When the cooling fan for the radiator kicks in, the RPMs drop and the car shudders. 1994 940 non turbo. Have cleaned throttle body, replaced throttle position sensor, did all mechanical replacements. Any ideas?

 

Are you sure it's not the AC compressor kicking in?

 

Could be happening at same time.

 

My car does the exact same thing. I've never considered it an issue though. When my AC compressor turns on, there's a slight RPM drop, and the car idles just a little bit more rough, but never to the point of shuddering like you describe. When the aux cooling fan turns on, there's another RPM drop, but it rises back to normal in a few seconds.

My guess is, the increased load on the electrical system causes a slight drop in RPM. With our old cars, if your alternator is the original, it may not be operating at maximum efficiency anymore, and an increased electrical load from the fan causes the voltage to dip slightly, but then return to normal once the electronics have had a chance to sort themselves out.

You should hook up a voltmeter and observe what happens when your fan kicks on.

 

Thank you. I did hear this was a possibility. My alternator is the original, so I just got a new one. I will let you all know how it works out next week.

 

if everything is properly adjusted and working right, the idle air control valve should compensate for any normal loads and maintain a steady idle at 700rpm. it might drop for just a blink when the AC compressor comes on but should bounce right back.