Turn off fan over, say, 30 mph

[Michael]

I am planning a circuit to cut out the cooling fan when the road speed is greater than, say, 30 mph - for the sake of argument. I have a suitable digital signal from a vehicle speed sensor. With my set up this gives about 7300 pulses per mile.
I have MS1 V3 board - not yet installed in vehicle but all checks out well on simulator.
Has anyone done this through MS software? I don't see a suitable digital input although I could envisage using one of the analogue inputs at a stretch.
Alternatively I'll make up a board with an LM331 Frequency to Voltage converter followed by a voltage comparator which will quite easily do the job. I can set the V to F range and the comparator set point with trip pots to get the cut off point I need.

Any comments anyone?

Thanks - Michael.

[MonzaRacer]

This is not a good idea. If you would run into a low coolant issue or air flow blocked(plastic sack over grill) without your knowledge then you would over heat .
Honestly set up your cooling fans by temp not from the computer. this way the ONLY time they are on is when the ENGINE needs them.
Trust me as a a tuner and professional master tech this is much safer and MUCH simpler.
Only reason manufacturers do it is THEY have to use fans to control engine operations on IM240 testing as a dip can cause an odd spike on rollers.
Your plans are not best idea. Keep the fans controlled by temps NOT speed. Trust me your engine WILL survive much better and they are not gonna run that much EXCEPT when the engine NEEDS them.

[Michael]

Good advice Monzaracer - its still an idea at present - so I will seriously keep your comments in mind. Thanks - Michael

[ol boy]

You could use a separate processor to handle this function unless you have enough IO pins on the MS1. It could measure either pulses or an analog value. You can drive a relay placed before the relay that drives the fan or place it on the ground leg of the fan. I like this idea. Would be useful for the drag strip too.

If speed <= 30 then
high fan_relay
else
low fan_relay
endif

[Michael]

Similar to my line of thought, only I wouldn't use a processor. I would use a quite simple circuit to take pulses from the Vehicle Speed sensor: feed the pulses into a Frequency to Voltage (V/F) converter using, such as a 331 chip, use the voltage output to trigger a comparator which could be almost any old op-amp ; 741, 3130, 3140, etc. This would drive a small relay to interrupt the supply (or divert to a different, high speed fan) to the main fan relay when the vehicle speed exceeds a set speed. The whole circuit would be built on a piece of circuit board (veroboard) to fit in a spare set of slots in the Megasquirt case extrusion. This would keep all the control circuitry contained in one box. I've already included a wire from the VSS to an unused pin on the Megasquirt DB37 to save me having to loom it in later.

My reasoning is that above about 30mph the cooling fan contributes nothing to cooling as the airflow through the radiator at or above that speed is greater than anything the fan could supply so keeping the fan on above about 30mph is a waste of resources. Most current manufacturers follow this principle. Could even envisage the possibility that the fan being engaged above, say 30mph, could actually disrupt the incident airflow and adversely affect cooling, but I haven't really thought that through and it should probably be the subject of another thread.

[ol boy]

I believe a lm1815 already has the capabilities to do what your looking for all in one chip. All the min and max ranges are set with a few resistors and caps.

[Michael]

Had a quick look at the datasheet for LM1815, but don't quite see how it will work in this application. I can see its function for pulse conditioning as indeed used in MS, but it only gives a one-shot output rather than a high or low transition above a given frequency. However, it might just be me - you know how it is when you've worked something out for yourself that you understand, then someone says, "how about XX?". I'll spend some more time on it later this evening. The idea of a single chip approach is good.

[ol boy]

I had my numbers mixed up.

LM2907/2917 is a frequency to voltage converter with features like automatic door lock threshold type of stuff(speed switch). Looks like the output sink current is max at 50ma. Won't drive the field coil of a relay directly.


**edit** if using 2 of this chip it can do traction control feed back.

[Michael]

Yes - Thanks spot on! a very useful and versatile chip and cheap too! I'll get a couple, knock up a circuit and let you know how I get on. Thanks - Michael.

[ol boy]

It looks like it can also do basic VR to digital conversion also.

yes, let me know how well it all works out.

[prof315]

Just so you know MS3 has this feature (turn cooling fans off above a user selectable speed). It also does vehicle speed and with the correct user input, gear detection.

[Michael]

Good to know Prof315, but I'm only running MS1 on V3 board. However, I think that the 2907/2917 suggestion provides the solution for this modification. - Michael