Dropping resistor

[tomj76]

OK, this is very unconventional, but I have the same problem and have planned (but not carried out) to fix it this way...

As far as I can tell, the motor is being run through the resistor to reduce the voltage on the motor to about 6 volts. This is actually a brute force method in electronics, the way things used to be done.

A "better" approach is one that avoids the resistor, but switches (at a very fast rate) between fully "ON" and fully "OFF", at just the right proportion that the average voltage on the motor is the desired 6 volts. This a version of what they call a switching power supply.

Of course, this is definitely a hack, and there's no telling what kind of problems will be caused by implementing it improperly, anything from electrical noise that causes radio noise to PCM issues, over voltage conditions caused by turning the motor on/off at a rapid rate... more.

In the mean time, I turn off the A/C at stop lights, or even don't run it at all unless totally necessary or driving >50 mph.

[12Ounce]

I just took another look at my wiring diagram. On the '99, there is a low fan speed relay, that uses the dropping resistor, and a separate hi-fan-speed relay that feeds the same two fan motors a full system voltage. The relays are fused separately. Therefore if the fans are not coming on at all ... there is more wrong than just the dropping resistor. The hi-speed circuit should offer a separate cooling fan supply voltage not dependent on the dropping resistor.

Note: I apologize for the mis-leading math in one of my earlier postings ... I must have been smoking something very good!

[tomj76]

Just a little help for those trying to calculate power dissipation for the OEM resistor...

Power dissipation for a resistor is the voltage drop across the resistor (not the supply voltage) times the current through the resistor.

Power = Voltage_drop * Current

Voltage drop for a resistor is resistance times current.

Voltage_drop = Resistance * Current

Therefor:
Power = (Resistance * Current ) * Current
Power = Resistance * (Current)^2
(e.g. P = I^2 * R)

Some comments have determined the voltage drop to be 6 volts. If the current is 40 or 50 amps, then the power dissipated is 240 watts to 300 watts (40A * 6 v or 50A * 6 v).

I measured the resistance to be 0.15 ohms. Also, my low speed fan circuit is fused for 40A maximum. Then the maximum voltage drop over the resistor is 6 volts (0.15 ohm * 40A) and the maximum dissipated power is 240 watts (6 v * 40A).

When you calculate power using the supply voltage (B+ or 12 v), then you've calculated the total power delivered to the resistor plus the fan motor. So, when the fan is running at low speed, if it draws 40A from the 12 v battery or alternator , then 480 watts ( 12v * 40A) are pulled from the electrical power source.

From the previous calculation, the resistor dissipates 240 watts and the fan consumes the remaining 240 watts. However since the fan motor is doing work moving air only a portion of the 240 watts is dissipated as heat in the motor.

[12Ounce]

... and just to supply some real "who cares" information: the resistance is somewhat dynamic, in that it will probably increase significantly with increasing temperatures. So who can say what the resistance is once heated up? (Anyone know how a thermistor works? I do. It was a freshman lab study eons ago.)

Back to my nap ...

[kevink1955]

So far the resistor in my 96 is good but if it was to die and no replacement is available I would look at putting the 2 motors in series for low speed. Should be able to do it with a couple Bosch relays.

Let the original High and low relays drive the added bosch relays and the PCM will not know the difference. Will have to drag out the manual to see how hard this would be to do.

[rlp71068]

000

[tomj76]

Adding the circuitry to switch the motors from parallel to series operation should also work. It seems like a good work around.

I don't think the resistor changes very much, even if it gets really hot.

The dropping resistor is made from Nichrome, which can be engineered for a low temperature sensitivity by controlling the ratio of Nickel to Chrome. Typical resistors of this type have temperature coefficients of < 0.05%/degC. So, if the resistor heats up 200 degC (hot enough to melt solder), the temp change from ambient will be about 1%. That means your 0.15 ohm resistor is now a 0.1515 ohm resistor.

On the other hand thermistors are designed to have a large and predictable change with temperature. They also are normally designed for a resistance that falls with temperature (negative temperature coefficient) although thermistors are available that have a positive temperature coefficient. Thermistors have a large resistance change so that they are good temperature sensors (hence the name). The negative change ones are normally used in sensor applications, while the positive change ones are used for heater regulators or initial on current sources (such as the degaussing coil of a CRT).

And since it was asked, the type used for sensors are typically made from silicon (like a transistor) with one or more impurities added to control the resistance and rate of change. In layman's terms, as they heat up, the thermal energy activates more electrons to carry electrical current which lowers the resistance.

The type used in heaters are based on a different technology (and materials), that have a "critical" temperature where the resistance increases very rapidly.

[12Ounce]

In most conductors: higher temperatures, therefore higher electron activity, is associated with higher electrical resistance. Hence, "Super Conductors" would usually make very chilly bed mates.

Thermistors are often conducting globules, such as carbide particles, suspended in a semi-rigid non-conducting plastic potting material. As the temperature rises, even though the individual conductive particles increase in resistance ... the over-all resistance decreases; because the size of the particles increase, making more conductive paths and decreasing the contact resistance between particles as they pressed together with more force. As you write, this is all subject to selective design.

[tomj76]

That's an interesting and unique approach. Do you know which manufacturers make them that way? I've only used the (NTC) semiconductor type, like that used for the engine coolant temperature sensors in the Windstar.

[tomj76]

"I would look at putting the 2 motors in series for low speed"

I have a couple of thoughts.

Make sure the design works with the logic of the controls from the PCM. I don't recall if low speed and high speed are complementary (high speed is false when low speed is true, high speed is true when low speed is false). If not, then a different approach is need than how I've envisioned it ( a switch on the positive of one motor, a switch on the negative of the second motor, and a switch from the positive of the first motor to the negative of the second).

One thing to avoid in your design is any possibility of connecting the B+ to ground through the relays, even if just for a moment during the switching. I think you'll have to make sure the controls and relays operate in a "break before make" so that the first two switches (used for high speed operation) are open before the one switch for low speed operation closes.

The other thought that occurs to me is that with this on-off-on sequence, there could be (maybe small) problems with inductive kicks when the PCM cycles between low and high speed.