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Discussion Starter #1
2003 Trailblazer EXT with 4.2l engine. I'm a skier so regularly drive from the lowlands to the local ski area which is at 4000ft altitude. The cruise control works flawlessly on the way up the mountain but after being parked at the resort for a few hours then hitting the road to drive home, the cruise control doesn't work. The next day it will function properly. If I make a stop at a store on my way home from the ski area, the cruise control will work just fine again. This has been consistent for several trips to the ski resort. Any ideas on what I should look for as the problem. There are no other failure indications.
 

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When it doesn't work,,, check that your brake lights work. If the brake switch is going out the cruise control won't work and will not set a code either.
 

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Discussion Starter #4
Finally got up to the mountains again today. As usual, cruise control worked fine on the way up but the doesn't work on the way back down. Per TJBaker's suggestion, I checked the brake lights prior to leaving the ski area. Brake lights worked fine so I need something else to look for.
 

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Well it was worth a look at least. Just today I had to slap my brake pedal several times to get my own cruise control to work! I am at 8200 feet elevation myself but I don't think that's got anything to do with it.

Have any sort of scanner? In particular one that can show 'pending' codes? A pending code is one where an issue has been seen once but won't set the check engine light unless the issue is seen on more than one consecutive drive cycle. These can disable the cruise control without enabling the check engine light.
 

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At present I don't have a scanner because mine died. I was wondering if it could be something as stupid as coasting down the mountain could cause a lean burn and sent a pending code that clears when I get back to the lowlands and more power on driving. Any thoughts?
 

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I was leaning more towards a wheel speed sensor issue. I once had a damaged speed sensor cable (wiring) that was OK when dry (or frozen) but gave trouble when wet.
 

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Discussion Starter #8
Interesting thought about the speed sensor. I live in western Washington so wet is normal but that doesn't exclude the sensor as the problem. I will have to do a visual inspection when I get the chance.
 

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Mine was readily apparent on inspection. A clip had come loose and allowed the cable to loosely contact the wheel & tire right at the bead. Wore through the outer insulation to expose the shield. I waited until an extended dry period then brushed on several coats of liquid electrical tape. Not exactly a classy repair but it worked.
 

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Another tidbit of information. I got a scan tool that I still don't really know how to use. I was able to pull fault codes prior to starting up after having returned from a trip to the mountains. One pending code, P0105 manifold absolute pressure. Not sure if that relates to the cruise control problem but appreciate any advice.
 

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The trouble we have is GM does not have to tell us what will cause the cruise not to work. Users have figured out some of what will do it but for other codes we can only guess. We know anything that affects the sensing of wheel speeds will do it, like the wheel speed or vehicle speed sensors. We know the brake light switch can also do it.

There is still a possibility that it's the brake pedal switch because that switch is actually 3 separate circuits in the one unit. One is for the actual brake lights (terminals A & B, normally open). One of the other 2, (terminals C and D, normally closed) tell the torque converter lockup to release when the brake pedal is pressed and supplies a signal to the cruise control system. The third circuit (terminals E & F, normally closed) is for the shift lock solenoid. So even if the lights are working (term A&B) the cruise control/TCC terminals C&D could be failing. Do you know if your torque converter is locking up as it should? Hard to say when your going downhill, I know.

As for that MAP sensor I cannot say that I have heard of that affecting the cruise control. Does your scantool read live data? Can you see the MAP sensor values? That particular code is a type B code that won't light the service light unless it happens twice consecutively. If the check engine light doesn't come on I wouldn't look into that any further. Just for curiosity,, what sort of elevation change are you doing? You mentioned the 4000 foot ski area but where are you starting at?
 

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You'd really need to have a scan tool with OE level access to start to diagnose the cruise control issue. You would also be able to look at the last stored reason for cruise control disengage. Most DIY code readers won't have OE level access. The CC logic is integrated into the PCM. The MAP sensor issue is likely a seperate and coincidental issue.

Things that cause PCM to disengage an active cruise control operation:
  1. CC On/Off switch
  2. Torque Converter Clutch (TCC) brake switch
  3. The stop lamp switch (integrated into the TCC switch)
  4. A circuit issue between any of these and the PCM
  5. A connector issue between any of these and the PCM
  6. A failing PCM
My suggestion? Take it to a reputable shop and pay them for an hour of shop time to diagnose the problem(s). You'll be dollars ahead with this investment because you will run out of money before your local parts store runs out of parts and you run out of guesses. If you then want to do the actual repair work yourself when armed with that information, you can choose to do so.
 

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Just for curiosity,, what sort of elevation change are you doing? You mentioned the 4000 foot ski area but where are you starting at?
Hmm.... good observation. Baro sensor out of whack?

Also a skewed ECT sensor can greatly impact this fuel modeling and set a MAP code if outside of expected values. Stuck open thermostat? Pretty cold at 4k feet.

I have to think on this.
 

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You'd really need to have a scan tool with OE level access to start to diagnose the cruise control issue

While there are a lot of points we haven't figured out yet we do have access to a good number of things. The state of the TCC/Cruise release switch (part of brake switch), at least some of the multifunction cruise control switch states, and the last 8 cruise control release history records are some of what we CAN see with a smartphone app and a $12 obd adapter.
 

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Hmm.... good observation. Baro sensor out of whack?
The P0105 code as pending at least indicates there may be something screwy there. I live at 8200 feet and a few times a year venture up the pass which is a tad over 11000 feet but don't recall ever seeing a code for my MAP.

MAP sensor is also one that we can see with a smartphone app and a cheap obd adapter. I can see both the legislated diagnotics PID and at least one GM extended mode 22 PID for MAP and another for BARO.
 

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Discussion Starter #16
The trouble we have is GM does not have to tell us what will cause the cruise not to work. Users have figured out some of what will do it but for other codes we can only guess. We know anything that affects the sensing of wheel speeds will do it, like the wheel speed or vehicle speed sensors. We know the brake light switch can also do it.

There is still a possibility that it's the brake pedal switch because that switch is actually 3 separate circuits in the one unit. One is for the actual brake lights (terminals A & B, normally open). One of the other 2, (terminals C and D, normally closed) tell the torque converter lockup to release when the brake pedal is pressed and supplies a signal to the cruise control system. The third circuit (terminals E & F, normally closed) is for the shift lock solenoid. So even if the lights are working (term A&B) the cruise control/TCC terminals C&D could be failing. Do you know if your torque converter is locking up as it should? Hard to say when your going downhill, I know.

As for that MAP sensor I cannot say that I have heard of that affecting the cruise control. Does your scantool read live data? Can you see the MAP sensor values? That particular code is a type B code that won't light the service light unless it happens twice consecutively. If the check engine light doesn't come on I wouldn't look into that any further. Just for curiosity,, what sort of elevation change are you doing? You mentioned the 4000 foot ski area but where are you starting at?
Starting at a out 500 ft above sea level
 

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Discussion Starter #18
went skiing again today and as usual, cruise control works fine on the trip up. Temp at 500 ft was about 40 °F and dropped to about 26°F at 4000 ft so not an extreme temperature drop. On the drive back down the cruise control was nonfunctional as has been usual. Stopped at a store at about 1000 ft and when I restarted and drove off, cruise control worked perfectly again. I honestly don't know if the torque converter is locking up properly but to my feel, everything seems normal. When I learn how to use my smartphone app scanner I will try to provide more info. Thanks for all the suggestions. I'm not to the point of spending money on this right now since it is more of a nuisance than a serious problem. It is a mystery that makes me want to solve it.
 

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I would suggest the next time you go up in the mountains, monitor the MAP and have the app save it to the data file. Also see if there is another parameter in the app you are using to indicate elevation in feet (yes, I know MAP can be used, but this way you'll have a way to correlate elevation in feet to engine pressure in PSI). Then when you come back down the mountain, have the app save the elevation and MAP data again, stop at the store around the 1000 ft level, shut off the engine, then start it back up (and hopefully a new data file). Hopefully the cruise control begins working again.

Then when you get home, look at the elevation vs MAP PSI date and see if it drops and how much it drops as you go higher in elevation. Then look at the elevation vs MAP PSI data that is stored when you start the engine for the trip home. It will be interesting to see if the how different the MAP data is for the the last part of the trip up the mountain from the data stored for the trip down the mountain

Good Luck!
 

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Discussion Starter #20
I still can't figure out how to get torque pro to log anything other than GPS positions. But then I'm not putting much effort into this. What I can tell all of you is that I get a pending code p0105 after descending the ski resort access road at which point cruise control stops working. This is a long steep mountain road so mostly coasting and braking. If I clear the pending code on the fly, cruise control immediately starts working again. So it seems that I need to be inspecting vacuum hoses and electrical connections to the mapp sensor and throttle. It will probably be quite a while before I learn anything more but will post if find anything.
 
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