Odd effect of defroster on GPH

[Kodak]

This is strange. I was idling today and using between .32 and .33 gallons per hour with a warmed up engine.

I thought it would interesting to see the effects of the defroster, so I put it on speed 1 (it goes to 4) and consumption spiked up around .42 to .44. Then, when I jumped to speed 4, it went back down to the mid .3's.

I plan on re-testing this, but that's what I saw.

What could explain better efficiency with a faster defroster speed? Was this some sort of fluke? Like I said, I'll retest this for accuracy.

MY SG-e is still in the process of being calibrated, but I trust it to at least be congruent with increases and decreases in consumption.

[gone-ot]

...if speed #1 is "slowest" and speed #4 is "highest" then it sounds like a fluke, because #1 should be drawing the least amount of current and and #4 the most, which would cause the highest "load" on the engine.

[Ladogaboy]

Unless the engine was still compensating for the initial surge?

[Frank Lee]

Maybe at the moment you selected "4" the compressor kicked off.

[JRMichler]

Originally Posted by Frank Lee

Maybe at the moment you selected "4" the compressor kicked off.

+1

The AC compressor turns on with the defroster at temperatures above about 32 deg F. Then it cycles on and off.

[Diesel_Dave]

Originally Posted by Frank Lee

Maybe at the moment you selected "4" the compressor kicked off.

That'd be my bet as well. A lot of vehicles automatically kick on the AC with the defogger (at least for a while). The idea is that the AC takes the humidy out to help defog. I personally think,the AC shouldn't be on unless I want it on. I'm working on putting in a relay on the compressor clutch so the AC compressor is disabled unless my AC button is manually pressed. In the meantime, I've unplugged my AC compressor clutch for the winter.

[slowmover]

The FE improvement will not offset the lack of lube being spread through the A/C system, IMO, if the defrost engagement is disconnected. Potentially an expensive lesson since the de-humidification is beneficial; a double penalty. Figure that an A/C system is good for 6-8 years tops before complete system replacement is favored well above component replacement.

I really can't tell the difference between A/C and non-A/C use for FE. Today's compressors are efficient, and the CTD has power to spare. The old RV2 compressors were another matter. The Owners Manuals in those days recommended the compressor be engaged and run while above 30-mph for a good while at least monthly in cool weather. IIRC, the auto-defrost engagement of the A/C cooling showed up in the late 1960's/early 1970's.

We also got in the habit of running the heater in the "summer" in the same way. Seals, manifold doors, fluids all benefit.

.

[Diesel_Dave]

Originally Posted by slowmover

The FE improvement will not offset the lack of lube being spread through the A/C system, IMO, if the defrost engagement is disconnected. Potentially an expensive lesson since the de-humidification is beneficial; a double penalty. Figure that an A/C system is good for 6-8 years tops before complete system replacement is favored well above component replacement.

I really can't tell the difference between A/C and non-A/C use for FE. Today's compressors are efficient, and the CTD has power to spare. The old RV2 compressors were another matter. The Owners Manuals in those days recommended the compressor be engaged and run while above 30-mph for a good while at least monthly in cool weather. IIRC, the auto-defrost engagement of the A/C cooling showed up in the late 1960's/early 1970's.

We also got in the habit of running the heater in the "summer" in the same way. Seals, manifold doors, fluids all benefit.

.

Sure, there's got to be some maintenence benefit to rumming the compressor every so often. When I have my relay in place, it'll be really easy to turn it on every so often just to exercise it. In the mean time, I'll have to re-plug it in every so often.

As far as the defog option, I'm sure the AC benefits the defog somewhat, but I personally don't think it's a whole lot. My defog has been working just fine since I unplugged. Growing up in a fairly cool climate (nothern PA), must of the vehicles we had until the last 10 years or so we bought without AC. I never remember the defog being slow then. Until fairly recently, AC has been considered optional equiptment in much of the country (I would imagine that's not so you you in TX, slowmover), and defogger ran fine without it. BTW, we usually called it a "defroster" not a "defogger"--because that's what we needed it for.

[slowmover]

I think I also called it a de-frost. Yes, winters down here call for de-fogging windows, especially as one passes through farm land. The fog is impressive, even more so with windows and exterior mirrors fogged up as a result of Gulf air moving over cold ground right about dawn.

[BurningDinosaurBones]

Another theory here is that the speed control on the fan can be controlled by resistors in series with the fan motor. The resistors' job is to drop the voltage in the line to reduce the fan speed, and release the extra energy as heat into the heated air stream in the duct. Three resistors of a different resistance produce the three lower speeds and then when switched to maximum speed the fan has zero resistance in series with the fan motor and runs at it's designed speed and current (Amps), and it's maximum efficiency. This could account for your drop in current at the fans highest speed.

As a side note, sometimes a vehicle defroster fan will not work on any of the lower speeds but will work on maximum speed when the resister module fails because full speed is on a different circuit.



All of this being said, I would think that a late model vehicle would have a more sophisticated electronic speed controller. But maybe not, since that would be much more expensive than a simple resistor.