SN/95 Fan Question

Black Sun 5.0 said:

I have an SN/95 fan installed in my '91 with a DC Control unit. In stop and go traffic tonight, the temp in the car seemed to rise a bit too high for my liking. I noticed that the fan has 3 points of connection: ground, low speed, and high speed. Which connector is for the high speed? I figure it's obviously either the center connector or the one just next to it (I know, one of them is the ground). I didn't setup the fan myself, so I'm wondering if the fan is on the right connector. Also, on a side note, a somewhat sailly question. I have a 180* t-stat installed. I was wondering whether this meant the t-stat would just start to open at 180* or if it opens at a somewhat higher temp to drop the engine temp down to 180*.

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i would be sure to include a circuit breaker on the wire to the 12-volt feed to the motor. there may not be one already. do it!

i might check in the SN sub forum if no one on this side knows. i believe there is some info in their FAQ sticky ( in the section for the fan manual switch wiring).

the stat: it depends. most parts store stats begin to open at the specified temp or a degree or two lower. they open fully 10* or more past the temp rating.

a balanced performance stat will open a bit by the specified temp and open fully sooner. im a big fan of Mr Gasket stats.

good luck.

If you are looking from the top of the fan down. The top terminal is ground the middle is low speed+ and the bottom is high speed +.

Black Sun - once again, I'd ask Baskin all these questions. That's what he's there for. He can tell you which is the high speed connection, and he can tell you if swapping the wire around while things are running will hurt the control unit.

Who put this in for you? Sounds like they didn't have a clue what they were doing.

T'stat temps vary all over the board by a few degrees. The only way to tell what yours is doing is to measure it. Put a pan of coolant on your stove. Drop the t'stat in, heat it up with a thermometer in there (don't let the thermometer sit on the bottom of the pan for obvious reasons). See where it starts opening, and where it's fully open. It will likely start somewhere between 178-182F and be fully open between 180-185F. Mine started at about 181F and was open by 184F. I selected the 187F control temp on the controller and it works fine. When I come to a stop after cruising, the fans are off. After sitting for 30 seconds or so, they gradually come on. About 30 seconds after I've left the light and hit 30-40 miles an hour, they ease back off. You want your control temp at least 3 or 4F degrees higher than your t'stat temp. Don't want the fan running all the time - kind of defeats the purpose of the controller.

That fan and the controller shouldn't have any problem at all keeping your engine cool - especially this time of year. If it's still exceeding the set point of the fan, then either your still not hooked to the highest speed setting, or something's amiss with your radiator.

Are you certain the system is full of coolant? Sometimes air in the system will cause gauge fluctuations. What kind of gauge/sender? If it's the factory gauge/sender, you really should get an accurate aftermarket gauge/sender. The factory unit isn't worth much.

The thermostat opening temperature is not very consistent from one to another. Most tend to start to open near the specified temperature and fully open over 10 degrees. I have a 180 in one of my cars that starts to open at 170. The cheaper thermostats also tend to have some hysteresis when the car is warming up. In other words, they won’t start to open until they’re past the spec temperature, fully open and then cycle a few times before finally settling at a partially open position. This might be due to the fact that the cheaper thermostats don’t have holes drilled in the housing and therefore don’t have some water constantly flowing through them. This also makes the system more difficult to purge the air from.

With regard to the fan, Ford is pretty consistent on using the two outside terminals for the high speed winding. Leave the middle terminal open.

Some of the advise in this thread isn’t that great. An sn95 fan generates its own voltage of about 10V when it's running. If you inadvertently reverse the leads while the fan is running, you will apply –10V to the controller output. It would probably survive, but it’s not a good idea with this or any other controller.

A “freewheel” diode is commonly connected across the fan in order to protect the relay driving the fan. This is actually a misnomer, in that a free spinning fan doesn’t damage anything. When the current is removed from the armature winding of a motor, the magnetic field that is generated by the windings collapses. This, in turn, generates a negative voltage spike which is far greater in magnitude than the 12V driving the fan, 150 V is not uncommon. The controller that you have has a high speed equivalent of this circuit that is built into the unit in order to make the diode across the fan motor unnecessary. If the fan is grounded to the chassis, and the ground connection to the controller is lost, there is no connection of this circuit to the fan and the controller output will receive the –150 V spike. By grounding the fan to the unit, there is no possibility of the unit becoming ungrounded while the fan stays grounded.

If you want to install the breaker, it won’t hurt anything, but the controller will protect the fan without any outside circuitry, so the circuit breaker is redundant. The sn95 fans catch fire for two reasons. One is a shorted armature and the second is frozen bearings. The factory Ford controller has no way of dealing with this, so the 120A draw of a fan with a seized bearing or the nearly infinite draw of a fan with a shorted armature causes a fire. If the fan seizes, the dccontrol unit will limit the current to about 40A and the voltage to 4V, so the power dissipation of the fan will be 160W, about the same as on a normal fan in lieu of the 1,440W dissipation using the Ford system. If the fan shorts, the dccontrol unit will limit the current to 40A, but at near zero volts, so the fan will actually run colder than when it’s not shorted


With regard to the controller temperature, your posts are somewhat confusing. The blue wire should be the AC input and the yellow should be the ignition input. Regardless, from your posts it sounds as if the shop that you had install the controller made quite a few mistakes installing the controller. It might be worthwhile to read through the installation instructions and double check everything. 187 is the preferred set temp and will result in quieter, stable fan operation and lower current draw for the reasons stated above, but either 180 or 187 will work (no jumpers for 180, 5-7 / 8-10 for 187) The Input should be connected to the battery positive, the output to the fan high, the ground to the battery negative and fan negative, the blue to the AC, and the yellow to +12V or ignition 12V. The probe should be connected to pins 1 and 2 and mounted downstream from the fan. Optimum placement is as close to the radiator output tank as possible.

With regard to the temperature gauge, the gauges tend to be internally regulated at about 5V. The problem is that it’s referenced to ground, so if your chassis ground voltage changes by just 1V in reference to your engine ground voltage, the gauge reading changes by 20%. The best place to ground the gauge is on the engine, where the sending unit is grounded.

I don’t check the threads here very often , but you can E-mail me if you have any other questions with regard to the controller or fan.