Question about 2004 Chevrolet TrailBlazer

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Rpm increases when you turn on the defrost fan

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  • Chevrolet Master
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Nothing dangerous; using the defrost mode in most vehicles turns on the AC compressor to effect the drying out/removal of moisture. The compressor represents an additional burden on the engine which can bog down the idle speed so there is a compensator that should normally just prevent the engine idle speed from falling. Yours is just overcompensating and this may not be adjustable.

Posted on Apr 05, 2011

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When i turn on my heat/air on defrost or auto my rpms go up... what could cause this


The basic concept is that in auto or defrost the air conditioner compressor is activated.
The compressor when activated via an electrical clutch adds an additional load that will reduce the engine RPM. The the RPM is increased slightly when the A/C is activated to prevent the engine from stalling. Power produced by the engine is increased by allowing more air and gas into the engine. When it is working perfectly the RPM will remain the same but the engine will produce the additional power to power the AC compressor.

Usually the bi-pass air valve is how the compensation for the additional power is accomplished.

Depending on your make year and model several methods to achieve the AC compensations have been used. In your specific case if the compressor is not working(no Air) and the compensation is open loop no engine control electronics I would expect a surge in RPM. With no additional load from the AC the additional air into the engine would increase the RPM.
26106712-qb20k32l4fvb2dr3l0mnapaf-4-0.jpg On older cars there is a solenoid that has an adjustment for idle speed when the solenoid is activated. See the image above.
Hope this helps?

Feb 24, 2016 | Cars & Trucks

1 Answer

Increase then decrease idle when engine is cold especially


cold engine idle surge. (problem)
RPM range of surge not stated
300-700 or 800 to 1500? (range is critical to diagnosis)

bad fuel
running summer fuel in winter (running old fuel?)
needs a tune up bad, see the list in the operator guide.
idle speed controller,sticking or EGR valve same. (clean them)
location not stated, so cant guess about local air temps.?
no other symptoms, lack of engine power? etc?
was it ok last week, or better, when was the last time it was OK?
1week, 1year?
surge , causes, on a tuned engine might be:
1: too lean. so lean the ISC can not control idle. rpm low, stalling
2: too rich, flooding, stalling, spark plugs foul (look at them)
3: weak compression in 1 or more cylinders.
4: bad spark.
5: air leaks, in the plenum causing , ISC to lose control.
why high RPM?

especially?
well that means it fails hot too.
and there is a simple tests. for hot surge.
is engine fully hot? 180F and holds solid there?
say rpm is about 800
and you turn on head lights, heater fan and (not AC) and defrost.
the RPM must hold or the ISC is failing.
ISC -= idle speed controls. (all EFI has this function)

Oct 28, 2013 | 1996 Honda Prelude

1 Answer

The blower fan seems to invert the air when the rpm's increase


you have a vaccume leak some where and all air goes to defrost

Aug 14, 2013 | 2003 Jeep Grand Cherokee

1 Answer

Finding the idle goes up n down/die out when using Defrost


Sounds like a vacuum problem. Do you have a seemingly hard brake pedal? If you do chances are your brake booster is leaking vacuum. If not check your vacuum lines. Your heater system is controlled by vacuum.

Feb 22, 2013 | 2002 GMC Envoy

2 Answers

Car does not heat up despite debater being on and letting ca run for 15 minutes


There are several things which come to mind. You have 2 complaints. A gas smell and a cold blooded vehicle.

Starting with the temperature problem. My initial concern is whether the thermostat is weak, but also that the automatic controls may be inoperable and working against you. The car is suppose to go into a temporary "cold start routine" which increases the RPM and makes the fuel rich. (This could contribute to the gas smell)

After the Temperature sensor and O2 sensors warm up, the idle is suppose to drop and the fuel mix should lean out. If the engine remains at high idle, the water pump will work faster with the engine spinning faster. Sometimes there is not a direct 1 to 1 ratio between the heat generated by the motor and the RPM versus the cooling produced. An idling engine may warm up faster than an engine running at higher RPM.

Another problem may be the Defroster setting. There are Thermal switches which are suppose to cut off the Electric Radiator cooling fans. When the Defroster is activated, some cars are built to spin the A/C compressor in the Winter. But the Electric Radiator cooling fans may also switch on as part of the A/C system. This would super cool the radiator at a time when you least want it. Check to see if the cooling fans are activating when the Defroster is turned on.

There can be 2 Temperature switches in your car. One would send the signal to kick out the "cold start routine" and the other would work the guages or indicator lights.

The O2 sensors can have heaters in them or they can just be slow. If you leave the engine idling like you do, they could foul out and be sluggish to warm up.

If running the Temperature at 80 degrees causes an additional 5 minutes of warm-up time, you may be better off setting it for say 72 and having it work sooner.

The thermal sensors are not that expensive. Work on a cold engine when changing them. Note what I said about your rich fuel mix contributing to the smell.

Please rate my info.

Jan 05, 2013 | 2005 Ford Five Hundred

1 Answer

I have an 04 galant when i am at a stop light and i have the a/c or heater on my car wants to turn off. What can be the problem?


The a/c clutch puts a load on the engine if the system is charged with freon. The engine should have a way of increasing rpm slightly to compensate.

The heater, on the other hand, simply operates a fan and maybe adjusts the door inside the dash board to make air flow to the floor vents.

I think that if the car is this sensitive to a/c or heat, then it needs a tune up.

Also, are you sure it is 'heat' and not 'defrost' that you switch to when it stalls?

Because the a/c compressor goes on when the 'defrost' selection is made, which would point to the actuator or iacv (idle air control valve) is not bumping up the air and fuel mixture to increase the rpm slightly.

Jun 23, 2010 | Cars & Trucks

1 Answer

2001 dodge intrepid ...rps suddenly rise when when fans turn on


how much? if a little this is normal computer compensates for the decrees in idle to keep idle at the right rpm. In the cold weather the RPM will increase while in idle for 10 mins to keep the engine at correct operating temp. =If you need more assistance write me, Lee

Jan 07, 2010 | 2001 Dodge Caravan

1 Answer

Just replaced my fan clutch 5 days later the enginge light came on. The engine will rev up then come back down. Any ideas?


Check to make sure that the electric fans are working. The computer will increase engine rpm to draw more air in attempting to increase air flow through the radiator as the engine temp increases unchecked. Also make sure the system is full and the the radiator is not obstructed either externally (leaves etc) or internally.

Nov 02, 2009 | 2000 Jeep Grand Cherokee

2 Answers

2003 Nissan Altima, Air condition makes loud noise when the unit is turn on. It slows down where u cant almost hear it after you turn the fan off.


When you put your A/C on, there is Vacum Motor or Solonoide, that increases RPM of the Engine to compensate the load of an A/C. If the vacum hose is leaking or raptured the RPM of Engine remain low and Engine Shakes. When you put the A/C on and the RPM of Engine Increases, but a little, then your Selonoide is working but not adjusted properly, so adjust it for the proper RPM of the Engine. Put all the loads on, I mean Head Lights and A/C on and then ajust the RPM.

Jun 02, 2009 | 2001 Nissan Altima

1 Answer

Engine revs realy loud


You have a faulty thermostatically controlled fan clutch. Same thing happened to mine. As soon as you start the car, the fan is always turning, and the speed increases as the RPM is increased.

Oct 19, 2008 | 2006 GMC Envoy

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