david Posted August 23, 2006 Report Share Posted August 23, 2006 Q: I have a 97 deville. I recently replaced the a/c compressor(every thing works fine). While repairing the car I accessed the trouble codes and to my surprise a current code was registered for the ACM (code U1104). This code is not in any service manual. I brought the car to a diagnostic shop and was told to ignore this code. I am unsure if the a/c compressor should cycle on and off in warmer temps (it isn't) and if this code may be the culprit. Maybe the programmer is bad? Any help is greatly appreciated. A: DTC U1104 Loss of Serial Communications for Class 2 Devices One of the items on the class 2 communication line, which is like the local network that the PCM uses to talk to the other computers and the radio, isn't talking back, or the line has a problem. bruce Q: Thanks for the info. where did you find this code? how come no one has this code in a factory manual? can it be ignored? the diagnostic shop said they tested all the computers and they all are communicating with each other. how about the compressor cycling issue? the darn compressor clutch won't turn off. is this weather related or could it be caused by the U1104? the a/c system is ice cold. the low side is 45 lbs. of pressure and the high side is 225 lbs. I am going crazy trying to figure this cycling issue out. (I hope I am not badgering you with all these questions) Thanks , David Graham A: I did a search on google for the code. I am not an A/C expert; that's a good question to post to the board. If the car is otherwise functioning fine, I would be less concerned with the U1104 message, as it may be intermittant and fairly meaningless. No worries on PMing, although PMs are great for things that only I can answer, but you get a lot more info from posting technical questions to the board as a whole. And that way others benefit who may have the same questions / problems. bruce ^^ Does anybody out there know anything about this situation? Link to comment Share on other sites More sharing options...
Ranger Posted August 24, 2006 Report Share Posted August 24, 2006 Most "U" codes are from pulling battery power. Clear it and ignore it unless it comes back again. The compressor does cycle on & off. The hotter it is the shorter the cycle times. I believe in very hot humid weather, it may run constant. Kevin can probably verify this. Link to comment Share on other sites More sharing options...
KHE Posted August 24, 2006 Report Share Posted August 24, 2006 The A/C system is designed to cycle the compressor on and off - if it didn't, the evaporator would freeze and there would be no airflow out of the vents. Kevin '93 Fleetwood Brougham '05 Deville '04 Deville 2013 Silverado Z71 Link to comment Share on other sites More sharing options...
david Posted August 24, 2006 Author Report Share Posted August 24, 2006 Thanks for the help gentlemen. I guess the compressor is working properly as I have very strong airflow thru the vents. I think that I may have a loose battery terminal or connection that may have set the code. I will check it out and report my findings. Link to comment Share on other sites More sharing options...
Ranger Posted August 24, 2006 Report Share Posted August 24, 2006 Don't confuse the compressor with the blower motor. The blower controls air flow. The compressor provides cold air and has nothing to do with air flow. Link to comment Share on other sites More sharing options...
david Posted August 24, 2006 Author Report Share Posted August 24, 2006 I am not confusing the two. What I meant to imply is that I don't think the evaporator freezes up because I have proper flow of cold air . I think I was confusing the code issue with the compressor issue. I now beleive they are unrelated. Link to comment Share on other sites More sharing options...
david Posted August 28, 2006 Author Report Share Posted August 28, 2006 Good Afternoon everybody, I found a loose negative battery cable. I disconnected it, checked for corrosion and tightened it back up. went to diagnostics ,cleared all codes and the U1104 is gone! That same day it was 70 degrees outside and the clutch cycles properly. Thank you Ranger for finding out the code and thank you to everyone who relied to my dilemma. Link to comment Share on other sites More sharing options...
nrositano Posted September 4, 2006 Report Share Posted September 4, 2006 Hello, Sometimes when a battery is disconnected and reconnected to fast, or disconnected to soon after key off. It doesn't allow a controller time to power down correctly. This will corrupt the program in control units memory that will remain corrupt util you disconnect the battery allowing it to completely power down, so it will reboot from the firmware in the controller. Anyway I am glad your vehicle is now code free. I wish I would have thought of that sooner, I was to busy trying to find the exact definition of the code. Nick, Bay Diagnostic Link to comment Share on other sites More sharing options...
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