Disabling Auto Heat
 

My 81 SC auto heat has never worked in the 9 months I've owned the car. It went in and had the motor removed for a rebuild and while I have had the car I began testing with all the threads on the subject. So, both of my temperature sensors have issues, the cabin one has a break in the wire(somewhere?) to it and it goes off after re-installing it and bending or pushing the wire. The one at the heater box is dead.

I have read that the selector switch on top of the auto heat box will still control the engine fan without these sensors but would like to confirm this with others. I tested the selector switch and it is working. If this is the case I will just run with the manual lever disconnected from the auto heat motor and the engine blower with the selector switch.

Last question, do I plug in the dead temp sensors for this plan or not? Thanks for any thoughts!

The heater control unit on my 1978SC had been modified by some PO. I bought another one (when they were cheap) and have since made one that works after a fashion by combining parts from each of them. Basically, the central control knob is now just a switch for the engine compartment fan but at least I can get some heat. I have also disconnected the rod that opens/closes the gates and use the manual lever to open and shut the gates. So you don't need the sensors; when you get too hot just turn the heat off manually.

Roy

Thanks Roy! Were any of the modifications needed to get the temp knob to control the fan or did that work from the start?

Call these guys, they repair the units; www.systemsc.com

No. From memory there are two yellow wires that come out of the back of the heater control unit that feed the blower motor relay. When I turn the control knob I get a whirring sound as the motor moves the lever to open/close the gates (disconnected on my car) and if the engine is not running I can hear the fan start. The whirring noise stops when the lever reaches the end of its travel. There are two micro-switches that control the movement of this lever. One of the cables was a bit seized (now fixed) so the rod connecting this lever to the cables would pop off. Now that I have freed the cable off I might try reconnecting the rod and see if it can open/close the gates without any manual intervention. But access is difficult to fit the rod and it's cold outside at the moment; when the weather warms up I'll try this out.

Roy

Thanks Roy, this is helpful! So with this I have the control box out out and on the passenger seat and I applied power from the connector to the motor and got it to move back and forward. This is the first time I have seen the motor move. But I have no power coming out of the 2 brown wires that connect to the 2 yellow wires you mentioned, which I'm assuming means that no trigger power to the engine blower relay which will keep the blower from running. I'm going to have to find a wiring diagram of the control box, because if it takes the 2 temp sensors and the circuit board working with them to power the relay trigger it will not work as I want it to.

Thanks again,

The unit doesn't supply power, it provides grounds. Read the wiring diagram.

I just found a wiring diagram and see that, thanks. But without the motor where this gets power from the starter solenoid I can't go any farther now. But my thinking is my plan may work?

From another relay, again, as shown on the wiring diagram.

On my car there are two yellow wires (under the carpet?) which need to be connected to the corresponding wires on the control box. Did you disconnect these to get the box out? Order of connection doesn't seem to matter. I'm speaking from memory here; I'll check tomorrow as it's 8.23 pm here and cold and dark! Ignition needs to be on, engine doesn't need to be running. Also need to have the other connections in place. I wouldn't worry about the sensors at this stage. I think the idea with the sensors is that you dial in a temperature and the system will attempt to maintain that temperature by opening/closing the gates.

Roy

Roy,

Yes, I disconnected the 2 yellow wires and they are negative and the continuity is broken by the limit switch on the electric motor when the lever is at fully off. I have everything connected and in the car except for the 2 temp sensors connections and it is working. It didn't work when I got the car and I have no idea what I did to make it start working other than disconnecting the 2 broken temp sensors? I'm happy with it as it is, if it will turn the fan off. Once I get the motor back in the car we will see.

Thanks everyone for the help!

No one has suggested simply removing the machine screw that joins the heat control lever to the motorized lever ?

My 77 came to me with that modification and the prior owner made sure I put the machine screw in a 'special place' so I could return it to factory.
(Takes all of 5 minutes to move the rubber boot, align the two levers, and install the screw)

Bill K

I had it that way but it would not turn the blower on that way. To get the blower on the Mico switch on the auto motor has to close.

This happens, i.e. max blower speed, with the knob at max temp. The footwell blowers function (low speed) at various knob settings, as the motor varies the flaps in the engine compartment.

There are also some excellent auto heat threads that have helped me immensely with auto heat on my 87.

My footwell blowers don't work either and they are going away. Not worth having those as the car will never be out in the snow or ice with me. Next guy wants them, I will have the broken ones for him....

Those blowers are also the main heating blowers for the front.
The system is very simple, the vehicle without it has less value, so why not fix it properly?

Also footwell blowers help a lot with keeping the inside of windscreen and side windows dry on rainy days.
The footweel blowers won't work unless the rear engine compartment blower is also working.
Or else you need to modify the heater relay to bypass the reed switch inside.

A lot of threads here about this.

Dave,

Thanks for your help. I have owned the car for 10 months and got to drive it for about a month or two before the old broken head studs gig was discovered. And it's been apart since. If the footwell blowers will not work without the temp sensors as I understand, I will not be taking the headliner and interior apart to run a new used wire to the interior one anytime soon. This is a 47K mile car and I will not be driving it in any type of bad weather so for me just having heat will work fine. I almost went the heat backdate route but just couldn't do that to this car.

I do most of my repairs that don't require too much room as I am space confined. But chances are once I get past where I'm at now and start enjoying the car I will end up repairing the rest of the system. Yesterday, I rewired the power to the radio since they had 12v constant and 12v from the ignition switch backwards from the Alpine 90's install, causing the loss of all programmed radio stations every time you killed the motor.

Thanks again to you and everyone else for the very valuable assistance, I understand the system now better than reading all the threads. as I did!

The OP's '81 911SC uses a different heating system than the 911 3.2, i.e. no heater relay like the 911 3.2.