Should engine run cooler when AC is engaged?

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b5cuda

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Did a road trip today and got to run my newly reinstalled AC system. To my surprise, every time I turned on AC the temp gauge dropped 5-8 degrees. I'd expected just the opposite. Can anyone tell me why the engine temp drops while AC is running? My system is factory inside the cabin, an older Sanden-type compressor under the hood (340 engine). Water gauge is aftermarket, I believe accurate, reading at the intake manifold.
 
Stock mechanical fan?

Could your AC be drawing enough current that your system voltage be dropping enough to make the temp guage show lower
 
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If you have electric fans that are controlled by the A/c it will run cooler. Otherwise not usually. If you have a thermostaticlly controlled clutch fan it may engage shortly after the a/c is turned on. The a/c condenser (by design) purges heat from the system and because it is in front of the radiator, it usually heats the cooling system up. A/C cars have larger radiators, larger fans and usually shrouds to aid in the cooling.
 
No. The AC is a load. Just like driving up hill.

I think you have a voltage drop causing the readin to change.
 
No electric fans - have a heavy duty clutch fan and shroud. So you guys figure it's a faulty reading?
 
the drop on the gauge was not instant, but gradual over several moments after engaging.
 
Is you temp gauge a sending unit with wires to the gauge or is it a mechanical gauge ?
 
No electric fans - have a heavy duty clutch fan and shroud. So you guys figure it's a faulty reading?

NO the reading on the gauge is correct. The thermostat on the fan clutch has a bimetal spring on the front that controls the rate of resistance in the clutch. The hotter the temperature gets, the more resistance the bimetal spring gives the clutch making the fan spin harder and pull more air through the radiator core. You are describing correct operation.
 
The temp gauge is mechanical. Thanks everyone for helping to clear up the mystery!
 
If you have a thermostaticlly controlled clutch fan it may engage shortly after the a/c is turned on
The hotter the temperature gets, the more resistance the bimetal spring gives the clutch making the fan spin harder and pull more air through the radiator

I think the mystery is solved, thermostat fan is doing its job.
 
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