Ok, now I'm confused.

A few months ago I had my fan temp on/off settings re-programmed. The results were amazing, car ran much cooler in stop n' go and never really got over 190 or so BUT this was w/ the a/c off. Well, the other day it was 98* in Phx and I used the a/c for the 1st time since the re-program. Car ran incredibly cool...even in stop n' go, never went above 194*. The car never ran this cool w/ the a/c on before the re-program BUT the part that has me baffled is, I thought that once the a/c was on the fans were on automatically...so how can the car run cooler? I thought that the fan re-program would only affect the car w/ the a/c off BUT it has absolutely had an effect on the car w/ the a/c on as well.
WTF?

Can anybody offer me an explanation for this? TIA

Jeff

FWIW...I have 160 stat w/ aux fan on @ 186/off @ 180 and secondary on @ 194/off @ 188.

 

A/C sends a different signal to the ECM/PCM for fan operation - generally, if the pressure at the Condensor equals a temperature of about 120 degrees (depends on whether it's an R134 or R12 charge) then the fan relay(s) are grounded. Below that temperature (pressure) the fans won't be running and you could probably see that if it were 32 degrees or less. Anyway, that's well below any thermostat you can stick in it (which is one of the reasons you need one) or any programming that someone might change for non a/c operation. The only reason you might change the a/c operation would be if you switched gasses - R12 to R134 - at which point lowering the threshold would probably help as R12 systems usually click the fans on at around 150. The signal (90 and up) is from a pressure sensor on the high side line. The ECM/PCM sends it a 5 volt reference and as the pressure increases, the voltage being returned goes up with it. Each volt is about 100 psi so if you were to test yours, expect the fans to come on when the PCM sees 1.2 volts. Additionally, assuming you have a '96, that system is designed to kick up the secondary fan to another speed when the pressure indicates an even higher pressure signal. I don't have that threshold info handy, but it should be outlined in the Service Manual under Cooling Fans and that's probably the main reason yours dropped a few degrees with the a/c on.

Most cars run cooler with the a/c on as the fans are on sooner and have to stay on longer. Not a whopping difference (about 5 or 6 degrees), but that and the richer mixture required to compensate for compressor load, exempts a/c operation from Federal and State Emission Standards (the OEM's can't meet the standards). What the car ultimately runs at is a function of cooling capacity and the ability of the radiator to dissipate heat. For Foreign and Domestic production (over the last 20 Years) that's about 195 (or stock thermostat) to 226 and regardless of what you monkey with, changing fan temps or thermostats won't do anything unless you increase capacity and thermal efficiency (which your system does to some degree already by speeding up that secondary fan under certain a/c conditions).

 

I appreciate the informative reply and, thx to you, have a better understanding of how the "sytem" functions. I was just under the impression that w/ the a/c on, re-programmed fan temps would not come into play. It appears that the re-programming has had an effect on the car w/ the a/c on as well. Who'd a thunk?

Thx again.

 

In the earlier cars there are 2 sets of fan settings. Those for a/c on and a/c off. I assume the 96 follows the same logic.

This can be misleading since the a/c on fan settings are actually used in all cases below 11 mph. The a/c off fan settings are only available above 11 mph.

Therefore in order to reduce the fan on/off temps for low speed operation below 11 mph, the a/c on settings must be changed. These settings of course, will also be active when the a/c is on which explains the behavior of your car.

There is an underlying assumption here that the a/c on fan settings will be lower than the a/c off fan settings, but that is not always the case.

It is not unusual for both sets of fan settings to have the same values, even in production, which eliminates the advantages of having different settings above 11 mph.

By selecting greater a/c off fan on/off temps, you can have the effect of turning the fans off at speeds greater than 11 mph which can be desireable to reduce alternator load.