no heat!

[J]

 hello all i have a '75 eldorado i drive often it was a bit chilly so i turned heat to defrost and no heat i know last year no other setting would work accept defrost but that was enough for me as it warmed it up enough and kept windshield clean but this year nothing at all at any settings any help as to where i might start to look for problems as i do want to drive untill first snow thank you, Mrjb

[Mike Josephic CLC #3877]

There is a sensor on the block that will not allow the heater motor to turn on until
the engine reaches a certain operating temperature.  This was one of the many
"pollution control" devices that hye installed on these cars.  My '73 has this feature,
I would asume yor '75 is the same.  Does your blower come on at all? Or does it just
blow cold air without any heat?

Some aditional info would help us sort this out.

Mike

[J]

last year just  the defrost setting  worked and was hot enough to keep me happy this year nothing i checked  simple things such as fuses and  and they seem fine.

[TJ Hopland]

The blower should run full speed on the HI BI and Defrost.  Other modes it runs through the speed control portion of the programmer.  The system needs vacuum to operate.  Its the same line that feeds the parking brake release.  It comes off a T on the #7 runner of the intake manifold.  In all cases the blower runs through a relay.   In my 78 and 3 others I looked at the relay and socket had failed, not sure if this was a problem on earlier models.  To find out if your blower even runs pull the wire off it and use a jumper wire (decent sized one) to go to the + side of the battery.  There will be a spark but the blower should run full speed, you may just have a bad motor.  The shop manuals are pretty good in the heat/ac department.  Lots of troubleshooting and testing that for the most part does not require special tools.

["Cadillac Kid" Greg Surfas 15364]

Jessie,
Just to back track a bit.  The heater turn on switch located at the front of the right side cylinder head, just behind the alternator is in the control circuit to delay the blower motor operation (except in DeFrost when it is not in the circuit) until the motor warms up so that the water circulating through the heater is warm so that in heat the system doesn't blow cold air and the automatic temperature control has heat and can reheat the air as required. 
After the heater delay switch has been warmed up and the bi metallic inside of it makes contact, that circuit provides the ground for the  "lo" and "high" relays.
The vacuum has no part in the blower circuit, other than provide a vacuum signal to the programmer which then moves the programmer actuator in response to the ATC and inturn controls the blower speed.

It sounds like you originally had either a bad heater turn on sensor, or what was also likely, a disconnected wire at the sensor. Look behind the alternator and you will find the sensor. Check the connection, and as a test you can take the wire that goes to the sensor and ground it to the exhaust manifold with the engine running and the ATC control set at either normal or high.

If when you turn the lever to "high" with the engine running and the AC compressor is operating (clutch engaged) you most likely have a problem in the blower  circuit only. Check the connections on the two blower relays which are on the blower housing behind the VIR for disconnects and/or burnt wires.  If you find everything okay but no blower, check and have someone ground the heater delay sensor wire (with the engine off and the lever in the High position). Listen and feel carefully and when this wire is grounded you should hear the relays energize (click). If they do, and the blower motor doesn't start check the blower motor as TJ suggested AFTER checking to be sure the connection to the motor is not the cause (disconnect).
All the above is assuming you have checked the fuses (again) there are two that play into the Heater/AC system.
You have an "honest" system and I am sure you can run down the problem.
Good luck
Greg

[J]

thanks for all the input i didn't have time like i would have liked today to check but i will tomorrow and hopefully come up with something i will keep you all  posted.