automotive electric fan controll

I am installing four twelve inch electric fans on my son’s truck. Wondering how difficult it would be to controll these so that two fans will come on at 165 deg F and the other two at 175 deg F all four shutting off at 150 deg F.
I am hopeing to do this somehow by paralleling into the existing temp sending unit and useing it’s linear voltage. I think I will also need some sort of on delay timer so that if at startup the temp is above setpoint the fans will not come on untill after the engine is running. Any advise or input would be greatly appreaciated. Thanks. Glen

If you’re asking if there’s a commercial-off-the-shelf unit that will do the job, the answer is: I don’t know. But this would be a fun design project for a junior electronics hobbyist. The circuit could be entirely analog (using comparators, op-amps, etc.) or digital (using a microcontroller). If you don’t need it right away, and you don’t feel you have the skills to design it, perhaps there’s a technical school in your town that might consider taking it on as a student project.

Couldn’t you just install two temperature units on the engine, setting one to come on at 160° and the other at 175°? Then connect the separate fans to each one so they come on at the appropriate temp.

Seems like just installing 2 standard temp units in the engine would be easier than trying to get a special purpose electronic circuit designed & built.

Thanks for the input. I may be able to design it myself as I work in the field of instrumentation. But mostly maintenance and troubleshooting not so much design.
I think I’m going to give it a shot.

As for just useing temp switches I really don’t have locations to install them as I would like to maintain operation of the temp gauge.

Any how- to info would help so all you electronic whizzes feel free to send any information.

Some information on the temperature sensor/transducer would be helpful. It’s probably a thermistor, but it could also be a thermocouple. If you post the vehicle make, model and year, someone can probably find some information on the sensor. Once we know its response characteristics, we can design something around it.

You can find power in the fuse box that is:
Key Off = no power
Key On = 12v +
Key On Start = no power

So get your main power to the relay from such a source.

When does the thermostat open? Seems like you got the fans coming on early. Do you have a way figured out to turn the fans off at speeds over say about 40 MPH? No need to keep all then fans on when the truck’s speed will force the air just fine.

I had a chip that turned the fan on right at the recommended thermostat temp - I thought it was not the best idea as it kept the fan on all the time, except fro the few minutes of warm up. Just off the top of my head I would add at least 15* to the thermostat. I do not see a reason to have the fan start at the engines minimum operating temp. - IMO

I know there are fan switches that screw into a water passage and are open until a certain temp then go to ground - this might be easier and safer than taping into an electric sending unit - electric gauges seem very sensitive - don’t know what effect taping in for a multi fan multi start temp set up will have on the gauge.

Any ideas for a delay relay to turn fans on after shut down if temps hit a certain range?

Good luck

What kind of truck is this? 4 12" fans seems very impractical. Also, if you want the engine to runs it’s best, it needs to atleast get somewhat hot. Turning the Primary fans on at 165ºF wouldn’t acheive much. If it’s a small truck, S10, or 2500 type truck. Install 2 12" fans, with the primary at 185ºF, and the secondary at 200ºF. 180ºF is considered an acceptable temp for getting the engine hot, it runs better this way, and is good for emissions. In newer cars, stock cooling fans are programmed to activate at 220ºF, some at 230ºF.

But anyway, I think TCI makes preset/adjustable thermostat fan switches.

Curious though, what kind of truck is, and what is it being used for? (Race, street, daily driver?) The 4 fans thing just seems unnecessary and inefficient for any of those types of applications.

This truck is a 1969 ford f-100 with a highly modified 351-c with the timing set at 20 btc static , a 160 thermostat. and sitting at idle the current radiator and fan setup do not cool enough I have ordered the four 12" fans but may not need all of them. Anything in excess of about 185 and the engine deisels when shut off.

It’s just the stock temp sending unit for the 1969 ford f-100. I beleive it just varies resistance to ground. I actually need to measure voltages from it at different temps to get a better idea of what I’ve got. I will try to do that today and let you know.

If it’s that old, who knows then? Is there an actual temperature gauge in the dashboard, or just an idiot light that comes on if the temp is too high?

There is an actual factory temp guage.

OK I took some reads and here’s what I got. This is the resistance to ground read, the voltage is also dropping as the temp rises but its so irratic I can’t get a good read
Im useing an external thermometer so the reads may not be real accurate.
90 deg 35 ohms
132 deg 24.6 ohms
150 deg 22.1 ohms
165 deg 20.6 ohms
170 deg 20.2 ohms
185 deg 17.7 ohms

That’s not right, for a thermistor. Are you reading the resistance across the sensor with the wire pulled off? A thermistor’s resistance increases with increasing temperature. It could be a thermocouple, in which case a voltage will be created across the sensor which is proportional to the temperature. In this case, resistance measurements are useless–you’d need either a microvolt meter or a voltage amplifier with a regular voltmeter reading the putput.

Just wondering what kind of radiator you’re running in that truck. The fans may not help much if you’re running a stock radiator. I’d just run one fan myself. Why do you think you need to have such a complicated set up?

Q.E.D.: FYI, almost all thermistors have a negative temperature coefficient, which means their resistance decreases as temperature increases. PTC thermistors are available, but are much less common that NTC.

But having said that, I don’t think it’s a thermistor. Thermistors usually have higher resistances, like in the 1K to 50K range.

I’m certainly not an expert on these things, but I believe most automotive temperature sending units are just a 2-wire potentiometer. As the temperature changes, a (temperature sensitive) bimetallic contact moves the pot’s wiper contact. It’s configured such that, as the temperature increases, the resistance decreases. If we excite the potentiometer with a constant voltage source (e.g. 5 V), then the current will increase with increasing temperature. And the coolant temperature gage (which is in series with the sensing unit) is nothing more than an ammeter.

I’m not sure if temperature sending units are still designed this way. If it were me, I’d design a sending unit using a thermistor or solid-state sensor. Perhaps that’s what they’re doing today? I dunno.

try killing some of that initial timing
what is your total advance and compression ratio?
what type of fuel are you using?

It does that at idle? Something isn’t right… I’ve seen highly modified engines in dirt circle tracks go with mechanical fans, and some even without, and are running fine at idle comming into the pits (these cars run HARD). Don’t mean to criticize, but if you need that many fans at idle, look into what Blown & Injected mentioned to see what you can find.

Hypertech makes a fan switch (Part No. 4026). Available through Summit Racing (800) 230-3030 for about $45.

Replaces the factory switch.
Fan turn-on at 176°, off at 166°.
Hypertech makes a fan switch (Part No. 4028). Available through Summit Racing (800) 230-3030 for about $45.

Replaces factory switch.
Fan turn-on at 200°, off at 185°.
Hayden makes a fan turn-on relay (part no. 3652). Available at Pep Boys for about $25.

Thermal switch mounted in radiator.
Fan turn-on at 185° and off at 170°.
Can be wired to augment factory switch, not replace it.
Hayden makes an adjustable fan turn-on relay (Part No. 3647). Available at Pep Boys for about $39.

Adjustable turn-on from 160° to 210°.
Can be turned on automatically when AC is engaged.
Can be wired to augment factory switch, not replace it.
Can be manually turned on with a dash-mounted switch.
Derale makes an adjustable relay (Part No. 16759). Available through Jegs (800) 345-4545 for about $42.

Adjustable turn-on from 140° to 260°.
Adjustable turn-off from 2° to 20° below turn-on temperature.
Can be turned on automatically when A/C is engaged.
Can be wired to augment factory switch, not replace it.
Can be manually turned on with a dash-mounted switch. LED indicator.
If running when ignition is turned off, the fan continues to run until turn-off temperature is reached.