hot air only through defroster

A sudden problem of air only coming out from the defroster vents. I believe they are the defaults. Yes there is some slight air flow from the vents but very slight and not heated. Is this due to the heater box mechanism, the bowden cable or the HVAC controls  mechanism in the dash? Thanks

Loss of vacuum to the control panel. Check the vac tank under the hood first, then the black hose going to the vac switch in the dash.

Thanks very much.

Hopefully it's an easy fix!

I hope so. I replaced the vac tank 6 months ago with a new one (hopefully still good)  but not the vac hose.

However I have been preparing for the day when things go south behind the dash. Belle is almost 50   and she really needed only basic repairs until several years ago when I started the renovation. I do have NOS bowden cables for the HVAC and trans shift, a rebuilt HVAC controls unit and heater box and a few other parts so at least I am prepared for the worst.

Sounds like you're ready.  I never could get that heater lever to move freely.

I agree. I didn’t have this problem on a 73-77 but on an 80 when it did that, it was the vacuum switch on the back of the control unit. There was 2 halves that were sealed by a diaphragm and the switch had broken.

Well, Leave it to Belle. The heat suddenly came back on through the vents again. Now I am unsure what is going on other than aging of parts. Things are starting to go and I will just have to replace everything I can in back of the dash.

76Chevelle2Tone wrote:A sudden problem of air only coming out from the defroster vents. I believe they are the defaults. Yes there is some slight air flow from the vents but very slight and not heated. Is this due to the heater box mechanism, the bowden cable or the HVAC controls  mechanism in the dash? Thanks

From reading the rest of the thread, I take it that you mean straight unheated air.

If you have a vacuum operated heater control valve, I would simply feel the hose that feeds hot coolant to the heater core to see how warm it is. That will give you an indication that the trouble may lie with the valve not opening.

Since the water shut-off valve was mentioned, be aware of one thing, too. Depending on the car, there are two valves made that look exactly the same and sometimes get mixed up by parts suppliers. One opens when vacuum is applied, the other closes then vacuum is applied. I’ve seen the wrong one installed more than once.

zucchi wrote:

If you have a vacuum operated heater control valve, I would simply feel the hose that feeds hot coolant to the heater core to see how warm it is. That will give you an indication that the trouble may lie with the valve not opening.

zucchi,

Sorry if I was confusing. There has never been a loss of heated air. The issue is where it comes out. It suddenly came out of the defroster vents only for about a week with the selector moved to defroster. On heater it was only weak cool air.. Now it is fluctuating. Some days the heated air comes out of the vents in normal fashion when the selector is on heater. Other days very little cool air comes out of those vents facing the driver  but hot air comes out of the defroster vents when the selector is moved to defroster.

Speaking only for myself, it seems like I don't have a complete picture.

76Chevelle2Tone wrote:There has never been a loss of heated air.

…but then you say…

76Chevelle2Tone wrote:On heater it was only weak cool air.

Without intending to sound argumentative, that sounds like a loss of heated air.

76Chevelle2Tone wrote:The issue is where it comes out. It suddenly came out of the defroster vents only for about a week with the selector moved to defroster.

If your temperature selector is set to HOT, that's exactly what it's supposed to do. When temperature is set to HOT and the system to DEFOG, hot air should only come out the defog vents. Regardless of what the temperature is set at, when the system is set to DEFOG, air is directed only out the defog vents.

76Chevelle2Tone wrote:Now it is fluctuating. Some days the heated air comes out of the vents in normal fashion when the selector is on heater.

Just to be clear, heater vents are at the base of the firewall where the transmission hump is. When the system is set to HEAT, air is directed only onto the floor.

76Chevelle2Tone wrote:Other days very little cool air comes out of those vents facing the driver…

When the system is set to VENT or BI-LEVEL, air is directed out the heater vents and the vents that face the occupants. No air should be directed out the defog vents with those settings.

76Chevelle2Tone wrote:…but hot air comes out of the defroster vents when the selector is moved to defroster.

At the risk of sounding redundant, that's what the system's supposed to do.

Zucchi,

it's a bit difficult to easily describe a series of if-then statements. For a week hot air  came out of the defroster vents when the selector was on Defrost. When the selector was moved to Heater there was no hot air from Any vent on the dash or under the dash or the defroster vents. In other words on Heater there was no hot air anywhere. But if the selector was moved to Defroster then plentiful hot air came out of the Defroster vents only (as is normal).

Now it is fluctuating. Some days putting the selector on Heater produces hot air from appropriate vents. Other days it reverts to the pattern described in the first paragraph.

I agree with S3TPI and bracketchev1221, sounds like a vacuum problem.

If you don't already have one, get a hand-held vacuum pump…


Systematically test all the vacuum circuits used by the climate control system.

That looks useful and I will get one. But I think the only real way forward here is to replace everything behind the dash. The car has never had a frame off or major renovation before I started 4 years ago. Behind the dash it's all original pretty much and one by one each part will fail sequentially if they aren't replaced at the same time with new parts. Fortunately I have been able to find new , NOS or professionally rebuilt parts for a lot of it. I probably am going to have to spend big moolah on a new dash harness too as I am getting a few quirks appearing. Thanks to everyone for diagnosing this.

I'm a huge advocate of getting to the source of a problem rather than replacing parts until the problem goes away. First off, replacing stuff that's ok is expensive and a waste. Second, without knowing why something failed, you risk subjecting the replacement part to premature failure.

There are only a few things that cause rubber parts like vacuum diaphragms to fail; physical damage (like a puncture from a sharp object), extreme temperature changes, ozone and UV light. The vacuum operated servos in my AC/heater distribution system have been in the car and in use since the the car was built in 1974. The only times I had issues is when there was a vacuum problem originating on the engine side (extreme temperature fluctuations and ozone) or an obstruction in the ducts or doors such as deteriorating foam seals or insulation. Some 38 years ago, the temperature cable failed at the point where it connected to the air temperature door and that was because the door was obstructed; the end of the cable is plastic.  So I liberally used JB Weld to "rebuild" the connector. I then dug a replacement cable from a wrecking yard for when the repaired cable fails. It hasn't failed yet.

I looked over the AC & heat air distribution ducts in the shop manual last night and there are a few diagnostic charts. At the risk of being redundant, if you don't have a shop manual, get one and follow the charts.

Since it sounds like the problem presents intermittently, I encourage to try and "make" it repeatable. For instance, when the system is working as it should, try to replicate the problem.

Anyway, after reviewing the diagnostic charts, and without being there to examine your car in person, a priori my guess is something is interfering with the operation of one or more of the air distribution doors. Since it's intermittent, it may be an obstruction that shifts when the car turns a corner or stops suddenly.

Keep at it and keep us posted.

76Chevelle2Tone wrote:I probably  am going to have to spend big moolah on a new dash harness too as I am getting a few quirks appearing.

If you're talking about electrical quirks, that's is a different topic. It's important to understand that current will follow the path of least resistance. If you have any circuit that's not fully grounded, current will flow through a related circuit in order to find ground. The GEN light is an in-you-face example of that.

Before you invest big money, I strongly encourage you to re-establish the connections on all your ground straps and cables. Often times the source of electrical issues is poor ground. Regardless of if the ground connections "look" ok, disconnect them, clean the terminal and the spot on the body, frame, engine, etc., where it connects, then reconnect. If you're keen on spending money, upgrade to thicker grounding straps and wires and add a few more here & there; you can never have too much grounding.

That is excellent advice in all cases. The problem, as related, doesn’t seem to be electrical. It would be helpful to know if this car had A/C or if it is just a heater. These systems are very vacuum dependent. The mode and recirc doors ( in the A/C version) are vacuum dependent. The temp door is cable operated. The default operation is to go to defrost. That’s MVSS.

Mine had heat out of the defroster vents and the floor but could not get it out of the dash vents. I used a vacuum pump on the manifold plug ( not sure what else to call it) on the back of the control unit it pulled the dash vent doors open. So I changed out the vacuum switch that was visibly broken and once I did it cam out the dash vents when I turned it to heat instead of just the defroster vent.

Thanks guys for all the comments. Yes the car has factory air, P/S, P/B. The A/C system was replaced in toto 4 years ago when I was forced to move to R134. At the time I also replaced the blower motor and its associated electrical connections along with broken flexible ducts. Those are working without issue.

Again understand that I am an outlier on this Forum. The car which is a sedan has always been used as a daily driver (and occasional commuter until 2015) since I got it more than 30 years ago, although it alternated with other cars over the years. The original owner, an elderly man, did very little work on the car while he had it. In addition it carried family, friends and future enemies on a fairly often basis. It's a sedan. Therefore no frame-off, no major restoration, no prior owners enhancing or customizing it etc - just repairs as needed.

Zucchi,

Up til recently I have followed the approach you state of diagnosing the problem correctly and fixing the part that is malfunctioning. I tried to explain why I was now going to a "system replacement" process. The parts are mostly ~50 years old and are failing sequentially. When I was having nagging performance problems I replaced the starter motor because that was going bad. It fixed a problem but not the problem. Then the fuel pump started to act up. So I tried replacing old parts one by one. Again each replacement fixed a problem not the problem. Until I replaced everything from the timing chain/harmonic balancer up to the carburetor and distributor there were nagging problems. Once the system was replaced, magically the car drove like its younger self. It turns on and shuts off in varying weather cleanly. My formerly wimpy 305 is now champing at the bit as it accelerates smoothly and roars down the highway so to speak. Were all those components other than the starter and fuel pump broken? No they still functioned after a fashion but were out of spec and not correctable except by replacement.

Re: the HVAC system on the cabin side of the firewall, there are signs of bowden cable wear as the sliders can't go fully to one side. Then there are the vacuum issues as diagnosed here. The electrical quirks have mainly been with the idiot lights but I think the speedometer and shift linkage parts (not the transmission) are starting to have a few mild issues too. I'll take at look at the grounding too which is a good point.

Regarding parts the damage has already been done haha. My storage room is filled with parts. The only thing I don't have are the wire harnesses which was the basis for my comment. They are highly expensive for what they are, I guess because of all the variants within and between years. Since it's not a "project car" I can't leave it sit for prolonged periods while I search or wait weeks for each part, get sent the wrong part etc. etc.

Thanks again for all the comments/suggestions. I will keep you advised. Hopefully I can get by until the weather gets warmer here. But the car is basically running OK currently.