96powerstroke

Hi Folks,

Newbie to the forum. Just bought my first '70 cutlass and having trouble with the blower motor; it only works on high. I know the stock answer is to replace the blower resistor however I'd hopefully like to get a little education on the HVAC wiring on this car as well. I just bought this cutlass and although I'm no greenhorn when it comes to wiring or mechanics, I am new to some of the practices GM used in these cars.

Seeing as how the fan only works on high I too would have just assumed the resistor is bad however it looks like there is a relay in line with the resistor? What is the purpose of this? (I know what a relay does...) Every other GM (and ford for that matter) i've had and worked on use a switch that sends power through that switch and to the resistor...so on and so forth - no relays involved. The car does have factory air conditioning for what it's worth.

I plan to replace the resistor but something in my gut says it's not the resistor. I had one in my Ford go (so I thought) and it turned out to be the thermal fuse. Easily fixed with a radio shack thermal fuse for $1.00 rather $30.00 the auto parts store wanted for a brand new resistor.

Your help is much appreciated.
Thanks,

MDchanic

All GM blowers in A/C systems from the early '60s through the '80s, more or less, ran the current for the fan directly from the battery, through a 30A fuse and a relay, for the high blower speed.

Lower speeds went through the resistor.

The switch routes the lower three speeds through the resistor, and sends the high speed to trigger the relay.

The lower speeds go through the relay after the resistor, so the relay routes the lower speeds to the motor when not energized, and routes full battery current to the motor when energized.

Since you need current through the heater fuse to trigger the relay to the High setting, a failure on that side cannot be your problem.
A switch failure that allows only one speed to work is highly unlikely.
Therefore the most likely bad part is the relay, which is easy enough to test.




You need to get a Chassis Service Manual and a Fisher Body Manual.
Originals are best, and easy to get on eBay, but you can also get digital copies, and a large amount of other material, for a roughly $25 membership fee to the Automotive History Preservation Society.

Welcome to ClassicOlds.

- Eric

rocketraider

Check and clean if necessary all the resistor terminals on both resistor and the wiring harness plug.

There's a remote possibility of it being the Master Switch on the HVAC control head, but as Eric has pointed out you have to have voltage thru high blower switch setting to seal in the high blower speed relay. So I agree the high speed relay is the simplest place to start. You should find it on the firewall. Look for the shorty 30a inline fuse holder for it and wire should be either black with orange or red stripe or red or orange with black stripe, depends on year and carline.

96powerstroke

thanks for the info. I found the relay located above the vacuum booster. Jumped the two terminals from the switch to the resistor and still no dice. Pulled the resistor and found the problem. Looks like someone tried to push the wires onto the resistor so hard it was loose in the fiberglass plate. I pulled the resistor checked resistance (which was good) and expoxied it back together. When I put the resistor pack back in and pushed the wires on my glue didn't hold but I managed to get the wires on the terminals and everything works!

I will get a new resistor but in due time.

I also noticed that the a/c compressor has been staying on even when the controls are off. I pulled the compressor clutch relay off the fender, cracked it open and cleaned the contacts. hopefully that'll work otherwise I'll need that too.

Thanks for all the help. It's a pretty simple setup but far more convoluted than my '84 and '85 cutlass's were. Oh well. Guess it's better to over engineer the system with relays than have switch contacts burn out.

oldspackrat

The '70 AC compressor clutch hold in relay is working the way it is supposed to. If you turn the AC on, the clutch stays engaged till you shut the car off. Stupid right??? They changed it for '71 forward to where it(compressor) can be switched off & on by the switch in the dash.

96powerstroke

I actually just stumbled uponthis in another post. I was hoping the member who wrote the reply was incorrect but obviously not! Very stupid IMHO. Won't this freeze the evap core if the blower isn't passing air through it? It also kills fuel mileage. Half the time I don't use the a/c because of that (i also like the windows down).

I think I have a simple (and reversible) solution though. If I add in a normally closed momentary switch I can run that to the power side of the a/c relay. When I shut that a/c - hit the switch and it interrupts the power to the relay thus disengaging the compressor. I don't plan to splice wires but use the factory connectors. I'm adamant about keeping this as stock as possible but also need to update some things that just aren't right.

Do you know why the relay stays latched? Maybe there is an easier fix that adding a switch? The switch idea will work but then I have to remember to push the button every time I turn the a/c off. That's going to drive me nuts. Oh well small price to pay I guess for an a/c car of this vintage!

rocketraider

Not sure on A-body since the compressor hold-in relay is more out in the open and visible, but I bypassed the thing on my 69 Toronado and everything is hidden underneath the relay and A/C hoses. National Meet judges never caught it...


The only benefit I ever saw to this design was that, by selecting an A/C position, the compressor would lock in and would then operate in heat and defrost positions too. So you'd get dehumidified air to clear fog from windows. On later design control heads they figured how to turn the compressor on in defrost separately, which, besides the dehumidified air, gave the added benefit of running the compressor occasionally during winter and keeping seals etc in good shape.


To my knowledge no other GM Division used the hold-in relay, and Olds used it only 1968-70.


Hey, at least you don't have a Comfortron car where the compressor NEVER shuts off. Can't tell you how many Comfortron cars I've seen with a toggle switch underdash to control the compressor.

96powerstroke

How did you bypass the relay? If the factory relay is a latching type I should just be able to swap the factory one for a bosh style 30 amp relay right? I've got handfuls of them floating around. Otherwise what keeps the relay engaged?

I do like the idea of being able to kick the a/c on for defrost. Guess the GM engineers didn't quite think that one through until later. Maybe I'll add a separate switch to engage the compressor I can only see leaving the compressor on while driving being detrimental to the health of the a/c system unless it's actually being used!

Thanks