1980s DIY blower control module?

[TJ Hopland]

Its looking like blower control modules for the 80's (and into the 90's) are even harder to find and more expensive than ever if you can find one.  Even if you do pay up and find one it may die because its crap or the replacement blower is overloading it.

With how common and cheap electronic speed controls are now for diy projects or maybe even could be harvested from dead cordless power tools? I am wondering if it would be reasonable to make one?  Even if its not variable like the original something like the 70's that used resistors could be rigged up with relays. 

Anyone know what the signal form the head unit is out to the module?  I would guess what ever it is it would not be hard to program an Arduino or Rpi Pico to read that and drive either a speed control or staging relays. 

[James Landi]

I can't speak with any certainty about the more modern ones, but the early ones were simply "bread toaster" ballast resistors that were set up in the air flow plenum to keep them cool.  I've performed "work arounds" that will provide full fan speed operation by simply bridging these connections to supply full speed fan function. I do agree with you that Chinese 12v dc control circuit boards available on Amazon might provide a suitable control for the fan motor, so you'd have to consider that the blower motor, on start up, pulls a lot of dc amperage, and that any electronic variable electronic controller would have to manage that initial start from dead stop with a suitable margin for a blower motor that is dragging due to age of fan blower cage obstructions.

[TJ Hopland]

When the control panels went digital in 80/81 the blower control also went to an electronic module.  I'm not sure what the actual device is/was.  Its a black box living under epoxy potting mounted to a heat sync and just like earlier designs its mounted in the airflow for cooling.  I believe into the 90's they more or less used the same design.

The 70's was basically the same as the manual systems where there were resistors that were either linked or bypassed depending on the switch position.  The switch was just remotely operated by a vacuum operated servo.  This could be done electronically using several relays and a resistor block from a manual car. 

I have not yet found any info that says what the control signal was.  I'm guessing its something fairly simple and its a single wire.

[bcroe]

The elegant solution is to use a BUCK DC-DC converter
to reduce motor voltage, and not generate any heat in
the process.  There are lots of those on Evil bay, etc. 
The devil is in the details. 

The first problem is protecting the buck from all the
supply and motor transients that like to destroy
electronics, recently discussed here.  Then, the
original speed control signal must be converted to
control the buck output voltage.  good luck, Bruce Roe

[smokuspollutus]

Many of these era cars have a lot of empty real estate on the dash that could be rigged up with a dimmer like wheel to provide for infinitely variable fan speed using a device like what Bruce describes. Probably wouldn't look too bad.

I had a screwy module that wouldn't keep a steady connection on the clutch drive circuit ie: hitting bumps would cycle the ac compressor off. As the fan control side of the module still worked fine, along with the rest of the climate control systems, I was able to rig up a simple Bosch cube relay to drive the compressor circuit reliably without any ill effects.

A good effort would be to find out what makes the magic happen inside the module and a good writeup here on the forum describing the "rebuild" process.

[TJ Hopland]

The more I think about it rigging up something that uses the manual style resistor with relays would likely be the way to go.  The little bit I was looking at speed controls they start to cost more in the amp range I think we would need for this and many are not clear if they are continuous ratings plus I have not yet found a lot of info on how well they work in electrically noisy and unstable environments like a car.  Resistor and relays are stable and should be easy to service in the future if there are issues.   

I just have to get my hands on a car and hook up a scope and see what that 'blower drive' signal is.  Once I know that I don't think it will be difficult to program something to read that and step through the relays.  I'm really not that good at that stuff yet but this is a pretty simple application.

["Cadillac Kid" Greg Surfas 15364]

Check the FSM on the individual car for the resistance in the circuitry for the various speeds, calculate the maximum wattage and buy a variable resistor (rheostat) to control the fans speed with a dial
Greg Surfas

[cadillactim]

The control head ran off of 8 volts, powered by the programmer. Most of the outputs controlled ground circuits. The blower module received power separate from the control, which I believe was battery voltage.

That being said the blower speed was likely a difference of potential, maybe varying the ground (I am not sure about that though).

Most likely a transistor is what fails in the module. Excessive current draws from old blower motors fried many of the modules.

But remember the blower module also controls the ac compressor. Sometimes just the blower circuit goes bad, sometimes the compressor circuit goes bad, and sometimes both. And I don't think you can just put a separate switch for the compressor since other components are tied in to the compressor circuit.

It's probably just too difficult to modify the system through the controller. The 80s electronics weren't very good quality.

Tim

[TJ Hopland]

A rheostat that can handle that sort of power won't be common or cheap.  Would also be fairly large like close to the size of the whole blower so cooling and mounting would be a project in itself. Electronics replaced high power rheostats in the 70's for most applications.

For a resistor I took a quick look at what fits full size Chevs and saw some that fit many GM's from the 70 & 80's.  They are under $10 and since they fit so many models should not be difficult to find.  These resistors are smaller than the modules so it would be easy to just make an adapter place to mount them in the same spot so they will also get the cooling from the air flow.  Its the same spot they are in in the cars that use them. 

I will have to pull up a non Cadillac manual and figure out how many relays will be needed to get the speeds and I will have to remember to include the compressor clutch one.  You can buy blocks with 2-8 relays and sockets on them make for automotive / marine / rv use so mounting under hood should not be an issue.  I have bypassed the clutch part of the modules before with a relay so I know that part is that simple. 

The car that has suddenly kicked this project into gear isn't working right and I may be getting it tomorrow.  Hopefully I will be able to probe the control signal and that part will be clear and working.  Back when I was working on these cars it always seemed to be the module that was failing and the controls were fine. Hopefully this is still the case at this age. 

I have found examples for programming a device like a Raspberry Pi Pico to read several types of control signals so as long as I can identify it I don't think I will really have to 'learn' anything new to make it work.  If its just a variable voltage that is really easy.  Say the voltage is 1-3 volts I tell it to engage relay 1 which makes the blower run on low. 3-6 volts I engage relay 1&3 or what ever combo gets me medium speed.  7-9 adds in relay 2 and blower runs on high. 

If I just wanted to fix this one car I would go about it differently but I'm trying to come up with parts and a design that the average DIY car person could do it themselves.  It really takes the fun out of those spring and fall days when you ain't got no heat and paying $300-500 for a module if you can even find one that will likely fail again soon ain't no fun either. With my idea the most likely failure point if you had a blower stressing things would be the resistor which is $10, 1 plug and 2 screws.  If you had to replace one of those per year?  Not the end of the world.

 

[TJ Hopland]

Good news and bad news I guess.  The car that I thought was going to be a good reason to figure this out wasn't as bad as as expected.  Other issues were easier fixes than expected so we didn't have to wait for any parts or spend extra time at my place. 

I started poking around the blower area and the blower kicked in.  Owner said thats how its been for the last 20 years, 99% of the time not working.  Its a summer car so just hasn't been that high on the to do list. Its not quite summer yet but because of an injury he needs a car with huge doors so its going to be his daily driver for a while. 

On these (this happens to be an 84) the module modulates the ground.  The module has a black wire just coming out of the plastic that leads to a grounding point where the cross braces hit the firewall. The 'ground out' is a spade terminal that runs to the blower motor.  The key on plus runs direct to the motor.  There is also a multipin connector that had the AC clutch and control signals in it. 

The ground wire at the blower is a factory looking ring terminal with the little teeth.  At the blower which I suspect is not factory there is nothing.  Other GM's I recall sometimes having a welded on spade tab and other times a spade tab under a mounting hole that isn't painted.  I recall that you could often look close and see where there was tape to protect the area from paint.

Some will guess where this is going....  Ring terminal didn't look too bad but when I hit it with the wire brush tons of white dust came off and it looked way better.  Blower end was slight rust around the hole and intact paint.  The bolt doesn't help because its threading into fiberglass unless it happened to be rubbing on the blower flange which I suspect was how it may have been working when/if it ever did.  Little sandpaper and 2 days and counting its been working.