Got a 87 base, auto. The heater works, but most of the time while in gear, the heater is about half as strong, or less, than before I put it in drive.
If the heater is acting weak, I can just stop, put it in park, and it will go back to normal again.
The previous owner said he had the logicon rebuilt, and the heater core/fan has been replaced. I tend to think it's some sort of vac. or solenoid problem, but somebody with more knowledge please comment.

 

how is it weak? not blowing, not hot?

 

It's hot and it puts out air, just not a whole lot, even at MAX. Eventually it'll get warm in the cab, but it really sucks because when it's weak it's not strong enough to defrost/defog the windshield.

 

If the fan speed is dropping in drive, automatic?, what is your volt gauge reading? Sounds like an alt problem.

 

I'll check next time.. Could very well be the alt. I first noticed it when it was raining the other day and I had my headlights and wipers going.
Voltage has been low. 12v at times. I just figured the gauge was off.

Yes, it's an automatic.

 

how long have you had your 7 and do you live by trees? The blower intake maybe clogged. It's located under a black trim piece at the base of the windsheid, outside that is. take a flashlight are look through the ribs and see if there is gonk down in that valley, if there is, remove passenger windshield wiper and all the little screws and pull it out, use a shopvac to suck it all out. the intake is located close to the pass. fender and has a metal screen to keep the bigger chunks from entering the blower.

 

Volts are definitely lower than they should be..

I've had this car about 4 days now. New owner.
A weird thing that happens is it I put it in park, from drive, the heater will behave normally for about 1 second, then go weak again.. Neutral makes no difference.

The car was garage kept by the previous owner, so I don't think it's clogged, but I will check it out anyway. I did have that problem on a car I owned a few years back...