Strange

[HotChiliRam]

A strage thing happens when I cruise. When I have my A/C on the MAX position, it suddenly change's to the Defroster position and come back to the MAX position. I know this have something to do with the vaccum acumulator box, but the box and lines are in great shape. Can my MAP sensor do this?, If not, what could be making this happen?

Thank You.....

[RichP]

It's a vac leak, you just need to track it down. Does it do it while cruising or under acceleration ? then work right after you let off the gas ?. Could also be behind the dash. What year ?

[HotChiliRam]

It does it under acceleration and cruising. It goes to Defrost and then back to where it was over and over, sometimes it stays longer in defrost and sometimes it stay's short. It work's when I left off the gas, but it work's if I don't too.
Sorry. The specs are 1990 XJ limited with some Add On's :)

[RichP]

The defroster position is the default position for the hvac system, safety thing as I understand it. you can generally find a vac leak by spraying wd40 or some other type of lube around the hoses and joints when it's running. You will hear the idle change when you find it as it momentarily seals the leak. The biggest culprit is the vac line that runs to the vac ball, it goes under the battery and battery splooge drips down on it gradually eating it away. Loose or leaking throttle body will also do it. On my 97TJ and 98XJ there is a vac tit on the intake that has a rubber plug over it, make sure thats not leaking or cracked. At the age of yours it might be time to just bite the bullet and start replacing one vac line at a time with all new ones. I had to do that with a 89YJ, the hoses all leaked 'a tad' but added up, tooke me about 2 hours, 1 at a time.

[HotChiliRam]

Thank you I will start to check my vacuum lines.