1978 Seville Heat control

[John Mann]

Please offer help if you can.
I have a 78 Seville that has only 41K. It is my favorite car but has a problem
I have replaced the dash heat/ac control twice with used modules.I am , again, left with the only active spot that I can put the lever on that works is the defroster.
The other positions do not respond. No blower and certainly no heat. The hot water is getting to the heater core, so it has to be the dash control.
Is anyone making a new replacement or do you have another suggestion.
I greatly appreciate your help as I love this car.
Best,
John

[Tod - North TX CLC]

John -

I have had the "Programmer" fail more often than the dash control.  The programmer is the black box with a cluster of vacuum lines, an electrical plug and a shaft attached to the recirc. door located down by the litter bin on the passenger side, attached to the firewall.  These units are always on e-bay for about 150.00 (new, AC Delco), but sometimes you will see them for less.  

Check your unit out - take out the screws (three, I think), remove the vacuum cluster, and loosen the lock nut on the shaft and take the cover off of the unit.  There are a couple of resistors inside that often overheat and burn out.  Also, check the condition of the electric contacts - I have often seen these burn out.

Let us know what you find.

[Denise 20352]

  First check the cylinder head temp metal switch.  The manual lists one for the Seville.  Im not familiar with Seville, but on other models equipped with the big block, there are two metal temp switches, one on each cylinder head.  The one on the drivers side rear turns on the metal temp warning light and key buzzer.  The one on the passenger side front turns on the blower when the engine temp is > 120 degrees f.  You can remove the wire from the switch and ground it, and if your blower comes on, theres your problem.  The spade plug on the switch gets corroded and loosens up, or the switch could be bad.  I usually just ground the wire...the last thing I need is for something to keep the blower from running.  When you put the selector in "defrost", this switch is overridden, which may be why your blower works.

-denise

[Dave MacGregor #18998]

Hi,

I have to agree with Denise that it may be the metal temp switch on the engine thats preventing the blower from operating. I had similar problems arise with my Eldorado a couple of years ago in that the blower always worked in the defroster mode, but in any other selection.

A longtime mechanic at Vallee and Bowe Cadillac diagnosed my problem site unseen for me.  The parts guy then looked it up, and found they had still had one of those switches left in their stock so I bought it.  (It really was their last one too!)  After a few minutes under the hood everything worked as it should in all settings.

I also have to agree with Denise that I too am not really a fan
of having the blower not running when I want it to.  Im sure the idea of the design was so that ones feet wouldnt get blasted with cold air before the engine warmed up, but I never was bothered by that in any other car or truck Ive ever owned.  If youre really bothered by that, a simple jumper as she suggested should work fine for you for just pennies.

I just wanted to keep my car as original as possible considering its nice overall condition.

Good Luck!

Dave

[John Mann]

Thanks friends.
I am printing these responses and will let my repair guy read them. I am too mechanically deficient to do much myself.
Again, I appreciate the help.
Best,
John

[John Mann]

Hello friends:
Update on the heat problem in my car.
You guys were absolutely correct that it is the heat sensor switch that was faulty and would not allow the blower to work. None available , so just grounded the wire and all is ALMOST well. The blower now works but I still do not have heat.
I can feel that the hot water is going into and out of the heater core but nothing into the car.
It seems to me that it has to be the control unit that opens and closes the doors. BUT, I can hear them open and close.
Any suggestions?
BTW---The Seville has that sensor switch in the front of the block and not on the passenger side. Must be that the big block is a little different.
Best,
John

[Denise 20352]

  I dont have a Seville here to look at, but on some other models, if you remove the glove box liner, there is a box behind the glove box called a programmer.  It has a connector on it with a big bundle of vacuum lines.  Out of the top of the programmer is a shaft with a lever that connects to a rod.  This rod goes to the valve that mixes hot air with the cold air.  As you go from coldest to hottest settings, this rod should move.  If not, very carefully undo the clip (dont break it) and push the rod in both directions, and the air should go from cold to hot.  If that is working, then either the programmer is bad, or its not getting a signal that it needs.  You could temporarily leave the rod disconnected and tape it into position or whatever, while you do some more troubleshooting.

-denise

[John Mann]

Thanks Denise.
I will try that tomorrow.
I sincerely appreciate all the help.
Best,
John

[hjlint]

Hello friends:
Update on the heat problem in my car.
You guys were absolutely correct that it is the heat sensor switch that was faulty and would not allow the blower to work. None available , so just grounded the wire and all is ALMOST well. The blower now works but I still do not have heat.
I can feel that the hot water is going into and out of the heater core but nothing into the car.
It seems to me that it has to be the control unit that opens and closes the doors. BUT, I can hear them open and close.
Any suggestions?
BTW---The Seville has that sensor switch in the front of the block and not on the passenger side. Must be that the big block is a little different.
Best, I bought on through NAPA. $32.00. Grounding the wire is good option.
John