88 Chevy 3500 no heat

[DJ's Lawncare]

I have an 88 Chevy 3500 with a 6.2L Diesel engine. I don't have heat. I have replaced the thermostat with a 195. The temperature runs between 190 - 210 most of the time. If it is sitting idling the cab will warm up a little bit. Once you start driving it blows luke warm air. I checked both heater hoses and they both feel warm. The radiator is full. The controls for the heater are very simple. It has the fan speed switch, warm/cold slider, and a slider for floor heat/defrost. I can't figure it out. Help. It has been way too cold to be driving with little or no heat. Thanks.

[comp]

when you move the control can you hear the doors move in the heater
box? the may be cable,vac.,or electric, the fan does blow correct??
if it quit check fuse first, also look for a control valve in one of the
heater hose line's if so feel hoses after the valve ,, (not opening)

[DJ's Lawncare]

where would the control valve be. what does it look like. the fan motor does blow. i will check to see if the door is opening and closing.

[comp]

not sure where it's at my book goes to 87, grab a cheap shop manual
it should have a location, follow both heater hoses to the firewall, to
start checking