Speed control issue

[SpartanBert]

We have the 2003 Liberty Limited Ed 4x4. We didn't have this problem until last year, even though we've owned it January 2003.

Problem: on the highway, set the cruise control at 72mph. Get pulled over by cops for going 81mph. I wasn't coming down a hill or anything. After I pull back on the road, I go back to the same setting, 72mph on the cc. The car felt fast, like it surges when engaged in cc. I just didn't really notice it before the ticket, as it was a long drive already that day, 9 hours. I take it to the dealership, they flash the computer saying they saw an error related to the cruise control. I don't know what the error code is or anything. This all happened last October.

Now, this week, my wife gets pulled for the same thing, 72mph set on the cc, pulled for 80/81mph; and other people have said they have seen her flying around town on the freeways the past few months. She isn't a speeder at all.

Something is wrong, and it didn't get fixed in October.

We don't notice this when CC is not engaged. Has anyone else had a problem like this? I have the 17inch tires on it, stock, but they could'nt account for a 12% difference in speed. Any ideas on what I should tell the dealer?

 

[roadrunner]

speed

Please forgive me if I'm missing something obvious here but, what does the speedometer read after you set the cc to 72? Presumably it's giving you some hint that you're speeding up?? If so, it may be better not to use the cc until the dealer repairs it. Otherwise it could get to be a very expensive problem. :-k

 

[Corwyyn]

I have an '03 sport and have had no problems with the cruise contol, but I usually set mine based on the indicated speed from my GPS; the speedo is usually within 1mph of that. Haven't noticed any glitches in the speedo display so I don't know what to tell you.

 

[2003KJ]

do you have any tires on that are different from the OEM size? Bigger or smaller tires will throw off the speedo, dont think by that much, but they do throw it off. Perhaps tell the dealer to re-calibrate the speedo for the hell of it to see where that gets you. And if it's still under warranty, i'd be making DC pay for the speeding tickets...lol.

 

[SpartanBert]

Original OEM tires, the 17" option. Speedometer will say 72mph the whole time. Kinda weird.

 

[grogiefrog]

I'd get in another car and follow the wife in the KJ when she's got it set on 72, and see what the other car says. If you both have cell phones, you can both talk to each other during the test to confirm.

I haven't noticed any speed up when engaging the speed control. I do figure that my speedo is about 2 mph over what it really is.

That's just weird... I would think that your speedo would have to be way off... not just the speed control. Crazy! :-k :-k :-k

 

[Schj98]

If you have a GPS unit that displays ground speed they are a great way to check you speedometer. While not that accurate with height they do a great job with ground speed.

 

[kegjeep]

I have an '03 sport and have had no problems with the cruise contol, but I usually set mine based on the indicated speed from my GPS; the speedo is usually within 1mph of that. Haven't noticed any glitches in the speedo display so I don't know what to tell you.

yea i love my GPS, my 05's speedo is within 1mph also. I've got a garmin etrex and its nice to see the mph on a truck that has larger tires since the speedo says that you are going slower than it actually is.

 

[4Factor]

I think it depends on WHEN you set the cruise. Obviously if you set it while accelerating, going up/down hills or while decelerating (foot off all pedals) you might actually get it set at a different value than you wanted, but I'd think you'd notice that when you glance at the speedo every so often.
Using a GPS unit for a quick test is a good idea, just keep in mind that the GPS speed is not 100% accurate.. my guess is the GPS with a few satellites "in view" should be accurate to +/- 5mph at the very worst. If it's showing 10mph (or more) off then maybe you should talk to a DC dealer about having it looked into further. Maybe the sensors that read the speed values (and send it to the gauges) are broken.

Just as a double check, your 17's should be 235/65R17

Sport: 225/75R16
Renegade: 235/70R16
Limited: 235/65R17

If somebody REALLY goofed up when putting on new tires and got all the numbers mixed up, the worst I could see is if they put 235/75R17's on. It would account for you reading 76.6mph (~77mph) when you thought you were going 72mph. Speedometers are not perfect so actually going 80 when it's set (incorrectly) for 77... that's easy to believe.

 

[SpartanBert]

The tires are the 235 65 17, straight from the factory, an item on the build sheet.

Yea, we will try following the Jeep with another car, see how it goes. Maybe there is something to the idea of setting it while still accelerating, maybe the cruise control is sensitive to something along those lines.

 

[Corwyyn]

I think it depends on WHEN you set the cruise. Obviously if you set it while accelerating, going up/down hills or while decelerating (foot off all pedals) you might actually get it set at a different value than you wanted, but I'd think you'd notice that when you glance at the speedo every so often.

From the 2003 KJ Service Manual:
OVERSHOOT/UNDERSHOOT
If the vehicle operator repeatedly presses and
releases the SET button with their foot off of the
accelerator (referred to as a “lift foot set”), the vehicle
may accelerate and exceed the desired set speed by
up to 5 mph (8 km/h). It may also decelerate to less
than the desired set speed, before finally achieving
the desired set speed.
The Speed Control System has an adaptive strategy
that compensates for vehicle-to-vehicle variations
in speed control cable lengths. When the speed control
is set with the vehicle operators foot off of the
accelerator pedal, the speed control thinks there is
excessive speed control cable slack and adapts
accordingly. If the “lift foot sets” are continually used,
a speed control overshoot/undershoot condition will
develop.
To “unlearn” the overshoot/undershoot condition,
the vehicle operator has to press and release the set
button while maintaining the desired set speed using
the accelerator pedal (not decelerating or accelerating),
and then turning the cruise control switch to
the OFF position (or press the CANCEL button if
equipped) after waiting 10 seconds. This procedure
must be performed approximately 10–15 times to
completely unlearn the overshoot/undershoot condition.
So it looks like this might be part of your problem. Its at least worth a bit of time to see if you can get the cruise control to behave properly. I just put a set of Geolander 235/70-16s on my Jeep and have only noticed about 1/2mph difference at 65mph; wonder if the shop actually had the ability to flash the memory for the larger tires :-k

 

[grogiefrog]

Nice post! Good info. =D>