93 Cg A/t Dual Air No Heat

[GNW]

Here is a tricky one only for the best.
1993, Grand Caravan, A604, 3.3L, dual air, no heat from vents.
Things already done:thermastat, cooling system flush, back flashed front heater core, back flushed the rear heater core, back flushed the engine, back flushed the radiator, checked the water shut-off valve, checked the vacuum to wsv, checked resistance to water flow through the front and rear heater cores, replaced the water pump, replaced the radiator cap, checked the temp. door adjustment and movement, bled the system of air according to Dodge TSB (I.E. loosening the temp. sensor).
Conditions: After bleeding the air from the system the vehicle heats okay for about 2-3 miles in stop and go traffic 30-40mph. Vent temp. never gets above 130f. In the next 2-3 miles the vent temp. drops up to 30f. It continues to drop the vent temp. the more it is driven. both heater cores blow cold air.
If you need more info. on what has been done or the conditions under which this accures let me know. If there is a special "Dodge" bleed method let me know. This is not the first Grand Caravan with the 3.3L that I have ran across with this problem.

[tazman]

Have you checked to make sure the exterior venting door is fully closed or not damaged? This will cause the blowers to blow cold. Also, if you have the heat seat to circulate the internal heat you may have a stuck or bad relay in the system. They are prone to rusting over time and will not generate continuous heat. Make sure the coils on the resistor are rust free and are not broken or worn thin.