AC Radiator pusher fan? Info?

Mike Goble

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ok
so to cut the ground of the sensor you use another relay that is operated by the +12v signal from the ignition key and that allows the sensor to have a ground or not depending on the previously mentioned operation, right ?
Yes. If you look at the schematic in post #11 of this thread the red switch contacts in the ground wire of the temperature switch represent the normally open contacts of a relay operated by the 12v ignition signal. In my case it's the signal to the old idle solenoids, which is 12v with the ignition on.
 

deQuincey

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Yes. If you look at the schematic in post #11 of this thread the red switch contacts in the ground wire of the temperature switch represent the normally open contacts of a relay operated by the 12v ignition signal. In my case it's the signal to the old idle solenoids, which is 12v with the ignition on.
that is fantastic idea, mike
yes the carb choke solenoids are +12v whenever ignition is on, and there is one free spade on the front one, so i will take it from there
well done

one question on the first speed of the fan:
i plan to use the original fan which has only one speed, this is the maximum speed from a straight positive cable
then i should put a power resistor in series with a second positive cable for a slower speed, right ?
any idea of how to calculate that resistance value ? it should be ohms and watts as far as i remember to define a resistor

thanks
 

Mike Goble

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that is fantastic idea, mike
yes the carb choke solenoids are +12v whenever ignition is on, and there is one free spade on the front one, so i will take it from there
well done

one question on the first speed of the fan:
i plan to use the original fan which has only one speed, this is the maximum speed from a straight positive cable
then i should put a power resistor in series with a second positive cable for a slower speed, right ?
any idea of how to calculate that resistance value ? it should be ohms and watts as far as i remember to define a resistor

thanks
No idea on the resistor. I would measure the high speed current to see what the dynamic resistance of the fan is, then calculate a resistance that would halve the power. Since P = I²R , it will not be a linear function. For example - if the fan current is 10A at 12v - that's 120W and the dynamic resistance is 1.2 ohms. If we want to cut the power to the fan to 60W, we'll need 1.2I² = 60, and I = sqrt 60/1.2 or 7.07A. 7.07A @ 12v = 1.7 ohms of circuit resistance, or an additional 0.5 ohms in the form of a resistor. The total circuit power dissipation will be 85W in low speed, with 60 dissipated by the fan and 25 by the resistor.
Given the above example, if you put a resistor with a value of 40-50% of the dynamic resistance of the fan you would be pretty close.
 

Ohmess

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I have a two stage setup on my e39 that was designed by Zionsville Autosport to go with the aluminum radiator they made for those cars (a bunch of us petitioned them for this so that we could remove the radiators with plastic ends and plastic expansion tanks that were a known weak point).

I just pulled my wiring diagram for the cooling fan relay circuit for my car, and the low speed circuit has a .5 ohm 85 watt resistor. This circuit was designed for the Spal puller fan Zionsville used, which may not draw as much power as the fans originally installed in our cars. Nonetheless, this certainly indicates that Mike's computations were quite good.
 

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It's really simple. The BMW switch has 3 wires, one for each switch and a ground. The fan has 3 wires, one for each speed and a ground. The Volvo Relay has 3 plugs, one for the 12v power, one for the two switch inputs, and one for the two outputs. The switch in the ground leg of the temp switch turns the fan off. I use a Normally Open relay contact on the ignition circuit so the fan shuts off when I turn the key off.

9gEM2xL.jpg
Dear Mike.
To my inexpert eye it seems very odd to have an earth cable to the sensor and then two outputs, i am more prepared to see an +12v input and then the 2 outputs one for 91 and the other for 99

Could you please confirm ?

What am i missing ?
 

Mike Goble

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Dear Mike.
To my inexpert eye it seems very odd to have an earth cable to the sensor and then two outputs, i am more prepared to see an +12v input and then the 2 outputs one for 91 and the other for 99

Could you please confirm ?

What am i missing ?

It is a switch, not a sensor. The switches are connected to a common wire on one side, so when each temperature limit is passed the associated switch closes and connects the input associated with it to the common wire. It would absolutely work with 12v on the common lead, but that's not what is needed in this case. In the Volvo link there is this schematic of the dual relay:

volvo_diagram.JPG

Note that the 12v needed to operate the circuit is present on A1, so we will need grounds to actuate the relays. When the coolant reaches 90°C, thermoswitch contact #1 closes and presents a ground to R1. This actuates the low speed ( left side ) relay with current provided through a NC contact of the High speed ( right side ) relay and the fan is started on low speed.
If the temperature continues to rise above 99°C, contact #2 of the thermoswitch will close, presenting a ground to R2 as well. The high speed relay ( right ) will actuate, providing 12v to the high speed winding as well as removing 12v from the low speed winding, resulting in high speed operation.
When the temperature drops below about 93°C, contact #2 of the thermoswitch will open, the high speed relay will drop out, and the fan will revert to low speed operation because of the NC contact on the high speed relay. When the temperature drops below about 84°C, the low temperature switch will open, the low speed relay will drop out and the fan will stop.
A serious advantage of this sequential operation is that the inrush current on the initial start of the fan is limited by the speed limiting resistor, much less likely to blow fuses and easier on all the contacts. Once the fan is running on low speed it is generating back EMF and the inrush current on the high speed actuation is also limited.
 

Ohmess

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Here is the diagram for the setup I have in my e39. The device on the left side with the T0, T1 and T2 connections is the temp switch.

Zionsville Fan Schematic.jpg
 

deQuincey

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we are building a very interesting thread,
i have just sourced a fan and two relay from a couple of volvo that sit in a scrapeyard 300 km from here, will receive it in a couple of days, really inexpensive, 30 euro shipping included
 

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Comparing the Volvo setup to mine, the Volvo fan has two connections, one for low speed and one for high speed. The low speed connection probably incorporates a resistor similar to my .05 ohm resistor that lowers the voltage to the motor, decreasing its speed when fed from the top connection. The stock BMW fans, and the Spal fans Zionsville used in their setups, have only a single connection for the fan voltage and therefore the resistor needs to be separately incorporated into the circuit.

I looked quickly but didn't find a 0.5 ohm 85W resistor. I did find this 0.5 ohm 75W resistor: https://www.mouser.com/ProductDetai...bXrIkmrvidNw3CpujCJIJ8ISkA43KWPqNiG9jGs9ddw==. Mouser has a 100W as well.

I believe the temp switch in my e39 is from another BMW, but I don't recall which one. My best guess is that it is a standard VDO switch, but I didn't find an 80/88 switch on their web site. The closest I found was this 81/86: https://www.vdo-gauges.com/sensors/...capacity-86-81-c-floating-ground-m18x1-5.html.
 
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