89 Mercury Grand Marguis AUTO temp setting not working.
Asked by scotttaylor1959 Feb 09, 2023 at 01:41 PM about the 1989 Mercury Grand Marquis LS Sedan RWD
Question type: Maintenance & Repair
I have a vacuume issue that when I accelerate when the AC is in 'Auto' it
stops blowing, deaccelerate and it blows cold again. Lately the 'auto'
selection stopped working, but when in AC 'high fan' position it now blows
HOT air on acceleration and cold when coasting, and the HEAT is winning in
Florida. Mechanic replaced the EATC (Electronic Automatic Tempature
Control) but this did not fix the Auto not working. I can live with this if I can
get the Auto working again. Any ideas? My mechanic has none so far.
6 Answers
Vacuum leak in main vacuum supply line, leaking heater servos motors or from the control head or supply lines to servos. A smoke machine sure comes in handy when tracking down small leaks.
Thanks Ed. I really appreciate it. Not many of us old timers left these days.
An old timer is anyone who can remember very large steering wheels, points, condenser ignition systems, AM only radios that had tubes in them hand crank windows and drum brakes on all four wheels without power assist.
You got me beat by 5 years, I’ll be 70 this August.
I like your definition and I measure up to all of these. I never had AC until I bought a 1973 Pontiac! I'm right behind you at 73.
What you're describing is called "mode shifting". Somewhere in the vacuum part of the automatic climate control system is a check valve. It looks like a little round plastic disc with a vacuum hose on each side. Intake manifold vacuum is strongest at idle and weakest on acceleration. To prevent things from going wonky while you're driving, like they are now, you have a check valve to help keep the vacuum level in the system as you're driving. There's also a vacuum reserve tank that helps too. The vacuum reserve tank will look like a square plastic box or possibly a coffee can with 2 vacuum hoses hooked on to it. Hope that helps! Jim
