ac control on 98 lesabre
 

At random the ac contol on my lesabre will divert all air flow to the defroster. This happens when I am using the ac, heater or vent. It is not effected by engine speed, engne load or road conditions. The indicator lights on the control unit remain on and eventually it will go back to the correct air outlet. Help!

Automotive heating and A/C systems are designed to default to the defrost position should anything go wrong with the system. Understandably in winter conditions you must have defrosters, with that said, lets see what can be responsible for the problem your vehicle is experiencing.
An insufficient supply of vacuum , an incorrect routing of the vacuum, or complete loss of vacuum can do this.
Also the settings on the control panel can do this too , therefore , the electrical section has to be checked out.
The first thing I would check is to make sure all the vacuum hosed under the hood in the engine compartment are in good condition and not possibly cracked and causing an intermittent leak.
The electrical control side requires some testing equipment and I don't know what you have to work with.
A good visual check of all the hoses is the best place to start.

Do you have any idea what Public Liability Insurance might be if the hoses all appear to be perfectly fine?