, check to see if it's working.
Chris.

[HotZ28]

You may have a problem with the cruise/shift interlock brake switch located under the driver side dash and above the brake pedal. I agree, the bulb in the rear/center high mounted brake light should have no effect on the cruise shutting off, it is controlled by the adaptive light monitor control module. If you do have a problem with the rear/high mounted brake light, this again could point back to the brake switch. If the brake switch is not functioning properly, you will not have power to the adaptive light module that controls the rear/high brake light. The white wire (W) from the brake switch feeds the module for power to the rear/high brake light. BTW, you have a PM with a schematic. Good Luck!

[happydog500]

Originally Posted by kelshm

I know that lifting up on the brake pedal doesn't change anything.

I remember looking at the break pedal now. I agree, it doesn't change anything. On mine, the 'Cruse" light comes on.
Chris.

[kelshm]

Since the brake switch was so cheap, I went ahead and replaced it. It didn't change anything.

[happydog500]

Answers that don't work are an improvement over not getting any replies at all.

With other stuff, there's a few basic things it could be. Ways to check what's wrong. Must be something with Cruse Control that makes it different then even the most complicated things on a car.

I looked all over the internet. I still don't know how to check what's wrong with Cruse. It's been two years.

Chris.

[pcmos]

Okay, just to fill everyone in, I've been chatting with happydog500 via private message.

I have some free time right now because I'm currently biding my time while waiting for this dumb economy to yield some employment for me.

In the mean time I'd like to finally put this cruise control issue to rest. The cruise control system is really simple. Out of all of the computer controlled systems in this car, the cruise control should be one of the easiest to diagnose.

I'm working from the shop manual for the 1999 LeSabre, which should be identical to the 1997 and 1998 models. Cruise control is also the same for both the Custom and Limited models 97-99. Hopefully I can figure out how to post some diagrams on here. I doubt GM will care if I copy their shop manual, they're busy doing other things right now.

[pcmos]

First and foremost we need to look at the primary component of the cruise control system.

The cruise control module is the "controller" or "computer" that handles all of the cruise control functions. The module itself consists of an electronic controller coupled with a small electronic stepper motor (like a hobby motor from an RC car kit). The electronic half of the module moves the small motor which literally pulls on the throttle cable to maintain the vehicle speed.

The cruise control module is located on the firewall next to the brake master cylinder and brake power booster assembly. It looks like a small black box bolted to the firewall with throttle cables going to the throttle.

Fortunately the cruise control module is highly independent compared to other vehicle modules. The cruise module is NOT connected to the class 2 vehicle network. It's a form of an input / output controller design.

Inputs:
Cruise ON Switch (located on the turn signal lever)
Cruise Set/Coast Switch (located on the turn signal lever)
Cruise Resume/Accel Switch (located on the turn signal lever)
Brake Pedal Switch - TCC (located above brake pedal)
Stoplamp Feed Circuit (part of the brake pedal switch)
Cruise Disengage Signal (comes from engine computer in case you go too fast or too slow)
Vehicle Speed Signal (comes from engine computer)

Outputs:
Throttle Angle (actuated by stepper motor as part of the cruise module)
Cruise Indicator Lamp (Located on the instrument cluster)

For each of the inputs the module expects a particular value. If the cruise module decides that any of the inputs are false or faulty, it switches into a "standby" mode. Based on my research, I believe that in standby mode the controller will illuminate the cruise indicator light, but doesn't actually move the throttle.

What we need to do is to figure out why the cruise module is failing to actuate the cruise by systematically checking all of those inputs listed above. If all of the input sources check out good, then the problem is most likely the cruise module itself.

[pcmos]

Before checking out the inputs to the cruise control module we need to check one really easy possibility. The cruise control module and most computer modules in the car depend on a really good ground to the vehicle chassis. Control modules in general operate by switching and manipulating grounds.

There is a ground post called G104 which directly grounds the cruise control module. The ground post is located under the air cleaner box on the vehicle frame rail. There are actually two ground posts near one another on the frame rail under the air cleaner box. Both grounds need to be absolutely clean metal to metal contacts with the vehicle frame.

Before trying to diagnose anything else, your first step should be to locate those two ground posts under the air cleaner box. Remove the nut, pull off the ground terminal and scrub everything with a wire brush until you have bare metal with no corrosion, rust or paint, then put the terminal back on and tighten the nuts. It's tough to tell which of those two grounds are G104, so do both of them to be sure, it can't hurt.

I've uploaded an image from the shop manual that shows G104. The image is pretty crummy. I don't have this car so I can't go put my hand on the ground post. It should be easy to spot, it's definately on the frame rail under the hood either under or near the air cleaner box. If you can't find the ground posts I'll go look on my car and see if the 04 has the same ground locations by chance.

In general when you have any problem with an electronic module on the car, its a good idea to track down all the ground locations required by the module and clean them. Make sure they're making good contact with the chassis. Grounding problems can cause all sorts of whacky behavior from the computer modules. I've learned my lessons about grounds the hard way.

[pcmos]

Here is a graphic image from the shop manual showing the location of the cruise control module itself. The cruise control module is labeled as item #2 in the image.

[happydog500]

OK, thanks for the information. I will try and do the ground check/clean in the morning.
I did a quick look but didn't' see the ground post. It was just a quick look. I didn't take the air box off at that time.

Chris.

[pcmos]

Okay sounds good. Clean those ground connections even if they don't look bad or loose or corroded, just to be absolutely sure. Wire brush everything so you have good clean metal to metal contact when you tighten the nut back down. Then it's easy to rule that out as a simple solution.

Do you own or have access to a good digital multimeter with a resistance mode? I'm going to try to think of a way to test those other input sources at the cruise module connector itself with nothing but a digital multimeter. Hopefully I can help you diagnose the problem without the use of a scantool system.