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automotive thermostate

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Hello all

I have just joined you forum and i am looking for a little help

I need a circuit that can be used to control an electric (12v) cooling fan in a truck.

I have seen several that use a bulb in the radiator and a mico switch but that is not what i am looking for.

I would like to construct on that can be hooked into the temp sensor on the motor, give a digital readout of the temp. and be able to set the temp at wich the fan comes on.

Is this posible or is it 2 much work.

For some reason we seem to be saying no more often then we used to.

I am not an analog guy but I think one could do a lot better then $50+S&H for this. But the cost is not the issue but rather a chance to learn.

It can be done with a $2 uC a temperature sensor, relay, and pot.

Seems to me you could do the same with 2 comparators, a transistor or two, and a wee bit of logic.

The question is, Is V8Ranger ready to learn what he needs to know and to do the work needed to build it.
For some reason we seem to be saying no more often then we used to.

I suspect its due to the safety Nazi posters that jump all over anything that could possibly be harmful or dangerous if common sense is not used.

If it uses line voltage it gets run down. If it could damage or destroy property it gets run down.
In this case I suspect the unlikely possibility that tapping into the temp sensor could burn out the ECC unit will cause someone will jump all over that! Or it could pick up some imagined 100 volt power spike from the starter or something else that will get this automotive thread run down just like any other automotive application circuit. :mad:

Just an observed trend.:mad:

Sounds like you have an existing "sender". Do you have a gauge to go with it? Would you be satisfied in keeping the analog gauge?

Can you determine what the cold/hot resistance is of the sender? Better yet, do you have a resistance vs temperature plot of the sender?

Would you be willing to pull the sender and determine its resistance vs temperature by heating the sender and measuring the resistance at several temperatures in heated oil vs a candy thermometer?

Could you replace the existing sender with a new one for which you can obtain the R vs T curve?
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SO what kind of truck is it?
And why doesn't the factory fan do its job well enough any more?
We will need a reference point to work form. Every manufacture uses a different method of operating sensors. Most of their sensor output information is available on line if you know where to look.
Without it we cant even begin to guess what it uses.

You may get lucky but being your name is 'V8ranger' and if that means your truck is a V8 ford Ranger, your going to be having some trouble reading anything from the sensor that drives the instrument panel gauge.

I have Fords, I know! I think Jimmy the retard designed Fords sensor drive system. Its horrible to work with. Its not voltage or PWM regulated its just a random on/off current/voltage limiting from a bi metal thermal switch/regulator/current limiter.

You will end up having to add a second sensor with a dedicated drive system and display. I have a 1985 Ford pickup I put a 460 big block in and an E4OD electronic transmission in. The transmission controller needed to know the engine temp. I ended up building a dedicated senor drive circuit just because there was no reliable way to filter and read the existing mess the stock gauge used.
A thermistor is a component which has a roughly linear resistance change (or at least predictable) response to variations in temperature.

A fixed current flowing through the thermistor will thus be altered depending on the ambient temperature. What I suggest you do is find a simple low cost microcontroller with built in ADC (analog to digital converter) ports. A simple external circuit (probably described in the microcontroller's datasheet) will allow you to digitally make measurements of the current through programming instructions. The resolution will be dependant on the bit depth of the ADC, but typical values in simple microcontrollers may be 8, 10, or 12 bits.

Then, regardless of whether or not you have a datasheet on the thermistor, you can make a chart of the digital number associated with a wide variety of test temperatures and ultimately form an equation which models the temperature.

Something such as the pseudocode ...
unsigned char measurement = read_ADC;
temperature = convertToCelsius(measurement);
... can be used in the program to easily allow manipulation of the data.

The a simple while loop can constantly check for new temperatures and respond via if and then control statements depending on preprogrammed thresholds.

You can simply end up making a device which triggers a relay if the temperature is greater than x but less than y, for example.

The relay can control the fan to begin the thermostatic process.


Jesse Randall
Electronic Engineer
**broken link removed**
dougy83 -------- thanks for the links but they are not what i am looking for, atleast not the first few...

3v0 ------- the reason that i am looking here is to do the work and try and build this circuit myself and make it do WHAT I WANT IT TO.

TCMtech -------- i agree..... I almost hate reading and posting because of the people that do nothing more than take up space and wast time.

Mike-----I do have an analog gauge. I can replace the sender if needed or I can install another but I need to take a look and see if i have a location for it.

Tcm---------A little history ........The truck is a 94 ford ranger which had a 4.0l. I have since removed the V6 and install a 331 stroker motor and the electronic trans on the computer from the full size ford truck. I gutted the wiring system in the ranger and combined with the full size ford trucks system. I am running 10 inches of lift 35 inch tires and axles fom a 60's bronco.

I am not new to electronics or wires or anything mechanical But i would like to try and find the best way to control my electric cooling fans.

I dont think that the sending unit is just a switch beause the gauge moves and it is not just on or off like an idiot light.

I will probably need to ad one like you said because if i remember it was tied into mytrany as you stated.

Maybe i should be looking at a circuit with a small processor that way i could bring the 3 fans in at diferent temp settings. just a thought.

Thanks for the help
Mike-----I do have an analog gauge. I can replace the sender if needed or I can install another but I need to take a look and see if i have a location for it.

How about the R vs T curve?

One idea. Leave the existing gauge and sender, and then record the voltage across the sender vs temperature. Feed that signal to a high-z input op-amp and maybe an ADC. This leaves the gauge intact, and you can build some external electronics to control your fan(s).
There is a problem with using the R/T curve on ford temp sensors. They are not linear. Its almost impossible to get a reasonable R/T voltage reference from them.
I believe the 1994 vehicles still used that lame ass retard design from decades ago.
For my transmission controller I ended up taking a ford temp sensor and drilling its guts out and epoxied a regular 10K NTC inside it so I could get a reasonable sensing reference out of it.
I ended up with two sensors on the engine then. One for the stock gauge and one for the transmission controller.
I think V8ranger will probably have to do something similar as well.
There is a problem with using the R/T curve on ford temp sensors. They are not linear. Its almost impossible to get a reasonable R/T voltage reference from them.

Is the problem lack of linearity, or is it accuracy (repeatability). I have dealt with the non-linearity issue. First, it doesn't matter how non-linear the function is, as long as it is repeatable. If all he wants to do is trip a comparator when the voltage reaches a preset voltage corresponding to an indicated temperature on the gauge (e.g. to turn on a fan), then it doesn't matter how non-linear the function is. (unless the function is not monotonic).

Second, if you go to the trouble of reading the voltage into a µcontroller via an ADC, then you can curve fit it and linearize it using math. I have done this by first recording the voltage (across the sender while being biased by the gauge) vs temperature, putting the voltages and temps into Excel, and then using the curve fitting functions to find a polynomial or exponential or power law to fit the experimental data. Record the coefficients, and then implement the function in the software.
When you say you "gutted the wiring" what does that mean?

Are you using a Ford ECU for fuel injection on the new motor? If so, then why not ignore the cooling fan temp sensor/switch and use the coolant sensor the ECU uses?

The only thing you need to be careful of is making sure whatever circuit you use to read the voltage from it has a high-impedance input so it won't place any load on the voltage to change the readings the ECU sees.

I'm doing something similar, except that I want to use a PWM controller for the fan so it changes speeds in relation to temperature instead of simply being on/off (electric fans can be noisy at full sped).

The noise in this case is not an issue since i will not hear the fan over the motor.

What dod you use to epoxy the the sensor in? does it take a long time to react to the temp changes.

Can i just get a sender from another auto and use that.

I have no problem using one from another car or truck i just want it to be reliable. i have a good penny into this engine and do not want to be over heating it. I am usually watching the trail and not the temp gauge.

tks all
I just used JB weld as the epoxy. Cheap and simple! I dont see any problem with just using a second stock unit as MikeMI mentioned about just using it for a reference signal of sorts.
My transmission controller is PCS unit and I needed an accurate voltage source from 0 5 volts for the full -30 to +240 range. The stock sensors are only linear near the normal temp range but way off below 150 degrees. That created a program problem for me being in a location that can get very cold.

If you spent a load of money on the truck already I would check out one of the PCS units. They beat the stock computer on every level! For off road you would love the versatility and adaptability too! two independent shifting programs from the flip of a switch, fully adjustable shift firmness profiles from a simple pot, torque converter lock and unlock in all gears giving you an 8 point shift feel! All up and down shift points are fully mappable too!

I dont recall but there are extra input channels and output channels available in the program an one may be user selectable to turn a relay on and off in relation to transmission or engine temperature, I think. I set up my pickup about 4 years ago and I dont exactly remember what all it can do now.

I would recommend checking them out! there software is free to download too so you can play with it before you decide to buy. Its what hooked me! Powertrain Control Solutions is the company.
A few thoughts or comments:
A. As a backup or fail-safe feature, you might want to consider adding a mechanical switch to bring the cooling on. You said you have a lot of money in the engine - it might be good insurance.

B. A variation of the thoughts in 'A' - to the extent that you can, design whatever you design so that it fails in a safe mode. You might think of it as designing your controller to keep the fan off so that if it fails to do what it's supposed to do the fan comes on.

C. Is there a situation where you might want to bring the fan on before the coolant temperature rises? If there is, how can you sense that situation? I was thinking of conditions where the engine is heavily loaded yet the vehicle ground speed is low (meaning no air forced thru front of vehicle). What I am thinking of is that there is thermal mass to the engine and coolant and there may be times when getting ahead, so to speak, provides you with some margin or extra capacity.

D. Something I had wondered about is evaporative cooling. Air conditioning technicians will often have a water spray directed on an air conditioning condenser coil as a way of getting some extra capacity. If there's a case where the air flow and radiator can't keep up (maybe on a really hot day?) then a reservoir and small pump might provide a boost for as long as the water holds out.

E. My experience with sensors is limited though I do recall attempting to measure the value of the sensor. Fortunately my manual said to test the sensor on the "diode" range rather than the normal ohms range - what I recall might be incorrect but I thought it was because the current in the "diode" range was far less than in the ohms range.

Good luck. Sounds like a good project.
TCMTech --- I had a unit like the PCM ( and its for sale ) that i was runing but I curently controlling all of my fuel injection and the trans from a factory ford computer.

steve --- thats just what i need. More ideas lol I like the toughts on a fail safe and also being able to just turn the fans on manually. I will be keeping that in mind.
Here is some info that may help

I am attempting to switch 2 thermo fans from an AU Falcon on the radiator for a 351 Cleveland in a 1982 XE Falcon. I got a few Ford ECT's with connectors from EEC IV setups from a wrecker today that screw directly into the top of the thermostat housing.

Here is some data on a suitable ECT (Engine Coolant Sensor)
Ford Fuel Injection Engine Coolant Temperature Sensor (ECT)

They have a logarithmic RT (Resistance Temperature) curve. there is some info **broken link removed** on calculating this.

I am going to work this info out and post a reply.

i found **broken link removed** on where to get one although you would be better off going to a car wrecker to get a second hand one with the connector. they are easy to get (they are used on almost all fords with fuel injection)
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