SC autoheat help needed

[dyount]

Before I tear into my autoheat to possibly fix the problem of being able to fire clay year round help me out a bit oh pelicans!

So, as I see it the SC years autoheat (mine is 83) does not have levers etc but rather just a 0-10 dial with no variable fan speed?

Currently all I get is no heat or max heat. My question is...... Does the screw at the top of the rheostat or whatever it is actually control how far the servo opens/closes the flapper boxes?

[wrxnofx]

My '87 has the same autoheat, and I believe the lever fully opens the flappers when turning the heat off (i.e. to 0), and fully closes the flappers when turning the heat on (i.e. 1-10).
As I turn up the number, the speed of the engine blower increases, thereby increasing the amount of heat.
I think at number 8, the footwell blowers each turn on, giving even more heat. When going to 9, then 10, the footwell blowers each increase in speed correspondingly.
All on or all off sounds like a problem with the rheostat. Do the footwell blowers turn on in the higher numbers?

Matt

[Josh D]

My '80 SC autoheat variably controls the flapper boxes based on setting. It has a single lever connected to the servo control that has a divided cable that goes to both flapper boxes. If I remove the screw linking them, I can control my flapper boxes manually with the lever.

When I first got mine working (cable was seized and servo rod was broken, which I replaced both), It would only go full hot when turned on. I blew the dust out of the interior temp sensor with compressed air and cleaned the electrical connection and its worked great adjusting for temp ever since. Even after backdating heat and getting rid of the engine compartment blower, it still adjusts heat well.

[aj88cab]

The SC auto-heat may operate differently than in a later Carrera (my car) so take this as a guide. Also I have manual heat so my auto-heat "expertise" may be suspect You said you have no levers so that is one difference.

The engine blower is a single speed (on-off) fan. It will come on when you turn on the dial (any setting other than "0").

The foot-well blowers have three speeds. I believe it is more like setting 1-4/low, 5-7/med, 8-10/high.

The rheostat controls the opening of the flappers. I do not believe it affects any fan speed.

Since you are getting heat and can turn it off, the flappers are opening and closing, so the servo in the center console is working, but does not appear to be responding to the temp setting (only full open or closed).

You need to confirm the foot-well blowers are changing speed as you rotate the dial, and that the flappers are opening partially to full based on temp (I guess we know they are not).

Sounds like it might be a problem in the temp sensing circuit.

[Josh D]

Just an FYI, SC's won't have footwell blowers.

[dyount]

As if today here's what I've got. Seems the dial only gets the servo to fully open or close the system.

Theory here?? the flapper boxes open based upon what the autoheat dial tells them and should be gradually opening from 0-10. Based upon spring back pressure from the flapper boxes?


So should the first step be to R/R the flapper boxes with correct springs to eliminate if they are fully closing then possibly replace the controlling rheostat?

Lastly, the top of the dial portion of the rheostat has a screw in it ... does it matter regarding the amount of heat based upon the screw? IE if I loosen or tighten the screw will it reduce the voltage/amperage that is being sent to the servo that moves the arm that pulls on the flapper box cables?

[dyount]

Oh, as well, the fan in the engine compartment.... it is single speed yes? in an SC. If it's multi speed I've already got enough heat to cook my shoes but would change the fan relay in the engine bay if it would help

[tirwin]

Donut,

I had the same problem! I fixed it in my '83 and I suspect that the fix for you could be very simple.

Right above the mirror is the air temp sensor for the autoheat. It is basically a thermistor if I remember correctly. If it fails or is disconnected, the autoheat system goes into the binary (on/off) mode you described.

I think I found an old thread on here that was extremely valuable. I remember doing a bunch of continuity and resistance tests. I remember testing the resistance on the thermistor and it was fine. Seems mine had just wiggled loose. It's been great ever since.

[javadog]

Originally Posted by dyount

So should the first step be to R/R the flapper boxes with correct springs to eliminate if they are fully closing then possibly replace the controlling rheostat?

No. Step one is to test the interior temp sensor. Do a search, the info should be out there.

Step 2 would be to test the temperature sensor in the duct under the car.

You can unplug the wires from each of these at the controller, once it has been removed.

JR

[dyount]

Update!

1 found old posts that had info regarding ohms testing of various parts of the system.

2 pulled the interior temp sensor and tested to 1.5k ohms , seemed a bit loose or that it had no connection when I disassembled it. Gave it a wiping and tightened where it fit into the connector.

3 Removed heater controller and verified good connection from interior sensor to control box and as well approx 1.5k at the dial turned to #5

4 Adjusted the lever on the heat box to 4.646" of length (it was 1/4" too long)

Reassembled and whoop! Autoheat dial opens the flapper box control arm gradually AND better yet the fan does not operate until the dial moves beyond 6 on the dial.

I do, however, have just a little heat still coming through that is probably the fault of the heater boxes. I should of R//R'd them when I had the engine out a few years ago. I'll soak the fasteners for a while and remove/repair/repaint them and at that point the flapper valve should work correctly and remove any lingering heat to the cab unless I call for it.

Wicked thanks to those that helped .... prior to posting I had "thought" I'd done a good search but with the variables of the system in different years I missed a few great threads.