2005 Mercury Marquis - A/c still intermittently blowing after blower replacement


Asked by May 13, 2014 at 07:14 PM about the Mercury Grand Marquis

Question type: Maintenance & Repair

2005 Mercury Marquis -  Complaint was no airflow from a/c ducts (regardless of mode)  Power at one point was not present at motor/harness connector.  Simple fix right, replace the motor.  Replaced motor, still not working.  OK, it's electronic controls, so let's reboot the thing, battery disconnected 10 minutes, reconnected, and wallah, it works...  (for a while, see next paragraph).  I'm still amazed and dumbfounded.  So just for grins, I test the old motor with direct 12v, and it works!  Owner / friend, 90 year old from my church, says leave the new one in, at least it works.  Makes little sense to me, but hey, wasn't breaking the bank.

Then my 90 year old friends have a $4k front end/side fender accident.  AC still works.  All repaired, all good.  Then they hit a bump and the blower quits again.  He hits another bump and it starts, then another and it stops.  (I know, avoid the bumps right, ...)  He taps on the evaporator in the engine and the blower starts.  Then he asks me to troubleshoot what's working.  No joy. It still works and nothing found wrong.

Murphy lives, and it will stop again.  Then what?  I tried disconnecting the ?sensor? on top of the evaporator, and the blower keeps running, but the cold air turns warm.  Duh. Put the connector back and the cold air returns.  OK no loose wire there because the blower kept running.  Since the blower runs at various speeds and voltage at the blower is speed setting dependent, I rule out the resistor pack.  Fuses are good.

What I need now are other ideas for what is going wrong.  Obviously I need to see it in failed mode again but that will come.  Thanks for any help.  - Rick

2 Answers

Rich Optional

run this test to see if the EATC Control head for you ac and heating is bad turn car off, close doors, start car w/EATC off, hold both off and floor buttons simultaneously, release both buttons and press automatic button w/in 2 secs. self test will begin, takes about 2 mins. If all lights come on, unit is probably good. If a code appears, need to check if unit is bad. If unit is good, Code of 888 is a pass any other code is a fail.

4 out of 4 people think this is helpful.

The KEY clue for me is the sensitivity to vibration. In my 45 years of electronic servicing I have found an appalling use of brass rivets as conductors on double clad copper PC Boards, typically it takes about 5 years for humidity and electrolysis to create corrosion and about 2K ohm resistance between these two dissimilar metals, in dynamic trouble shooting the technician would find an 12V voltage drop between the two sides of the speed control power transistor PCB, in static troubleshooting you would use a VOM to locate the 12Volt drop, however I always just clean all of the rivet connection on the PCB with a wire brush and carbon tetricloride and solder each rivet on both sides of the PCB... The Power transistor PCB is mounted on an aluminum heat sink an tucked away inside the AC flume located on the fire wall below the windshield of the engine compartment, you will need a 3/8 socket to remove the two bolts it to the flume. In my 45 years as an AVIONICS technician on aircraft I have never found this insulting practice used in fixed or rotary wing aircraft but only in land mobile vehicles and marine craft. Good Luck you youngins, n 73s to ya. Charlie AT1 USS KITTYHAWK CV63 / W8EIV / FCCP1 / FAA Cert.

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