79 FW HVAC/blower weirdness

mdmsp1 · #1 (**permalink**) 03-02-07, 11:19 PM

When I bought this car ('79 Fleetwood Limousine), the blower only worked on DEF. I read some of the threads on the forum, and checked some of the things mentioned. The connector at the switch looks new--no signs of heat damage. All flaps appear to be working correctly. The flap to the heater core was not positioned properly on the rod that opens and closes it, but I fixed that and I can see it working properly now. I was going to look at the fusible link, when the weirdness struck: I was driving the car this afternoon and the blower came on. I checked the different switch positions and the blower was working correctly. The blower would run faster when the thermostat position was at one or the other extreme, and slow down when the thermostat was set around ambient temperature. It worked the whole time I was driving around in the afternoon. I was overjoyed until I parked it for a couple of hours and came back to do an errand--Once again, the blower runs only on DEF, same as before. Anyone have any idea where I should start? Obviously the system CAN work right--but there is some intermittent issue. I'm stumped. Thanks. Mark

pnichalos · #2 (**permalink**) 03-03-07, 12:24 AM

I have the same type of problem on my 77 coupe deville, but i have not been able to fix this probelm yet. It only works whenever it feels like it, but always works on def. Thats weird that you have the same problem. Somebody should know how to fix it.

carnut · #3 (**permalink**) 03-04-07, 08:21 PM

This system also has a high blower relay in a bank of relays above and to the right of the glove box. Pull the glove box to gain access. The relay usually can be identified by the melted socket that it plugs into! Years and years ago GM offered a repair kit to remove this fan relay from this relay bank. You received a seperate socket and a short pig tail of wiring to reconnect it. If I remember this power wire to the relay was a fat orange wire. Follow it from the control head plug-in harness. The fan speed increasing at each end of the temperature scale is not really true. What happens is the outside air door shuts at these extremes and the blower is blowing in a smaller box giving an impression of higher speed. Once you locate the bank of relays, wiggle the relay with the car on and I'll bet the fan will start up. Mark where the wires locate on the relay, cut them out of the relay bank, install spade connectors to each wire and plug directly onto the relay. Then secure the relay up under the dash! Please understand this is from memory from my 11 years with Cadillac during this era.

mdmsp1 · #4 (**permalink**) 03-04-07, 08:47 PM

Thanks for all that info. I wish I could find someone around here who has any years of experience with Cads of that era! I like taking care of routine stuff on my cars, but I always feel like I am out of my league when I get into something like this. Now that the weekend is over, it will be a few days before I can get back to this, but I'll post what I find. Thanks again! Mark

mdmsp1 · #5 (**permalink**) 03-07-07, 06:08 PM

So...Today I went in to look at the blower relay panel. Everything there looked like new. There was no evidence of cooked wire connectors. I drove the car today, and the blower did its intermittant thing again. The thing I have noticed is that the blower works correctly when the car has been sitting in direct sunlight all day on a warm day. Being a black car with black interior, it gets pretty warm. This is the only time the blower works. Once the interior cools down, the blower only works on DEF again. Is it possible that the IN-CAR SENSOR is malfunctioning? I also checked the cylinder head sensor, by the way. I grounded it out, but it made no difference--so I am guessing it is working right. It is my understanding that if the interior is hot, but the engine is cold, the AC and blower should come on immediately--Could it be that the in-car sensor has to get really hot before it tunrs the blower on - then switches it off again when it cools off?

Just a thought.
Mark

nick stratta · #6 (**permalink**) 03-07-07, 06:18 PM

Mark,
Had the same thing on a '79 CDV. Couldn't get much heat until I turned the dial right round to 85!

Could be your sensors are dirty?

I feel for you. Hope you can sort it. Clean all the connectors you can find with aerosol switch cleaner. They may look clean but aren't always clean enough to conduct voltage properly without offering resistance, particulalry when the car is in a damp climate.
I also suggest getting a test sequence from somewhere and reading the voltages from the sensors and other connectors at a different temperature setting to see if they are sending normal voltage. If not then you know you have a bad sensor/ switch.
Worked for a L*ncoln Town Car I have.

regards
nick