In reading the Owner's Manual, it does not appear that there is a low fluid level warning light. The only way to check the level seems to be to remove the transmission fluid plug, which according to the manual should be done by the dealer. If the level is low, use a pump to add the fluid until it begins to overflow and then reinsert the plug.
Location of transmission fluid level check for 2004 mercury moutaineer. Reply 1: Hello, This Mountaineer does not have a transmission dipstick. ...
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Peter