1968 DeVille Heater query

[rustytractor]

The heater in my '68 Deville has never worked properly - all I could get out of it was tepid warmth which wasn't enough to unfreeze my fingers from the steering wheel so today I decided to have a play around.

I found some info on line showing the vacuum line routing and function and after about 2 hours of head scratching and going round and round in a loop as to what does what and when it started to make sense.

When I got the car I bypassed the tugboat because one of the electrical contacts was broken so I had fited a simple on/off switch to actuate the blower motor.

There were a few reasons why no heat was coming through, the first being that the lever inside the Power Servo that (I believe) opens and closes the various vacuum ports had been disconnected so that's now back on. The second was the tugboat wasn't holding vacuum so I bypassed it with 2x3-way connectors and joined all 4xvacuum pipes together.

These measures made the passenger side footwell vent flap close and I checked that all other vents were working (I believe most are) but still no real heat.

I then disconnected the power cable to the transducer which made the vacuum go high and hey presto - heat like the gates of hell had just been flung open !!!

With the power to the transducer disconnected the temperature control has no effect as the power servo vacuum valve doesn't move but at least the car's hot. I can turn the fan off by the switch when I've lost so much moisture I begin to turn to dust so at least I can now use the car on cold days without dying from either hypothermia or heatstroke !!

My questions are as follows:
The transducer seems to work (voltage present = no real heat, no voltage present = loads of heat) but how do I test the inputs that give the transducer more or less voltage ? Is the transducer likely to be at fault or the control head or sensors ?

Also, can someone please outline what each position on the control head should do (some are obvious, others less so).

I've got this far with a bit of logic but may need some advice to finish the job off. I know I at least need a new tugboat but want to work out exactly what else is needed.

There's far more in my head about his than I care to write as it'll bore the crap out of everyone but the above should give you an idea of where I am so far.

Thanks for your advice in advance.

[Jeff Rosansky CLC #28373]

Everyone is going to tell you to get Cadillac Tim's book.
Full vac is full heat. I would say the transducer is fine because the vac changes when power changes. You probably have an open circuit somewhere in one of the 3 sensors....... Or a closed one. More resistance is more heat..or vice versa. I forget which.
I am sure Scot will weigh in here on this. He is your man for answers.
Jeff

[Scot Minesinger]

Jeff is right, you need to buy Cadillac Tim's book.  I have restored the climate control system on all three of my 1970 Cadillacs and over the last ten years a minor problem has cropped up which was cured very quickly with the aid of Tim's book.  The shop manual has some of the same info, but it is not condensed and well organized like Tim's book.  The price of his book is a fraction of your hourly wages and it will paid for on your first repair.

The climate control system sends full vacuum (like 10" or more) for heat and no vacuum to servo for full a/c.  If you set the dial to 80 and it is cold outside and you have 10" or more of vacuum on the servo than all the stuff inside the car is probably OK.  You have a servo issue possibly.  Make sure the heater valve opens. 

You can check some basic things:

1.  Power blower fan with battery voltage (make sure ground is good and fan runs).
2.  By-pass heater valve with straight thru connector.
3.  disconnect servo arm and move the arm closer to the driver side (full vacuum position). 

You better have heat after this.  It may blow out dash vents, but heat nonetheless. 

Tim's book takes you thru this trouble shooting process.  He sells the rebuilt parts at good prices too.

["Cadillac Kid" Greg Surfas 15364]

Russel,
If I am reading your post correctly you are asking what controls the electrical input to the transducer.  It sounds like you have checked everything else out.  Basically there is the sensor string (out side air sensor, discharge air sensor, and in car sensor in series with the temperature control dial.  This circuit inputs to the amplifier to generate an output to the transducer.  Again, if I am reading your post correctly you have a factory service manual covering the AC system.  Locate and individually check (with an ohm meter) the individual sensors.  Resistance against temperature as on the graphs in the manual. following the wiring diagram check all the sensors. Check the connections and then with the sensor string connected (to each other) check the resistance of the complete string as you move the temperature dial from 85 to 65 degrees.  The range should be somewhere around 100 to 200 ohms.
Cheers
Greg Surfas

[rustytractor]

Jeff is right, you need to buy Cadillac Tim's book.  I have restored the climate control system on all three of my 1970 Cadillacs and over the last ten years a minor problem has cropped up which was cured very quickly with the aid of Tim's book.  The shop manual has some of the same info, but it is not condensed and well organized like Tim's book.  The price of his book is a fraction of your hourly wages and it will paid for on your first repair.

The climate control system sends full vacuum (like 10" or more) for heat and no vacuum to servo for full a/c.  If you set the dial to 80 and it is cold outside and you have 10" or more of vacuum on the servo than all the stuff inside the car is probably OK.  You have a servo issue possibly.  Make sure the heater valve opens. 

You can check some basic things:

1.  Power blower fan with battery voltage (make sure ground is good and fan runs).
2.  By-pass heater valve with straight thru connector.
3.  disconnect servo arm and move the arm closer to the driver side (full vacuum position). 

You better have heat after this.  It may blow out dash vents, but heat nonetheless. 

Tim's book takes you thru this trouble shooting process.  He sells the rebuilt parts at good prices too.

Thanks Scot, I'd already managed to get heat by going through the system and checking the obvious - I really needed to understand how the transducer inputs worked to generate a voltage.

I will probably buy Tim's book when I order the parts I need from him but by that time I'll understand the system and may not need it (vicious circle really). Coincidentally today I found a help guide on line that Tim posted which helped me to understand the inputs to the transducer and how they work. By a process of elimination I'm pretty sure that the sensor in the passenger kick panel is faulty as there's a dead short across the a/c switch contacts which is probably confusing the input resistance to the transducer.

If this is correct I will  need both a new passenger sensor and tugboat switch so will be in touch with Tim for these items. 

[rustytractor]

Russel,
If I am reading your post correctly you are asking what controls the electrical input to the transducer.  It sounds like you have checked everything else out.  Basically there is the sensor string (out side air sensor, discharge air sensor, and in car sensor in series with the temperature control dial.  This circuit inputs to the amplifier to generate an output to the transducer.  Again, if I am reading your post correctly you have a factory service manual covering the AC system.  Locate and individually check (with an ohm meter) the individual sensors.  Resistance against temperature as on the graphs in the manual. following the wiring diagram check all the sensors. Check the connections and then with the sensor string connected (to each other) check the resistance of the complete string as you move the temperature dial from 85 to 65 degrees.  The range should be somewhere around 100 to 200 ohms.
Cheers
Greg Surfas

Thanks Greg, I didn't have the '68 manual, just some time and natural curiosity.

I've only just seen the info you've posted but this will no doubt help.

If you read my response to Scot I was pretty sure where at least part of the remaining fault lied, in that the a/c switch is constantly shorted but maybe this is wrong. When I look at Tim's help guide it says that the switch is open below 32 degrees (inferring it's closed above 32 degrees) but where does it get this temperature reading from ?

["Cadillac Kid" Greg Surfas 15364]

Russel there are two circuits in the "outside air sensor". One is in the compressor clutch circuit that prevents the compressor from operating when the outside temperature is below (approximately) 30 degrees F. The fan and the control circuit side of the system is still operative, allowing, of course heat and defrost.  The other is the sensor in the sensor string controlling the power module via the amplifier and transducers.
If you PM me with your e-mail address I will forward a copy of the hand out I have been using for the seminars I have been giving at the Grand Nationals.;  It includes a section on the controls and is not year specific, but the information there is applicable to your car and perhaps will help.
Cheers.
Greg Surfas

[cadillactim]

The sensors send a resistance signal to the amplifier, which in turn varies the voltage to the transducer. The transducer then converts that voltage to a vacuum output to the power servo.

All you have done to this point is bypassing things to get full heat. Without fully understanding how the parts work together you will spend a lot of time guessing and getting frustrated.

From experience you probably have more than one problem with the system. A shop manual will help some, but unless you have the special factory tools it is not real helpful in diagnosing real world problems.

That is why I wrote the manuals, no special tools needed.

Tim