sumpanzer

Hello all,
I have a 2002 Tahoe 2wd, 5.3 L59 FF. A few months ago cruise stopped working and I diagnosed it to be the center high mount light, replaced that, and since then cruise only sometimes works. Sometimes it will work until you cycle the ignition, sometimes it will work for about a minute and disengage, sometimes it will never engage, seems to be dependent on ignition cycling. There are no codes relating to VSS, brake pedal position, etc. I did replace the VSS after eliminating the rest of the possible faults and it did not make a difference. VSS output on my scantool looks accurate, brake pedal sensor not sticking, just says cruise is disabled by ECM with no fault given on my Autel 808 when the behavior is present.

More recently I noticed on a particular slight hill it felt sluggish and i noticed it was not exhibiting a 3-2 downshift normally. You have to give a lot of throttle input, ~50%+, to get a 3-2 downshift even at 30mph or so. I investigated this and found that you can't get a 2-1 downshift unless literally WOT, it will upshift to 2nd instantly if you are not WOT. Driving in D all upshifts feel normal at normal points, it will stay in 1st normally during hard acceleration, even under say 75%. But once it upshifts to 2nd it will not downshift to 1st normally unless at a complete stop. This issue is persistent but I feel it is possibly related to the aforementioned cruise issue. Nothing in the scantool jumps out as an issue so I am looking for some direction here.

FranksCustomTrans

I would be suspect of the TPS and or MAF myself and no they may not set a code , In particular if a tunes been done . These can both be out of calibration but inside functional range and not set a codem And with tuning some go nuts turning off codes sometimes just because other times out of nessesity.

bbond105

Have you checked the TPS. MAP and MAF sensor readings? If any of these are off it could cause transmission shifting issues.

sumpanzer

I put some new tires on the truck and had it aligned, Firestone tried to sell me a battery by just putting it in the truck and charging me for it, had them put my old battery back in it (12.5v), and for two days those symptoms were gone. I was about to post back here and say it was fixed but yesterday we went to an estate with a 3k gvw enclosed trailer and less than 5 miles out it broke up, backfired, turned off cruise and traction control by itself at this time. This was really weird and on the way there, mostly 2 lane 55mph, it only got 11.4mpg. On the way back it behaved fine and got 17.8mpg. So today I took it out on some backroads and beat the **** out of it, put almost a hundred miles on it. It's perfectly dry out btw. It would not do a 2-1 kickdown and the low traction light kept coming on, and it really didnt like doing 3-2 kickdowns, mind you it wasn't even close to breaking traction. Traction control off and it seems to drive fine. I could not recreate the condition where it turned off traction control by itself or backfired, and cruise has continued to work. Cruising down the highway the low traction light never comes on, only when it would usually kickdown, this was not present in the previous condition. I guess I need to get the autel hooked back up to it. It's a DBW truck, last time I checked requested and given throttle were within 3% of eachother at all times, and I cleaned the MAF in December and only saw a .1-.2g/s increase in measured flow.

sumpanzer

I had been driving the truck like this with TC off until last week the A/C quit. I checked head pressure and it's fine. Fuse and relay are fine. All of the controls work, have illumination when the A/C button is pressed, low pressure switch is functioning, nothing at the clutch. I plugged in the autel and we have no com with the hvac module. Wiring diagram is saying for manual A/C setups the pcm is the controller for the compressor. P1571 is present with P1574 and these codes instantly set above 15mph with TC on. ABS and traction control are functional and prevent breaking of traction when tested. Center high mount is new and all other lights functional. With seemingly multiple internal com issues I am smelling a bad ECM.

Full Power

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The rig is 22 years old. MUCH more environmental damage likely OUTSIDE the PCM than inside it.
Remove, CLEAN, and refasten ALL ground connection points.
One: Battery chassis ground front left, just behind bumper.
Two: grounds: a pair on the body mount, just below drivers feet.
Next: TWO rear of engine, one passenger, one driver's side.
Main battery to engine block, a bitch to reach, kind of behind the steering pump.

sumpanzer

I got to all of them, the passenger side of the engine did not have a strap to the firewall like the driver's side. The straps to the frame under the floor did have rust on the surface but all surfaces were clean when reinstalled. The pcm still delivers a no com message from the hvac module. I went to the local salvage yard and couldn't find any 01-02 truck pcm's. I have another place to look later, I think it'd be worth it for $60 to rule it out or not.