Checked the search engine first. Couldn't get any hits on "heater". Weird....

My '89 has the auto (electronic) heater control. I have proper fluid levels but it rarely heats any more. I read something about a valve underhood.

Found a vacumn hose leading to a "T" in the heater hose. Don't know if it's working -- but both hoses get hot. This tells me yes.

The electronic control seems to work since adjusting the temp slows, speeds fan and opens ventwork behind dash.

Seems like the temp sensor must be bad? I notice a small vent in the pillar bar (behind driver's head). Is this where the sensor is located?

Anyone have other thoughts? Can I bypass sensor to test for proper heating?

Thanks!

 

Cars that don't blow any heat usually have a plugged up core - the Vette is no exception, in fact it's probably worse than average, particularly the '89 since it seems to have a high rate of head gasket failure which leads to galvanic corrosion and plugged up cooling systems. You can remove the hoses and try to flush it with a garden hose or compressed air. Before you do that, you can check out the controls by removing the blower module mounted on the left side of the Evaporator Case. Inside you'll see a door. Have someone operate the controls between 60 and 90 and the door should move. If it doesn't, the motor that controls it may be shot. Otherwise, the linkage between the motor and door is connected with a plastic clip and sometimes the clip breaks. However, if it's broken, the door will flap around and you usually get hot, then cold, then hot again. More probable then not, you need to flush the core or replace it.

PS: There isn't a valve controlling coolant flow to the core - the tee in the outlet sends coolant to the rear of the manifold and onto the heads. The manifold gasket is restricted at the rear cooling ports - supposedly to provide a little more cooling to the rear of the heads.

 

Ah and what a lovely and enjoyable job it is to replace the heater core.

Yep I concur with SunCr could be the mixing door, the core or perhaps a dirty head unit edge connector. The latter would have symptoms of not responding to temp settings.

If you decide to attack the heater core I have a PDF that will help you understand the job involved.

 

I had a similiar problem and when the blend door motor goes bad it will either go full cold or full hot. Mine went cold but go stuck in between. First check to make sure the door is opening or closing all of the way.

 

I don't suspect a heater-core clog. That's because I've had heat working, shut the car off, then restarted the car to find cold air. This has repeated several times over the past six months. Wasn't a big deal during the summer. I don't think it's flapping either. Mostly, it would heat when the weather was warm. In other words, warmer temps helped cure the intermittent problem. A weak motor/sticky door would probably come alive in warmer temps. So this theory seems plausible.

I have my dash bezel off and there is a door that opens and closes behind the upper a/c ducts. I'm guessing this is not the "blend door". This is probably the vent/lower air routing duct?

Where is the evaporator case? Is it on the firewall in the engine compartment near the heater hoses? Is it difficult to disassemble to the point I can see if the blend door is working?

Why not suspect a temp sensor? Am I wrong in guessing there's one located in the pillar behind the driver's head?

 

Evaporator case is on the right firewall. A/C inlet, the smaller line, enters at the bottom. A/C outlet, the larger line, exits at the top. Easiest to view the Temp Door by removing the Blower Module mounted on the left side of the case. Only 3 or so machine screws to remove and then lift it out. Otherwise, you need to remove the passenger's side hush panel, ECM, Disconnect the Temp Door Motor Linkage, Heater Plenum and then the Heater Core to see the door (which you are going to have to do to replace the core, but I wouldn't start until you see if the door is working). The door you're looking at is for the defroster.

Inside (Interior Roof Pillar) and Outside Temp Sensors (Radiator Mount) control Blower Speed and Discharge Air Temp between 65 and 85. 60 or 90 overrides their input. A fault in either should generate a trouble code which would be indicated by a flashing LED below the outside temp readout. An open or short in the Temp Door Motor should also generate a fault code. If the LED isn't flashing, don't worry about it. Coolant Temp Sensor and Gage Sender have nothing to do with HVAC so no need to consider them.

 

Diagnosis............http://members.shaw.ca/agent86/AC%20Electronic.pdf

 

[QUOTE=SunCr]Evaporator case is on the right firewall. A/C inlet, the smaller line, enters at the bottom. A/C outlet, the larger line, exits at the top. Easiest to view the Temp Door by removing the Blower Module mounted on the left side of the case. Only 3 or so machine screws to remove and then lift it out. Otherwise, you need to remove the passenger's side hush panel, ECM, Disconnect the Temp Door Motor Linkage, Heater Plenum and then the Heater Core to see the door (which you are going to have to do to replace the core, but I wouldn't start until you see if the door is working). The door you're looking at is for the defroster. QUOTE]

Thanks for ruling out the temp sensor (raising the temp to 90 does not create heat and I don't have a fault light).

I will try viewing the door by removing the blower as you describe.

If the blend door is sticking/failing, is the actuator motor easy to replace? If repair of the blend door (or related components) requires a/c disconnect or a lot of interior disassembly, I might be inclined to let the pros have at it.

P.S. Whatever's wrong, the car generates less heat in the summer than it used to. Even with the thermo set on 60, the car used to blow luke-warm vent air on cooler days. Now the air is always as cold as the external air. Sounds like my door may have had less than full travel before failure (or the core is plugged). I hope that fixing it doesn't bring back warmer vent air -- cause I do like the cooler vent air (since I don't drive it much on cold days anyway)!

 

Thanks!

This is one fine piece of documentation!

 

"If the blend door is sticking/failing, is the actuator motor easy to replace? If repair of the blend door (or related components) requires a/c disconnect or a lot of interior disassembly, I might be inclined to let the pros have at it."

Some might think it's difficult - you'll need to be laying down to access it, but there's no reason to disconnect any a/c components. For me, I just keep taking off parts until whatever I'm trying to fix is easier to get at - For that motor or the core, that would include the Dash Pad, Bread Box, the right side a/c duct work and probably a few other items. A bigger problem might be finding a new motor, assuming you need one. The clip is still available. Motor, link rod, and door may only be available used.

The symptoms still sound like a plugged core, but you'll know for sure once you see if the door is moving.

 

Looked under the hood tonight. If I understand you correctly, the Blower Module is kinda on top of the Evaporator case (and on the half closer to the engine). Basically, it's a harness going in the top, right?

If you remove this, everthing will still work? And, I should be able to see if the door is moving. (May not have time til the weekend to check this out).

Sounds like you're saying the blower door must be accessed for replacement/repair thru the interior?

Thanks.

 

That's what you need to remove. Blower Module controls blower speed and compressor ground, but doesn't interact with the Temp Door. Shouldn't matter if it's connected, but you can plug it back in once you've removed the Module. Just have the key in Run and cycle between 60 and 90 to see if the door moves.

Door is accessed from the interior and/or by removing the Evaporator Case. I've never seen the door itself fail. It's aluminum soldered to a rod. If there's a problem, it's usually the motor or the plastic clip holding the linkage together. Both are accessed from the interior.

 

I gather the blend door controls the flow of hot vs. cold air. If it moves and the heater hoses get hot, does that prove the car should be heating? If so, I can see why the heater core would be suspect!

If I find the door working and decide to try a flush/flow test, I have a couple of questions...

Does it matter which hose I force water thru? If so, please specify direction most likely to unclog....

Since the hoses are original (50K miles), I'd probably cut them for ease of diagnosis. Then temporarily reconnect (with patch pipe) until final repair. If flow seems restricted, are there any products that can "boil" out the clog? Which ones are safe? Are there more aggressive products that might work? Drano, acid, use your imagination!

Thanks for the advice!

P.S. Since both hoses get hot, I would have expected that water is flowing thru the core.

 

When the core is plugged - coolant flows over the top, but not through it. If it didn't, a hose or something else would break.

I wouldn't cut the hoses unless they show obvious fatigue. They seem to last forever (I have 58k on mine) and the outlet also tees off to the oil cooler. It's not usually in stock at the Dealer, nor is it cheap.

It doesn't seem to matter which way you flush it. Inlet is on the bottom, outlet on the top. After I replaced mine, I put 125 psi on one of the tubes, my thumb on the other. When I removed my thumb, there was a loud pop. I then ran hot water through it at the sink and all the junk came out and it got nice and toasty on both sides of the coil. I wouldn't do this with it in the car - Design limits are probably no more than 25 to 30 psi and if you put a hole in it, you won't know it until it's under pressure and there's coolant all over the interior.

If you haven't flushed the cooling system, I'd do that too. To get everything out of it, pull the oil cooler hose at the oil filter and the knock sensor on the other side of the block.

There are products that can be used to clean the cooling system, but I don't have much faith in them, especially with the head gasket problem that seems prevalent in the '89. Create any moisture path that includes metal to metal contact between the aluminum heads and the iron block and the coolant will quickly become a corrosive mix that will eat away at the internal components. I definitely wouldn't use Drano.

 

Sounds like directly testing for a plugged core might be difficult. If I disconnect the hoses at the firewall and shoot water through that point, nothing would tell me whether the water is traveling soley through the bypass. OTOH, if I connect to an indoor faucet where I can vary the water temp, I would expect to feel an interior air temp change -- with the fan blowing thru an unclogged core, right?

It also sounds like the best chance of clearing a plugged core would be to force water back through the outlet -- since this would reverse the normal direction of flow.

I had a dealership change the antifreeze when I bought the car in 1999 (@15K miles). I was getting ready to change it again. Would you expect a dealership (or quick lube place) to sufficiently flush the fluid as you described? If not, could you elaborate on the location of the knock sensor?

In almost six years, I haven't seen evidence of antifreeze in the oil and/or a fluid loss. From this, I'm hoping I haven't got a car with intake/head gasket issues.

 

I just reread your post. What (and where) is the head unit edge connector? (I assume it's a connector on the main control head, but I did not see an "edge connector" on the electrical PDF supplied in this thread).

Even if I don't need to tackle the heater core in this round, I would like to get a copy of your documentation as well. (It might not be so handy if I ever need it!)

Thanks
gp

 

I'll need your email address and the files about 5MBs.

When you remove the head unit from the console and remove the front panel cover it attaches directly to an edge connector on a PCB. If you ever have to replace the LED lamps they screw right into the PCB. If you remove it, and take the front panel off the edge connector can be cleaned with a pencil eraser.

My suspicion is its the door, but either the motor or the linkage.

Have you ever had a scent of anti-freeze inside the car?

 

No leakage. No smell.

I'm leaning toward the door function myself. If the core plugged up, I would have expected a gradual (or even sudden) stoppage of heat. Since it worked intermittently ('til death) and it seemed to work well -- when it worked -- I have been leaning toward an electronic problem. This blend door certainly qualifies though.

I can hear the defroster door open and close -- even with my eyes shut. I can not hear any other door functioning while varying the controls. If you should be able to hear the blend door open/close, then I'm really leaning toward this being the problem. BTW, is this door an open/close thing or can it be partially open based on temp setting?

 

You are just going to have to pull the Module and see - if the door is moving there should be heat.

Door position is determined by Temp Setting and inputs from the Outside and Inside Temp Sensors - except at 60 or 90. At those settings, Sensors are ignored and it either gives full a/c or full heat.

If you disconnect the temp door motor harness with the battery connected, nothing will work until you disconnect and reconnect the battery. Once you do this and hit the Auto Button, door will cycle from fully open to fully closed so that Programmer can learn it's position. Takes a couple of minutes and there will be a no blower condition while the learn takes place.

Even with minimal driving, if the coolant is dirty, time to snoop around and find out why. Could be that the Dealer simply drained the radiator and refilled it - could be exhaust leeching into the coolant too, which unfortunately, a high number seem to have had this problem shortly after leaving the factory. Also, if the mix is corrosive, it isn't going to matter whether it's sitting or being driven. Plugged cores are one indication - higher than normal operating temps are another; ie, if it needs the aux cooling fan to maintain idle temps, you might want to look into it a little further.

 

Bottom of page 11 of that PDF Gregg is a picture of the mixing door. Kind of rusty looking.