altima heating problem

Hi:
I have a '93 altima SE. The heater is works to a certain extent. when I turn on the heater, heat will come out of the dashboard and that it. If I change the flow control knobs to direct heat to come out of the vents or the ducts (on the floor) it won't. the blower motor appears to be working. I have also check the hose going from the thermostat to the bottom of the radiator and it is warm when the heater is on. I did also check what fused I could find and them all look good.
any idea on what the problem is? mechanical or electrical? I bought a haynes repair manual but is doesn't go into to much detail on this. is does mention a few components related to the heating system (fan switch, control head).
any reply would be much appreciated.
thanks
BA

sounds like a bad mode door actuator, common problem
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is that the box that has a round white plastic part sticking out of it that connects to a vertical rod?
I broke that plastic part about a year ago (on accident). I don't beleive I did any damage to the actuator.
I wondered if that might of been causing the heating problem I've been having.
thanks a bunch for the response NissanTech.
BA
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I went ahead and replaced the mode door actuator and had no luck in fixing the problem.
I was told there is a air intake actuator related to the heating system. could this be causing the the problem? if not, any ideas what else it might be?
all responses are greatly appreciated.
thanks
BA
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A few thoughts come to mind, BA, based mainly on personal experience.
Firstly, you mention that the heater works "to a certain extent". If you mean that it warms up, but does not get as hot as it used to, you could be looking at a stuck thermostat. If it is stuck open, it is allowing coolant through to the radiator before it gets time to reach normal temperature.
Replacing the thermostat can be a DIY job, and the Haynes manual should tell you how to go about it. It is normally a simple procedure.
You mention also that re-directing the air flow to the ducts does not work. If you mean that you get the air coming out, but it is cold, then it could be the thermostat again.
If you mean that you get no airflow at all, this suggests a mechanical problem with the controls, cables, levers, etc inside the heater. I've never had this problem in 30 years of motoring, but I guess that it is going to happen sometime.
There is just an outside chance that the problems are not the result of a jammed thermostat, but are caused by loss of coolant.
You might think that this would lead to the system getting hotter but, if you've lost most of the coolant, there is not enough to circulate to the heater. Usual caveats apply - make sure that the system is cooled down before removing radiator caps, etc.
If coolant loss is the problem, there is a high risk of the engine over-heating. The block / head may have warped. A blown head gasket becomes a greater risk, but it might not happen immediately. If you know what you are doing, this can be a DIY job but it is not simple. I would recommend getting an expert on to it if you are not certain what to do. The Haynes manual can help, but I think that most would agree that it is not as helpful as you would need if going through this procedure for the first time.
Let us know how you get on.
Ken County of Kent, Garden of England, next to the compost heap