1979 Cadillac DeVille - intermittent fan motor

[fasted55]

Hi, Back again with another question.  My climate control was fine until my mechanic was rummaging underneath the dashboard and now it works intermittently.  When working all the functions work correct - fan only, AC/heat, and bi-level.  When not working, the fan motor only comes on in the bi-level setting.  You can hear the compressor relay click in the AC/heat mode, and nothing at all in the fan only mode.  Any ideas?  Thanks, Ed

[TJ Hopland]

Dont know 79 for sure but 78 had major issues with the fan relay.  The relay would overheat and deform.  Eventually it also overheated and deformed the contacts and the socket.   Its one of the metal encased relays in the relay block which is above the fuse box IIRC.

["Cadillac Kid" Greg Surfas 15364]

Ed,
I think you answered your own question. It sounds like something behind the dash got knocked loose.  Since the problem is intermittent I would look at the major connections behind the AC control head.  The best way (in my experience) is to unplug and re-connect the connectors, checking the terminals for tightness and wire contacts.
Greg Surfas

[Gene Beaird]

That car has the climate control programmer, right?  If so, I'd hold the main electrical connector suspect.  It's loose-fitting at best.  The HVAC on our Seville went into 'failure mode' and I discovered that the connector was loose, yet allowed just enough of a connection to let the main power wire (the big brown one) get hot enough to melt the plastic connector. 

I've since located another connector, but have yet to transfer all the wiring over to it and test it.  As others have said, unplug that connector, inspect for hot spots/melting, then plug it back in, making sure you verify everything's securely seated.

[joecaristo]

I would check the relay first since its easy to do. Its on the drivers side near the fuse box. Its metal and square. If the plastic around the relay is melting then that's your issue. Try to pull the relay out and inspect the plastic casing and contacts. I had a lot of 78-79s over the years and that relay usually burns out. My air conditioner would work but wouldn't blow any air.
Joe

[fasted55]

Hi guys, More data.  The fan relay was just replaced and rewired for the reasons TJ mentioned.  It was melted and poorly wired by the previous owner.  And there might be some ambient temperature dependence.  This morning it was 70 when it didn't work and then at lunch when I next started the car and it worked it was in the 80's.  I'll drive it some more and try and notice the conditions of when it works and doesn't before trouble shooting for a loose connection somewhere.  And I misspoke in my 1st post as it's the defrost setting that works when the rest of the settings are dead.  What's weird is that the fan motor works in defrost although this is a data point too.  The joys of collector car ownership.  :-).  Thanks for the inputs.   Regards, Ed

["Cadillac Kid" Greg Surfas 15364]

Ed,
Check the connection at the "heater turn on" sensor at the front end of the passanger sid cylinder head behind the alternator.  If the wire is loose you would likely experience what you are seeing.
Greg

[Scot Minesinger]

The female connectors for the mid-late 70's on climate control are notorious for becoming loose and unreliable.  Often components are replaced as a result of these poor connectors.  Even if you had items replaced such as fan relay, re-crimp all female connectors, especially serving the programmer.  I suffered through a climate control problem of this type until this problem was discovered.  This solution costs nothing in parts and not much time, so give it try - no downside risk, big upside gain.

Good luck

[TJ Hopland]

The works only on defrost points to the temp switch on the head behind the alternator like Greg said.   Its a metal temp switch that grounds when temp is above 120*F.  This is so us Cadillac owners dont have to suffer with cold air blowing out our heaters in the cold days.  It keeps the fan on low till the temps come up.  Easy enough to test just pull the wire off the sensor and connect it to a ground.  IF the fan runs normally then you found your issue.   There are several varieties of sensors that look like that one that all have different temp settings including the one in the rear of the left head.  This one turns on at like 280 and is for the stop engine temp light. 

[cadillactim]

There are three ground circuits that allow the fan to come on.  One is the heater turn on switch mentioned earlier by Greg, the other is in the dash control itself, and the third is a direct body ground. 

The heater turn on switch is for cold weather, allowing the fan to come on only when the engine has warmed up.

The dash control ground allows the fan to come on immediately in hot weather (above 75 degrees).

The direct body ground is when the control is set on Defrost.

So, in cooler temperatures if the fan only comes on in Defrost after the engine is warm, the heater turn on switch is suspect.  If in warmer temperatures above 75 degrees, the fan only comes on after the engine has warmed up, then the dash control ground is suspect.  In warmer temperatures the fan will only come on in Defrost after the engine is warm, then both the heater ground and dash ground are suspect.

Tim

[fasted55]

Wow. Didn't check in again until just now.  Great advice and I'll check things out tonight.  I'm thinking/hoping it's the heater turn on switch.  Boy, what a rats nest of vacuum hoses under the dash.  As a recent owner of this car, I can't say thanks enough for the advice.  If you guys were local I'd sure owe lots of lunches (or beer) as payback.  Thanks and regards, Ed

[fasted55]

Goodmorning guys, I went out at sunrise this morning, the Gambel's quail family that lives in my front shrubs were just waking up, opened the hood and found the heater turn on switch.  It's a tight fit but I was able to disconnect the wire from the switch and ground it to the alternator.  And the fan motor started right up!  So later today I'll head over to my local NAPA store and see if they sell it.  A BIG thank you to Greg, TJ, and Tim for nailing this one for me and to the others that took the time to respond.  Until my next question.  Thanks and regards, Ed