1989 stock, except no egr or cats

What causes the engine temp to increase when AC is turned on? Is it just the extra drag of the compressor when it's engaged, or is there something else. I was caught in bumper to bumper traffic on the highway today for about 45 minutes at about 88 to 90 degrees. I was playing with climate control and watching engine temp. I also noticed that I think digital read out are not that accurate My fan is set to come on at 210, but noticed it comes on at 218

 

The condenser for the A/C sits in front of the radiator. With the A/C off, the condenser is at ambient temperature, so the air entering the radiator is also at ambient temperature. When the A/C is running, the condenser is hot (removing heat from the refrigerant), so the air that enters the radiator is also much hotter.

 

You've compared the 210° vs 218° using a scanner? All senders have ± tolerances and do also the IC devices so ................

 

Thanks, I will have to check it with scanner. With the Ecu getting its temp reading from the sensor on the front of the intake manifold, and the temp gauage getting its input from the sensor between cylinders 6 and 8, shouldn't there be a difference between temp fan is set to come on and gauge reading. In other words could fan be coming on at the setting I programmed, but reading on dash be higher then programmed setting? If I had temp gun I could compare the temp readings of the two different areas to each other

 

You can't compare and I actually doubt that an IR will get you accurate readings. You scan for temps and if you insist you check the resistance of the sender in the cylinder head then compare that to published charts in the FSM (and other sources), allow for the various ± tolerances and then make decisions.

 

One thing that goes against the logic of the AC heating up the engine is the fact that when I turn on the automatic climate control on my'93, it forces the fan to come on immediately.
So, when my engine starts to warm up (say 220) and I want it to come down, I turn on the AC, and sure enough, the fans that had been sitting idle at 220 will turn on and bring cooling air through the radiator, bringing the temps to the low 200's.
Except for the computer being programmed to turn on the fans with the AC, I would expect to see temps go up a little with the AC on.

 

I did scan today using MT2500
I have re-programmed my ecu for a lower fan setting

My fan comes on at 210 on dash, scan reading was 201
My fan cuts off at 196 reading on dash, scan reading was 192


Just for grins, I started car from cold condition and compared readings in dash to scan readings as the engine warmed up

for the most part there was between 10 and 8 degrees difference between them through out the complete range with the scan readings being lower then dash readings. Scan reading was taken from coolant temp sensor in front of manifold through mt2500, and dash readings coming of sensor between cylinders 6 and 8 on pass side head and read on temp read out on dash

Of course I don't have any idea how accurate cylinder head sensor is

 

My 87 had the dash sender in the head and the ecm sender in the intake. The dash temp was always 15* higher than the datalog numbers. My infrared gave similar numbers.

 

Don't know much history ... don't know much geography, or cars for that matter, but I know a bit of physics.

AC's work by consuming work and using that work to move a bit of heat from a cold place to a hot place, a direction heat does not ordinarily go: in the process, all that work is converted to waste heat. SO, you run the AC, first of all the motor is doing a little more work, so that means more combustion and more waste heat direct from the cylinders. THEN, some of the work being done by the motor which would normally go to friction elsewhere and not trouble the radiator must be removed by the radiator: so, with AC running more direct waste heat from cylinders, more secondary waste heat from work converted back into heat in addition to whatever heat is actually removed from the interior of the car. That's ONE prong of the answer, second is that heat flow increases with temperature differential - so at a given speed of the car and the fan running there is a given air flow over the radiator or radiators and a maximum amount of heat which can be removed for a given coolant temperature. If the cooling demand exceeds that capacity then the coolant temperature increases until the rate of heat removal again balances - or you overheat.

Not sure if the fact that I can add a comment to a four year old thread means that I should - the software lets me, but as they say, not all that is legal is ethical.