Ice cold a/c then warmer, then ice cold and so on

Maintenance/Repairs
1

I was driving today on the interstate and had the a/c cranking like normal. It was colder than the artic then all of a sudden it went cool, then a little warmer. When I exited the interstate and stopped at the first light, it went ice cold again, and went cool to cold and so on. 1994 grand marquis gs 4.6 v-6

2

Most likely it’s a little low on charge. Have the system pressures checked.

3

If that’s a V6 and not a V8, then you have a very very rare Grand Marquis.

4

Electronic or manual controls?

5

Sorry V-8

6

Manual controls

7

Note that the control of the blend-air door is still electronic.

8

As others suggested, check the obvious things such as the refrigeration charge. However, I had a similar problem on a 1990 Ford Aerostar and it turned out to be the fan clutch. I believe your Mercury also uses a fan clutch on a belt driven fan. My great independent shop figured it out. They connected the gauges to the system and noted that when the air conditioning quit functioning, the pressures went way up. The technician rolled his shop fan over and directed the airstream through the grille. The pressures dropped and the air conditioning started functioning again. The engine had never overheated, but apparently the additional load of the air conditioning condenser was too great for the marginal fan clutch.