89 Mercury Grand Marguis AUTO temp setting not working.

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.

Hey, GA... 300,000... Congratulations; well deserved. You're arguably the most knowledgeable contributor here. Ed

Thanks Ed. I really appreciate it. Not many of us old timers left these days.

Who's an old timer? I'm only 75.

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