air speed fan cutoff

Guest · Jul 27, 2002

When was in south Florida not long ago, I had plenty of time to stop by Micro Pnuematic Logic which was right on the way to Lauderdale airport. They wouldn't sell me a pressure sensor switch since they sell to OEM concerns only, but they did send me a sample. There are several articles on using this kind of sensor switch on www.autospeed.com: If you go to http://www.autospeed.com/cgi-bin/browse.cgi?category=706& there is a pressure switch description at the bottom of the page (their new one, not the one I got in Pompano that they were using and will be shown in the articles) and a series of links to several articles on projects they did using it. There are any number of uses for this kind of device beyond shutting off the radiator fans when the car reaches an adjustable speed. I'll let you know how it goes.

Bob Putnam · Jul 31, 2002

Lynn,

An interesting piece. Jeez - now I'm going to have to go back into some 30-year-old college textbooks to figure air velocity vs static pressure...

CCX33911 · Jul 31, 2002

Good idea. Something that would also work is a pickup off the speedometer or wheel speed sensor. I have been instrumenting my Cobra with a ton of sensor including a drive shaft speed (thus vehicle) sensor. It goes into a data logger and can be set to trigger an output of the logger (to turn on/off the fan) based on drive shaft speed.

[ July 31, 2002: Message edited by: CCX33911 ]

Guest · Jul 31, 2002

Yes, Robert and I will have to pull out my old circuit design books to use the proper capacitor to delay the cutoff from turning back on for 1 minute rather than the three minutes the circuit is designed to provide now. A delay of some sort is needed to keep the sensor from bouncing on/off if the speed varies slightly either side of the set speed.

CCX33911, if I had used a 94 or 95 computer (which has a speed input as does the electric speedometer) this function would have been built into the EEC-IV computer. There are a number of devices and speedometers that allow for adjustable speed triggers. The aspect of this approach is that it is based on airspeed which is the critical factor for radiator cooling. The car may be traveling at greater than 45mph, but, if it is traveling down wind, the airflow through the radiator will not be 45mph; therefore this device measures the most appropriate parameter for this application. It is indeed an electrical pitot tube airspeed sensor.

CCX33911 · Aug 1, 2002

Lynn
Is it possible to give your circuit a "dead band" such that it would turn off at say 45 and then not turn back on until the speed droped below 40 or 35. That would eliminate the problem associated with driving at the turn on/off speed.

Guest · Aug 2, 2002

With the price on these pressure switches, and the fact that they added the delay circuitry, I don't think they are sophisticated enough to provide this function. The delay can be set for as many minutes as desired by changing the capacitor. In the articles online, they used a 3 minute delay to avoid the flutter. Another thing that could be done is set it at a speed not often used as a speed limit like 40mpg. In the US speed limits are most often either 35mph or 45mph.

CCX33911 · Aug 2, 2002

Lynn,
Do you have a copy of the circuit or article discussing it? I know enough about electronics to get into trouble.

CCX33911 · Aug 7, 2002

Sorry Lynn I did not realize that this was a switch (duh! I should read next time) which means that it can not tell you what the pressure is just whether it is above or below the switch point. If we find a pressure sensor with an analog output then I could add a op-amp with hysteresis this would allow for turn off at one speed and turn on at another.

For curiosity I'll see if I run across anything.

Guest · Aug 8, 2002

The autospeed web site has several articles using a pressure or vacuum switch. Check out this URL: Autospeed"]http://www.autospeed.com/cgi-bin/browse.cgi?category=706&]Autospeed Pressure Switch[/URL]
There is a pressure switch description at the bottom of the page and a series of links to articles on projects they did using it.

Fatal Attraction · Aug 12, 2002

IMHO, you guys should be more concerned with
the fan being on/off at a certain temperature, rather than a particular speed. Due to different driving conditions,ie., weather, amount of throttle being used, city vs. highway, etc., use of the fan may be necessary at higher speeds.
What harm is the fan doing, if it is running anyway? A few less AMPS available maybe, but
that's all.
The fan DOES make a difference in engine
temperature at speed. I speak from personal
experience with my GT40. Perhaps yours may be
different, but better safe than sorry.

Bill

MK -IV J6 · Aug 12, 2002

Bill is 400% correct. In the real world you need all the fan you can get. In the summer I never run with them off. 190F goes to 220F
in a heartbeat. When they raced they didn't use fans but they adveraged 135MPH. It's hard to maintain that on the street.
Best
Jim

Guest · Aug 12, 2002

Bill, I have implemented a variable temperature control switch like yours (it was you if I remember correctly.) If I had used a '94 EEC-IV, rather than a '93, this fan cutoff would have been built in but based on the speed signal from the transmission. The fans do draw a fair amount of current which translates to HP from the engine. I always have a manual override if it becomes a problem. If it is a problem, I will take speed switch off. But, I see your point: if forward speed alone will maintain the proper temp, then the fans will cut off anyway. However, what if forward speed maintains the temp at just a few degrees over fan cut on? At this point, I would live with the few degrees and would rather have the fans off.

Bob Putnam · Sep 24, 2002

Lynn,

I've been looking at alternatives to the air-pressure sensor. (I have my doubts about the reliability of running those switches at their minimum range, plus practical placement problems.) I think a simpler system may use a simple speed sensor between the speedo and cable. There are a couple available from Ron Francis for $55-$87. All I need to finish it is someone to translate pulses to a control signal for a relay...

petek · Sep 27, 2002

You might look into Motorola's MPX4115 pressure sensor (comes in all sorts of different packages). One of these days I intend on taking a couple of these, some flexible plastic tubing, and a tiny micro (Atmel or PIC ... depending what I've got on hand) and do some testing with a tiny air dame under the snout of my Montage.

Bob Putnam · Sep 28, 2002

I recieved the pulse generator from Wireworks. In spite of being advertised as fitting "aftermarket speedometers", it has GM threads, not 5/8-18 as I expected. Fortunately, our speedo cable has a junction with the same threads at a junction, but I'll have to run wires from the rear of the car.

Now I just have to get a friend of mine to design the circuitry to drive a relay.

petek · Sep 28, 2002

Bob,

Email me, I'll betcha I can cobble together something pretty quick. Just need to know the pulse rate.

Bob Putnam · Sep 30, 2002

Petek,

Thanks a million! I'll do that.

Mark IV · Oct 2, 2002

True story....

A friends kit Cobra at Pocono some years ago running the SAAC open track suddenly started to overheat. This car had never ran hot before and could idle in 90 degree weather with the fans running and be "cool."

He was running the track with his fans on figuring it would help. What was actually happening was the fan speed was less than the airflow through the radiator at the speeds he was running and the fans were impeading the airflow. Turned off the fans and the temp came down. We assumed that when the airflow exceeded the running speed of the fans the motors became generators and the resistance caused the fan blades to rotate slower than they would have in a "windmilling" mode. This slowed airflow enough to cause higher temps.

Not a real scientific theory but thats what happened.

Rick

Guest · Oct 3, 2002

That makes sense MkIV. The main brakes on an aircraft are the propellers. (Next is the side of the aircraft which is used by slipping the airplane side ways - kind of a induced powerslide.) When the engine is idled back the propellers (unfeathered) have the same effect as two plywood discs with radii only slightly smaller than that of the propellers. This becomes very important in energy management with heavy twins which have a much higher ratio of inertia to parasitic drag that smaller single engine aircraft.

Bob, if you look at the article I referenced (I hope it is still there) they use a transistor to step up the very low current from the switch and then use the output from the transistor to activate a relay. This relay isn't quite up to automotive currents, so that relay triggers a standard 30amp automotive relay. Pete, one of the projects on the website was choosing a location for the external intake of a car's induction system. Like you they were looking for the highest pressure area.

[ October 03, 2002: Message edited by: Lynn Larsen ]

petek · Oct 6, 2002

Yup, kinda. Frankly I just want to instrument up a bunch of stuff and see what's going on (learning every minute of the day

Now that my Cobra is done (not finished mind you, but done) I should have some more time to play around with electronics and tiny computers.

air speed fan cutoff

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