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Old 04-21-2011, 06:48 PM
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Auxiliary Fans on 350sdl

The auxiliary fans will not come on on my 1991 350sdl. I checked the fans with 12V and they work fine. I jumped the temp switch on the A/C and could hear the relay closing in the fuse box but the fans still do not go on. The fans will come on or start to come on at high rpms. If I rev the engine when it is warmed up they start to come on (I do not keep it at high RPMs long enought to see if they are on completely.) Any help would be appreciated.

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Old 04-21-2011, 07:00 PM
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There is a temp-sensor switch on top of the engine next to the fuel filter, jumper the two wires and the fan should go on. This is the hot-engine mode.

Jumper the high-pressure switch on the A/C dryer and they should come on low speed, if not then there is a ceramic resistor nearby that often fails.

Check also to be sure that they turn freely. It sounds like they might be freewheeling from the airflow when you're revving the engine.
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Old 04-21-2011, 08:22 PM
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Aux Fans

Quote:
Originally Posted by babymog View Post
There is a temp-sensor switch on top of the engine next to the fuel filter, jumper the two wires and the fan should go on. This is the hot-engine mode.

Jumper the high-pressure switch on the A/C dryer and they should come on low speed, if not then there is a ceramic resistor nearby that often fails.

Check also to be sure that they turn freely. It sounds like they might be freewheeling from the airflow when you're revving the engine.
When I jump the temp switch on the top of the engine the fans run. When I jump the temp switch on the A/C dryer the fans do not turn on. I can hear the relay under the fuse panel closing but the fans to not work.

Where is the ceramic resistor you are referring to? How do I test it?

Thanks
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Old 04-21-2011, 09:23 PM
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I am not sure about 350SDL but I believe it should be similar to the 300SDL. The resister is hidden underneath the brake booster. It is not easy to see. It is about 3" long and 3/4" across. Just measure continuity with an ohmmeter, it should measure less than 5 ohm. Do not remember the exact value.
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Old 04-21-2011, 10:38 PM
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2x on the resistor being bad. Edit: yup, that's what the FSM says it's below the brake booster.

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Auxiliary Fans on 350sdl-screen-shot-2011-04-21-10.55.53-pm.jpg  

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Last edited by compu_85; 04-21-2011 at 10:56 PM.
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