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please help me with a circuit!

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Reloadron

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Have you done this part yet?

The first step in the installation is to back-probe the connections to the coolant temp sensor, either at the ECU or near the sensor itself. (On the Maxima we did it near the sensor because it was quicker and easier to find the right wire without having a dedicated workshop manual). As mentioned, you’re looking for a voltage (normally between 0-5V) that decreases as the car warms up. So you can easily see this change, start off the measuring process with the engine cold. Here the voltage shown on the meter is 2.369V – it was falling rapidly as the car warmed up.

Before doing much of anything you need to see what you have for a temperature sensor signal. Once that is done a comparator circuit can be designed including hysteresis. However, what you have needs to be known as well as how the fan(s) will be turned on.

Ron
 

fatdaddy

New Member
thanks.
So if I locate the temp sender and let the engine go from cold to normal what do I check for. I have a basic Multimeter.
I was thinking of using relay to switch the fan. A switching relay rather than a on/off so that the normal state can be going through the circuit I am asking about and the switched state can be to switch the fan on manually.
 

fatdaddy

New Member
further to that.................... that relay wouldn't work quite like that thinking about it, but still, the circuit I am asking about would trigger a relay that would power the fan.
 

Reloadron

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Most Helpful Member
thanks.
So if I locate the temp sender and let the engine go from cold to normal what do I check for. I have a basic Multimeter.
I was thinking of using relay to switch the fan. A switching relay rather than a on/off so that the normal state can be going through the circuit I am asking about and the switched state can be to switch the fan on manually.

You would look at the voltage at the sensor as they show in the link. You will likely see a constant 5 volts and another leg will be a voltage that varies. You want to note what happens as the engine warms up on the voltage that varies. like at what point does the fan turn on. That voltage is the signal sent to the ECU telling it what the engine coolant temperature is.

Now if you just want to be able to manually turn the fan on, that is another story. As the engine gets hot the ECU is monitoring the temperature. When the engine reaches a preset temperature based on the signal from the temp sensor it sends a signal to the fan relay turning on the fan. So waht you want to do is find the existing fan relay and see what the computer (ECU) is sending to the fan relay coil. Generally this is just 12 volts but you want to make sure. If for example the ECU sends 12 volts then a switch could be used to send 12 volts to the fan relay and bypass the ECU. The ECU could also be placing the fan relay coil at Ground depending on the system.

Ron
 

fatdaddy

New Member
I think I may have confused you now....................:eek:

Going back a bit further.
As standard the car has a viscous fan coupling. This has failed.
I want to replace it with an electric fan. Having seen the Simple Voltage Switch that seems like a much better way of controlling the fan than a standard capillary unit. So I want to use that switch to control the fan under normal use but I would also like the option of switching it on manually just as a safety thing.

I should have noticed the voltage instructions.

I am proficient enough to handle soldering etc. but cannot come up with the circuit design or components.
 

Reloadron

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Most Helpful Member
Now I get it, the existing engine does not quite have an electrical thermostat controlled fan but uses a capillary clutch type system. There are no existing electric fans and the existing fan is driven mechanically by the engine via the clutch. That is what you have correct?

Now if that is the case what you want to do can be done but will take a little effort. We will need to come up with a temperature sensor to sense the engine temperature. That is what the article was about, measuring the output of an existing engine temp sensor. The sensor would drive a temperature controller, those circuits are not that difficult but finding a good sensor may be a problem. The control is built around the sensor. Then a simple bypass switch is added so the fan can be manually turned on anytime.

What we need is a UK type motor head person in this thread. :) We need to figure out what would be a good sensor choice for the car and engine you have.

Ron
 

ronv

Well-Known Member
Most Helpful Member
I think this is similar to a problem I had once where my ECU wouldn't turn on the fan. I built a little gadget with a thermistor that "stuck" into the radiator. The circuit was just a simple comparator and transistor that turned on the fan relay. I could adjust the temperature at which it turned on so I watched the temperature gage until it got to 170 and turned the fans on at that point. Is that kind of what you want to do?
 

Reloadron

Well-Known Member
Most Helpful Member
I'd rather do it with the sensor voltage than a Thermistor if I can....................

That would be fine if you have a temperature sensor that outputs a voltage. I thought you had a mechanical capillary tube sensor?

Ron
 

fatdaddy

New Member
no, by "standard" I meant a "normal" type capillary, not that I have a capillary sensor :)
Standard on my car is a viscous coupled mecanical fan. Elec sensor is present for the ECU
 

Reloadron

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Most Helpful Member
OK, the electrical sensor should have an output it sends to the ECU. You need to take a look at the output of that sensor under cold and hot engine conditions. There should be a changing voltage there.

Ron
 

fatdaddy

New Member
from a search I get
136deg C = 0.29 volts
-11deg C = 4.96 volts

I have located the wire colour for coolant temp feed to the ECU so I will physically check it tomorrow (it's dark now)
 

Reloadron

Well-Known Member
Most Helpful Member
Good start, let's see what you get tomorrow. Nice span in there between -11C and 136C.

Ron
 

fatdaddy

New Member
I have a dual temp sensor (one for ECU, one for gauge). I have monitored the gauge wire.
I monitored the temp from an OBD device.
Having been parked overnight (ambient 9c) voltage was 3.89
normal running temp (99-106c tho the gauge shows no visual change) voltage 0.59
113c voltage 0.49
117c voltage 0.46
119c voltage 0.45
120c voltage 0.44
125c voltage 0.36

125c triggers a Temperature warning.
119c is where I would want a fan to cut in.
 

Reloadron

Well-Known Member
Most Helpful Member
from a search I get
136deg C = 0.29 volts
-11deg C = 4.96 volts

I have located the wire colour for coolant temp feed to the ECU so I will physically check it tomorrow (it's dark now)

Those numbers made good sense and could be worked with using a comparator circuit. Give this link a read and you should know where I am going with this. The problem is in the second set of numbers there is little change? I don't get that? Not that they can't be used but they do not offer what that first set of numbers does. Also, when you read the link note and pay attention to what is called "hysteresis". I would use a single comparator like the LM311 and use a pot to set the reference input. The output could drive a transistor used to drive an automotive relay. That would turn the fan(s) on and off. I would also power things from a LM7805 voltage regulator.

Ron
 
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