: Rig up a manual switch for '78 Deville blower fan



pompste
06-26-10, 08:01 PM
Long story short------i`m done with replacing "blower relays" for my '78 Deville.I now have "low speed" only and it`s not enough for very hot humid days and it won`t blow enough heat in the winter.
I got a full service manual on ebay,complete with electrical diagrams.It`s made real clear early in the manual that for the '78 Deville`s,a new 25 amp fuse was added to the fusebox for the blower relay circuit----indicating they had problems with this on the '77 models.
I`ll check the fuse,but since it does blow on low speed,the fuse is probably ok.

I know when i had no fan speed at all when the first blower relay blew,i asked my mechanic if he could rig a manual fan "on-off" switch i could operate from inside the car.He was hesitant to do so.He said he could just run a direct feed from the battery to the fan,but then i would have to open the hood everytime i wanted to turn the fan on/off.I wanted no part of that.
Maybe he could rig something with the electrical diagrams i now have----who knows----might be labor intensive and costly.

What do you all think of this?

CdnCaddi
06-26-10, 08:33 PM
I put a toggle switch under the hood on the ground side of the blower motor on my '84 Park Ave when the touchscreen in the dash quit.

Bro-Ham
06-26-10, 08:37 PM
You're beating yourself up on a seemingly unsolvable problem. You're not the first person to have this vintage of car and to be totally frustrated by this universal issue. Find a distributor to sell you blower relays in bulk and learn to live with it otherwise best of luck to you reinventing the wheel. :)

pompste
06-26-10, 08:45 PM
I put a toggle switch under the hood on the ground side of the blower motor on my '84 Park Ave when the touchscreen in the dash quit.

Did to you have to open/close the hood everytime you turned the toggle switch on/off?

pompste
06-27-10, 01:18 AM
You're beating yourself up on a seemingly unsolvable problem. You're not the first person to have this vintage of car and to be totally frustrated by this universal issue. Find a distributor to sell you blower relays in bulk and learn to live with it otherwise best of luck to you reinventing the wheel. :)

Actually man,the last 2 relays have NOT totally fried yet.As mentioned before,i now have LOW speed only and believe it or not every once in a while,about 5% of the time, it works on high speed and auto.
You`re right when you say this is a "totally frustrating" universal issue.Apparently the '77 thru '79`s have this problem.
IF i got high/auto fan speed consistently,i would just get a bunch of relays in bulk and tolerate it.Since i just get low speed,i need to figure a way to rig a switch i can mount inside the car that will give me high fan speed when i want it.
I`d really hate to get rid of this Caddy because the blower fan won`t blow on high speed.Sometimes i have passengers that want AC cooled air on high speed and i can`t provide it.
I hoping someone chimes in here that rigged up a switch to get high fan speed.

I`d like to know what the hell the Caddy designers did on the 1980 and up models to correct the problem.

sven914
06-27-10, 02:53 AM
What about bypassing the relay. The wiring for the newer ones is pretty rudimentary. I know they're completly different systems (analog v.s. digital), but in the newer system they use a fusible link to connect the motor to the power input. The ECC controls both the power input to the blower (fan speed) and the on off switch. Check the wiring diagram in your manual to see if there is any way for you to put a fusible link running from the part of the Climate Control, which controls the relay, and connect it to the blower's power feed.

I can't guarantee my idea will work; it's two different types of signals. If it proves infeasible, my only other suggestion is to replace your Climate Control with a the newer ECC and hope the lack of a computer doesn't interfere with it.

deVille33
06-27-10, 09:36 AM
It has been awhile since I studied this system, but I think your relay actually selects blower speed from the signal it gets from the control unit, which determined it's selected signal from the lever position and the thermal sensor. If you set your selector lever all the way over to defrost, your blower should go to high speed.
If when you first start your engine, the blower doesn't come on and turns on later when passenger compartment temp rises, your control unit may be functioning properly. If your blower is on low all the time, your problem may be back at the control unit.
Determine the lines from the control unit to the relay. Run a lead wire from the battery or from the feed side of a fuse connection to one or the other of the control unit connections at the relay. If you get high speed at one and low speed at the other, your relay is functioning properly and your problem is back at the control unit.
I would like to mock up a system, so I could better understand it. There is so much under the dash, it gets confusing.

pompste
06-27-10, 09:24 PM
It has been awhile since I studied this system, but I think your relay actually selects blower speed from the signal it gets from the control unit, which determined it's selected signal from the lever position and the thermal sensor. If you set your selector lever all the way over to defrost, your blower should go to high speed.
If when you first start your engine, the blower doesn't come on and turns on later when passenger compartment temp rises, your control unit may be functioning properly. If your blower is on low all the time, your problem may be back at the control unit.
Determine the lines from the control unit to the relay. Run a lead wire from the battery or from the feed side of a fuse connection to one or the other of the control unit connections at the relay. If you get high speed at one and low speed at the other, your relay is functioning properly and your problem is back at the control unit.
I would like to mock up a system, so I could better understand it. There is so much under the dash, it gets confusing.


When i go to defrost,the fan only goes to high speed about 5% of the time.
The relay DOES select blower speed when on AUTO.
The fan comes on only when engine temp reaches a certain temp.
Where is the "control unit" located?
I agree,there is much under the dash and it gets confusing---which is why i want to bypass all of that and just have a manual switch mounted under the dash that will simply turn the fan off and and on and hopefully also be able to control the fan speed.
I`ve been thru the archives here and there have been 2 people that have actually rigged a manual fan switch---but they give no details on how they wired it up.

Bro-Ham
06-27-10, 10:49 PM
I think this thread has jinxed my car - relay went out tonight...again. This one lasted, I think, about 3 months maybe. The relays from Advance say they have a 1 year warranty so that's where I'm getting my next one. All part of the fun! :)

pompste
06-27-10, 11:13 PM
I think this thread has jinxed my car - relay went out tonight...again. This one lasted, I think, about 3 months maybe. The relays from Advance say they have a 1 year warranty so that's where I'm getting my next one. All part of the fun! :)

All part of the fun! You`ve got a great attitude dude!

You`re lucky you just have to replace relays "occassionally".My dilemma is more complicated,as you have read.
As you said ADVANCE auto parts has a 1 year warranty.When, not if, the next relay blows--you will get a new one for free.
When one of mine blew out within just 2 months----i got the next one free.
Make sure you keep your receipt.

Bro-Ham
06-27-10, 11:17 PM
It's super hot in FL right now...not surprising...

pompste
06-29-10, 01:17 AM
I can see i`m going to have to talk to my trusted mechanic on this issue.
I appreciate all the suggestions here for a solution,though.
I`m looking for the simplest solution -----running a direct feed from the blower motor to an under dash mounted on/off switch with rheostat to control fan speed at my choosing.
Might be a bitch running the wire/wires thru the firewall though.
According to my long long searches in the archives here,many have/had the under dash switches mounted on the '77-'79 Deville`s when they owned them back in the day---but they don`t give any details on how they wired and mounted the switches.

sven914
06-29-10, 02:01 AM
A good mechanic should be able to rig something up after looking at the wiring schematic.

pompste
06-29-10, 02:35 AM
A good mechanic should be able to rig something up after looking at the wiring schematic.

I just got a good 1978 Cadillac service manual from EBAY---complete with the wiring schematic.I`ll bring it all along with me if my mechanic even needs it.
I`ll keep this classic Caddy if i can get control of the fan speed back.I just hope it does`nt cost me too much to get it done.I`m tired of complaining passengers bitching of no cold air in the back seat and not enough heat in the winter.

deVille33
06-30-10, 09:27 AM
When i go to defrost,the fan only goes to high speed about 5% of the time.
The relay DOES select blower speed when on AUTO.
The fan comes on only when engine temp reaches a certain temp.
Where is the "control unit" located?
I agree,there is much under the dash and it gets confusing---which is why i want to bypass all of that and just have a manual switch mounted under the dash that will simply turn the fan off and and on and hopefully also be able to control the fan speed.


The "control unit" is your in dash climate control selector.
When you say the defroster comes on 5% of the time, it suggests a worn selector or dirty contacts.
Your control unit is made up of several components, like a vacuum controlled manifold which selects which doors to open to direct air to different ducts. It contains the temperature selector which allows you to determine the ideal cabin temperature. There is also an electronic control unit which determines the control lever position, selected temperature selector position, and feedback, the cabin temperature. It turns the fan on high or low or off to maintain the selected temperature. It's like a small CPU.
I have replaced this electronic control unit, on a whim, to observe the reaction and restored the unit to proper operation. Most people buy another complete in dash unit on ebay and hope for the best.