Defrost not engaging in 86 560SEL

[PPlomaritis]

Basically when it is cold out the defrost (or heater for that matter) will not engage. The car needs to warm up for quite a while (up to 1/2 hour - long after the temp has risen to the 'Normal' range) before it will kick on.

Any ideas on what may be causing this?

[BenzMatic]

May be your monovalve is not functioning correctly and opening up a tiny bit and the heater core is taking a long time to get warm up. Fast lane sell the repair kit, fairly easy job.

[meltedpanda]

cold temp interlock switch circuit, could be sender.
may also be recirculating pump