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2008 Grand Marquis

maintenancem3maintenancem3 Posts: 2
edited April 2016 in Mercury
I have a bizarre electrical issue I'm having difficulty diagnosing. At times, everything is just fine and works 100%. Without warning, while driving or even when just getting in the car (key on), I will get TPS monitoring system fault. Then it goes away and the power locks stop working. Next time the trunk release won't work. Next time interior lights won't turn off or maybe they won't turn on... There's no real rhyme or reason for it. When scanned, there are many fault codes present but when the issue clears up, the codes all clear. I also noticed the HVAC be it on heat or AC will randomly switch. If I have it on defrost, it will go to floor, then vent, then back to defrost. When on AC, they same thing occurs. The most consistent issue is the HVAC issue. The locks, TPMS, truck, driver 'warning' messages are very random and at times work just fine for a week or two at a time. The HVAC issue is pretty much any time it is used. Odd as it sounds, (and I can't explain why), but at speeds above 70, it will go to defrost and pretty much stay there. At speeds under 40 it mostly stays on floor and speeds in-between, it will change between floor, vent and defrost. It makes me think of a module or data bus issue. I've not seen anything on line that mirrors this type problem. Anyone come across this type issue?


  • thecardoc3thecardoc3 Posts: 5,418
    Don't think of this as "a single" issue. There is nothing that suggests any of these are related at this point, so don't make that assumption. What is the exact model, year and engine? We need to know that just to determine if the HVAC controls are electronic or vacuum controlled. From there a solid routine would be to take what ever of the above symptoms are the most repeatable and analyze and solve that one first, then look for the next issue to deal with. Trying to solve all of these issues simultaneously even if they are related will just make the effort more difficult.
  • Thank you Cardoc! It is a 2008 Grand Marquis with the 4.6L engine. It has ATC for the HVAC system
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