Question about 2004 Chevrolet TrailBlazer

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With truck running, when defroster is turned on it makes the idle drop off. what could the problem be?

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When the defroster is on, it also turns on the A/C to clear the fog from the windshield. This is normal.

Posted on Nov 16, 2014

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When operating vechicle when using rear defroster wind shiel wipers and the fan rpms drop What is the problem


It depends on several factors. The rear defroster draws a huge amount of power from the charging system just like the blower motor does. So if everything is working correctly, it can be a normal thing. The alternator will not make enough power to cover the draw at idle when the other accessories are being used, but should at cruising speeds.

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Loud idle running hot in park but not in drive


double posted
post RPM in both modes,
it must be the same
the ISC idle controls system controls idle at all times.
post RPM. 800 is normal. the exact valve is in the FSM
and posted under the hood in USA.
no country stated in 4000s posts so.... well, gee....
here, open the hood, look up, see EPA vacuum map
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1 Answer

Car stalls when ac or defrost on


Sounds like the compressor is dragging the engine down. The compressor runs in ac and defrost mode. You might try unplugging the compressor to verify.

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My envoy idles up and down when i turn my defroster is on


When you place your controls on defrost it turns your air conditioner on - depending what the heat level is set on the air conditioner will cycle on and off to regulate temp. Anytime the A/C runs the idle sensor will bring the idle up

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may have something to do with climate controls in the car.. some make the car sped up to run the compressor for the defrost cycle..

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When i turn my heater fan on does the air compressor come on and why would the car shake


The air compressor will come on if you have the defroster on. Most vehicles run the ac with the defrost to dehumidify or dry the air blowing on your windsheild. The compressor being on at idle could cause the car to idle slower and rougher which could be the shaking you are talking about.

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1 Answer

The idle drops when i put it in gear almost stalling the engine


That is normal, in park or neutral there is no resistance on the engine when you put it in gear there is resistance because the car wants to move. Also when you turn on the ac that turns on the compressor that is run off the belts of the engine so it puts more resistance on the engine. Low idle is not a problem unless the engine dies.

Oct 22, 2009 | 1992 Honda Accord

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2003 montero sport. will not idle since battey boost


sit with foot on gas at steady 3000 rpm, then turn on all your lights, hazard warning lights, heater, and window defrost, if the power drops , then you need a new battery....sometimes the battery power can control the engine idle, the alternator is only for charging the battery, not for running the cars electrics, this is the battery's job

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1 Answer

Idle Revs With Front Defrost


when you use the front defrost in the winter the cycling of tha a/c compressor will cause this condition.

when you use defrost the a/c is run to de-humidfy the air that is blown onto your windshield.


in the summer this is not a big deal as it is hot enough outside to keep the a/c system pressure above the compressor clutch cycle threshold. but in the winter the general lack of heat in the ambient air and the lack of humidity results in rapid cycling of the ac compressor.

the idle is bumped up to compensate for the load of the a/c compressor, or else your engine would stall out. but when the compressor is disengaged the idle is dropped. but it raises again when the compressor cycles back on.

there may be a good chance that your ac is under charged after all these years. when the weather gets warmer have the charge level in your ac system checked.

a low charge will also increase the on off on off cycling of the ac compressor, usually resulting in the complaint that you have. so the base issue of your problem is low ac system charge.

Apr 11, 2009 | 2000 Jeep Wrangler

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