Unlike "normal" heat-related posts, what the Lizard (1972 numbers-matching K-code coupe) needs is MORE heat (for the Winter, anyway) in the passenger area! SWMBO (nearly the sole Lizard Driver) is half lizard herself, so this is a Really Big Deal until Global Warming becomes real.

In Summer, the cabin isn't "cool" by any stretch, but it isn't too hot either, even on 200+ mile road trips. The car came with A/C, but Bubba got it a long time ago.

Air blows from the cabin motor from all ports on the dash. I know that means we have vacuum issues (ALL vacuum issues) and the doors are at their "default settings". That's acceptable for now, we just want the air to be a lot warmer until Spring.

The radiator is New, and we have made every "upgrade" to the engine-cooling system the "Numbers-matching" designation will allow. (We even added an electric fan & I mickey-moused the power feed to make it pop out the Fast Idle Solenoid so when the fan is on, the idle goes up (to ~1200) so the Alternator can stay ahead of the fan as well as to increase the oil & water circulation.)

During all driving, the temp gauge almost never strays far above ~200. I've checked the block w/ a hands-off thermometer and the dash gauge is "close enough", especially since it never gets scary. Never boiled over, new anti-froze, etc. etc. all to assure a "cool" driving engine.

None of that matters, just trying to forestall the usual "they're all hot" posts...

The water in the system is perfectly warm for our use, and AFAIK except for the A/C bracket & compressor, Bubba has been kind to us in this regard.

Growing up in the Deep South, I've never really had a car I needed to make warmer! And this one has this "too hot" reputation...

How can I "reverse flush" the HC (Heater Core)? I see the hoses and have NO shutoff valve (!!!???!!!???) to keep the hot water out of the cabin in Summer (??!!???!!), and the A/C "box" is present on the outside of the firewall... What I'm curious to know is, which way does the water in the heater core flow, "normally", so I can hose it out the other way (i.e. "backflush")?

Also, assuming there is good flow through the HC, assuming bad vacuum (many leaks - manual headlights & wiper cover, no creaking air doors under dash, etc.) is it possible that there's just no air flowing over the HC into the cabin?

I just found the post where the unfortunately cold new owner found naught but a loopback pipe where the HC was. (!!!!!) Needless to say, I'll be looking to open the HC cover ASAP!!!

Are the dimensions (esp. mounting points) of the HC available??

Lastly, does anyone have an INDEX into the AIM??? I've slogged through the apparently-random pages & can find A/C stuff galore, but so far no Heat.

SWMBO's birthday is tomorrow, so it's starting to look like getting her back in the Lizard may not be on her Birthday Thank-you List...

Speaking of which, THANK YOU in advance for any useful information or questions you care to post in reply!!

 

Hi LL,
I believe the AIM is only geared toward installation of the components, not operation or trouble shooting of the system.
The best information about heater operation I'm aware of is in the GM CHASSIS SERVICE MANUAL in Section 1A pages 1 through 8. Since the astro-ventilation system plays a small part in cabin heating you should take a look at those pages too (pages 8-10). Lots of diagrams and notes.
I hope this gets you started.
Regards,
Alan

 

FWIW: I've heard/read never to reverse flush a Corvette heater core. I cannot vouch for the veracity of this statement. Could be another Corvette myth.

 

You are correct, sir! However, not to brag, but that's all I need, really. Troubles I got, by the boatload, and lots of 'sperience shooting them! (My male parental unit had zero mechanical skills, so I got to keep the family car running even before I could legally drive it) I was hoping for an INDEX into the AIM, since mine was apparently thrown into an A/C duct before being put together.

THANK YOU, ALAN!!!!

SWMBO has been bugging me since we first bought this beast to get something she can read (ABA : "Anything But AIM"), as she gets as much pleasure from greasy hands as I do, but hasn't had the experience yet.

And I trust other peoples' opinions exactly ZERO, other than those such as yourself who are only commenting on their experiences and direct knowledge. I've seen enough in your posts to not worry about asking this:

We would like to buy a copy of that book. Can you please help once more by telling me the ISBN number so I can order it? If you could suggest one or two others, equal-to-better in your opinion, as well, we'd both be VERY grateful.

Since this is a public forum, if anyone else has experience with other books ...

No, wait, not now. Is it still hijacking if I do it to my own thread? ...

THANK YOU ALAN!!

 

Easy Mike,

I'm betraying my antiquity here...

It's not really about "Corvette" so much as "general cooling system maintenance". Rust and other gakk in the coolant "go with the flow", and tend to impact on protuberances thusly. A powerful reverse flow can sometimes get a little more particulate matter out.

Or not.

Never having seen a Corvette HC (as such -- I've seen a lot of them, but not with "This came from ___" tags), I only assumed it was like most of the rest of the "normal wear" parts, Generic. (if that's supposed to be secret, I'll promise you, Bubba already knows!)

I'd really like to know why you said that. Again, I've seen enough posts to trust your experience, so I'd be remiss to miss an opportunity to NOT screw this up!

 

Update: A lovely afternoon cruise on a frosty, sunny day got enough hot water moving to get the HC housing box warm to touch.

(As for me, cold means I'm making more HP, so I should drive faster. This causes more adrenaline, which makes it easier to ignore the temperature. But that's just me...)

I already observed the hoses getting hotter, and squeezing one hose makes water come out the other, so the flow was obviously there. SWMBO had the heat-level wheel at the Cold end (and I had a hard time finding a picture of a clear one to check!!), but correcting that didn't help. I'm leaning toward deterioration in the air-box joints & seals... which means R&R the dash (in a coupe), which is right at the top of my "To-DON'T List" this winter ...

Thank you all for your patience, reading this far. I'm open for any suggestion that doesn't start with removing the dash, but it's looking pretty inevitable. SWMBO will be Warm!

 

Hi LL,
The GM CHASSIS SERVICE MANUAL for each model year is available from many/most of the larger vendors and even from the NCRS 'store' on their web site. MORE than worth the $$.
Good Luck!
Regards,
Alan

 

First we should determine the root of the problem, whether it's a volume or temperature issue.

What are the temps of the air at the vents?

 

Weeelllll... It's hard to say, since there's very little air moving through the vents, even while driving <= 55mph.

The LH wheel points to Heat or farther toward the rear of the car (needs refurbishing -- "to do" with the rest of the interior someday) and the RH wheel is all the way back, although SWMBO did keep it all the way forward. The fan power switch works all the way up to "High", where it finally finds "OFF" (as opposed to "Off" which is actually ON in GM-think). IOW, it goes from offlow - low - med - really off. Electrical. I'm down with that sickness. But still, at Med., some air should flow...

The fan motor turns, which we can hear when parked w/ the switch on. If you wet your finger you can just feel the thought of an "airflow" from the center vents, and maybe you could imagine how a little air in the footwell might feel, if you hold your hand just right... and no sign of Defrost (the main reason this quest started), air or temp, so there may be a dead squirrel in there somewhere...

PS: the car sat atop a hillside overlooking one of the largest lakes in the state for most of a decade before we rescued it. Outdoors. Never covered, never cranked, with half a tank of gas ........... <sigh> So I fully expect a thriving ecosystem of spiders, trolls and goblins in there. Not begging for pity, just trying to reinforce the idea this isn't "factory-fresh"...

But the wee hint of a baby's breath at the top-center dash vents seems like you might call it "warmer than ambient" -- think "heat pump", not "wood stove" -- if you had to call it something. But mostly I'd call it

BARELY BREATHING.

Volume, which must be addressed in order to test Temperature.

Any hope I could fix that w/o pulling the dash? Or worse?

(PS: I like your root-cause-analysis technique. I live by that for troubleshooting, until I have to fall back on dumb luck (by that I mean the advice of trusted friends and associates)!)

Unfortunately for me, all the best troubleshooting I can muster just keeps shoving my face back under that dash... Yeah, right, like there's an "under" there!

I may have found one page in the AIM which shows some of the inside of the firewall, but as I said, mine arrived in random order (p M5 is followed by A8, e.g.) so it's taking me some time to pull the documentation together.

Thank you all for pitching in here!!! Luck is knowing whom to trust, and I'm lucky to be here!

 

I appreciate you coming back. I noticed the '71' in your nom-de-plume, which I assume is NOT the reason your status is "SENIOR" member...

And on the list of manuals I found at Zip, for 1971, the manual they show does have the word Chassis in it. I think that's what threw me. The 1972 one has a different name.

And both appear to hew to the "one thin manual fits all GM cars" plan.

As if this documentation issue weren't already a train wreck...

SWMBO and I were really hoping for a good (specific) manual.

 

Well hopefully we can come up with a solution that doesn't involve pulling the dash.


From your description it seems that your blower motor is not engaging into high-speed mode when you select "high" like it should be. It's actually fairly common and the difference between medium and high is about double the airflow (in my opinion, anyway)

A bad blower relay (or connection to) will cause the fan to operate only in low and medium modes. It works by bypassing the resistors and applying a full 12v to the blower motor when you select "high" on the control.

I'm familiar with troubleshooting the later C3 blower relays but not so much on the earlier ones. Maybe someone can chime in with the pinout of the relay?

Check and see if your car is equipped with the 4lug or 5lug relay. I beleive 72 is around when it changed.

 

Man! Reading your post is like listening to the the soft and gentile rusting of leaves on a brisk Autumn morn.....!!! Eloquent, eloquent, eloquent! You must be a college professor or English major?!

 

......

Is that non-Jewish Ironwood leaves rusting?

Sorry, the devil made me do it.

Thank you for your kind compliment:o, unless you hate college professors or English majors!!

As English is my first language (of no more than 3 so far - English, Bad English, and Profanity), I do try to act like I've seen some of the words before. All I hope to do is not embarrass myself in front of such august company. Or bore someone away who might otherwise have an answer. If I can provide a modicum of entertainment, maybe that'll help get more replies.

It's awfully nice of you to take time to pay such a wonderful compliment! Thank you!

 

Doh!!! Is that "genteel"......jenteeeeel? Or perhaps it's gen3teel.....the 3 is silent. LOL!! Talk about embarassing one's self with the lack of spell check, or the discernment to at least CHECK what I've written before I post it! I'll blame the faulty keyboard, as it can not defend itself, but CAN make my life a veritable hell! And you are indeed most welcome on the compliment....well deserved, my friend. Well deserved.

 

With the luck we've had with this car, it will be seventeen lugs, of which thirteen will be connected, four to each other, but with different-colored wires, and two of the rest will have such elaborate protective covers it will be scary to behold the bare ones. There will be a part number, plainly marked, which can be found in 57 catalogs, none of which reference this part, and tracing the wires will lead to more mysterious "stuff"........

Ours was made 12/24/71. By elves. Sick, twisted, evil demented elves with a single-digit (collective) IQ, a chip on each shoulder, a bone to pick and an itch to scratch. Union elves! And we look like scabs on tree bark!

And as soon as the light comes back on in the Big Room with the Blue Ceiling, I'll put on all the clothes I own and go find out the answer for you. SWMBO has had the entire console apart (Woodland Camo shifter boot, inner boot, shifter, carpet, HVAC controls), so if I can convince her it's "not that cold", she'll be most helpful.

I had assumed that even "medium" would be feel-able at the vents, by memory of her old Camaro and other GM cars we've owned. The idea of hitting the fan with all 12 volts is a Good One. Resistance is futile. Maybe I'll wire it as "offlow - OFF - Med - High"...

 

Enjoy a 'Happy New Year' with so much heat you'll have to open the windows.
I'll give you some information about my 74 and maybe it will apply to your 72, there are TWO fuses in the circuit, a 25 AMP in the fuse box. That fuse gives you Off/Low-Low and Medium speed at the blower motor.
The other fuse is 'In-Line' as shown by the arrow in the picture.

This fuse gives Full power to the fan is engaged by pushing the 'Switch' all the way to the right, 'Hi'. (not a second off switch.)
This fuse is 'In-Line' and located under the drivers fender, should be in front on the master cylinder and connected at a junction box (a few orange wires).
That would be an easy fix if that's all that's wrong.
If that fails the as previously mentioned then see if you have voltage at the relay.
If all is good there than check that you don't have a shut off valve in the heater hoses, one hose should go from the water pump to the heater core and the other from the manifold to the core.
The thumb wheel on the right is attached to a cable to full open or close the heat door, that will be the next thing to check. Mine also has a "Vacuum' operated valve (under the hood and spliced into a heater hose). If you did the heater hose check you will see if you have a cut off or not.
Alan 71's post about P12-8 in the 'Service Manual' just what you need for the full repair. It shows the Left wheel which is the 'Vacuum' side.
That wheel controls which vents open or close, vent floor etc. but lets get the electrical side 100% and then we can deal with the vacuum side. Pete.

 

That put a huge smile on my face

Did I see camo shifter in there?

Yeah Medium should be feel-able but not by much. In my car with a brand new blower it's barley a wheeze at medium. These cars were notorious for the poor ventilation system.

If yours is clogged at all it's going to make matters worse for sure. But that's another venture we should fiddle with getting full air flow first

 

With the engine warmed up completely, get under the hood and feel how much heat is at each of the heater hose connections coming out of the firewall. I would surmise that one would be pretty warm and that the other is very cool. If so, your heater core is plugged up OR the water shut-off valve (vacuum operated) is switched OFF all the time [or permanently 'frozen' in OFF position).

From what you've described there are three possible causes (or any combination of all three) for your problem:
1) the heater core is plugged; 2) the water shut-off valve is 'locked' in OFF position; 3) the vacuum signal going to the water shut-off valve is not functional. I think the odds are very high that your heater core is plugged. You can flush it by removing the hoses from the heater core inlet/outlet tubes and plumbing a [home] water hose with valve to one of the fittings {and an exhaust line connected to the other}. Start by just cracking the valve on the hose so that you can hear it running through the valve. If you get ZERO flow [and the hose is 'charged' with water], shut the valve off, as the heater is blocked and will get pressurized to full house-line pressure quickly; that could blow out a weak heater core.

 

Dear Mr. Lizzard,
I have a 68 and would guess there are disign differences between mine and yours. That said, my fan has not worked since I bought the car. The vent is open and quite a bit of air moves through the system while driving. (Bubba took most of the vent, fan and control hardware) So much hot air that I needed to install a shutoff on one of the heater hoses for summer use. During tha late fall (I live in Chicago) I open the shutoff valve and wear a boot on my right foot to keep it from burning.
My point is that I suspect your squirrel theory may be correct or you are not circulating water. The water circulation can be checked by disconnecting both HC hoses and running a hose in either direction. I would not use high pressure but would rather check for movement. If no movement, the core could be blocked or the valve closed.
If the water is moving I put my money on the squirrel. (bird nest, rat, etc.)

PS; I also enjoy reading your post...
Hope you get it fixed!

 

LOL...................That is good! My wife is looking at me laughing like I'm crazy.Or am I? I have no heat...bad heater core(bypassed) I'll be damned if I tear the entire dash out to change it. So it stays in storage till spring.