k2500 with Manual A/C

wierd issue...

Bought my K2500 Sub used...noticed that when A/C was on high fan / cold (without recirculation enabled) the the A/C blower fan would increase speed at random times.

Had the circuit board (reheostat) replaced...no change. Turn on A/C normal mode, fan high.. at times the fan will increase in speed purely at random...it does not maintain this speed, it will maybe have a dealy of 10 to 15 min where it slows back down, and then it runs faster for while.

Only guess now is that the fan motor potentially has an issue. Only applies to the front A/C, not to the rear. No issues noted with ALt output.

Anyone experienced this ?

wierd issue...

Bought my K2500 Sub used...noticed that when A/C was on high fan / cold (without recirculation enabled) the the A/C blower fan would increase speed at random times.

Had the circuit board (reheostat) replaced...no change. Turn on A/C normal mode, fan high.. at times the fan will increase in speed purely at random...it does not maintain this speed, it will maybe have a dealy of 10 to 15 min where it slows back down, and then it runs faster for while.

Only guess now is that the fan motor potentially has an issue. Only applies to the front A/C, not to the rear. No issues noted with ALt output.

Anyone experienced this ?

What year?
Fan speed is controlled with blower switch positions?

But I would make sure blower motor is good and grounded good and has good wiring and plugins and power.

Also blower motor speed and voltage runs threw a resistor block.
GOOD PLACE TO START CHECKING FOR LOSE OF POWER OR BLOWER SLOW DOWN.

wierd issue...

Bought my K2500 Sub used...noticed that when A/C was on high fan / cold (without recirculation enabled) the the A/C blower fan would increase speed at random times.

Had the circuit board (reheostat) replaced...no change. Turn on A/C normal mode, fan high.. at times the fan will increase in speed purely at random...it does not maintain this speed, it will maybe have a dealy of 10 to 15 min where it slows back down, and then it runs faster for while.

Only guess now is that the fan motor potentially has an issue. Only applies to the front A/C, not to the rear. No issues noted with ALt output.

Anyone experienced this ?

remove the blower mtr and check how the fan rotates...any binding the motor is defective..

as the motor heats up it will usually start to bind up and this will cause your problem..sometimes you can lube the bushing and get it to work again...since its not that expensive replacement is best..

as stated the resistor block was replaced with brand new GM item.

its a 2001 K2500 Sub with 496 engine.

No issues with A/C control head that I know of. This happens only at high speed, thats why I suspected the resistor block. It does this at random. Get in truck turn on A/C , and set fan speed to high. Fan works...then all of a sudden you hear the fan speed increase by itself (almost like it went into recirculate mode) and it will run for 10 to 15 min like that, and then randomly it reduces speed by iself...back to say normal for the high speed.

If I set A/C to high fan and recirculate, the fan/ blower speed remains constant and it does not randomly change speed at the high setting.

No codes set.

What I assume at this point is there is somethign wrong with the blower motor or potentially the A/C control head.

There is nothing wrong with your system- it automatically goes into recirc mode when head pressure hits 350 psi--this makes it sound louder wshen there is less air flow restrictions-- system is performing as designed. We call this "poor man's climate control"

Per the service manual-- recirc door shut when system pressure returns to 250 psi or below- ther is a timer involved as well, see if when you slow down, (head pressure rises) that recirc door opens- then closes after a given amount of time after driving at highway speeds-- You may also have a fan clutch problem- or air flow across condenser problem-- dirty condenser would definitely cause this.

I don't think there is an issue with the fan clutch as its under a year old, brand new at the time GM item. No crap in the radiator (I'll check between the condensor and the radiator to make sure).

First time I've heard about a manual A/C system having an automated feature. Mine has the manual a/c controls...not the automatic ones, where you just set the temp and the system maintains that.

I'll check to make sure no blockage at condensor. Truck has always run under 210F and the A/C is probally one of the best I've seen. It works very well and will freeze your @$$.

I may not have an issue. Its not a big deal to me..I was just wondering why the fan speed seemed to increase by itself as if in re-circulate mode, but with manual controls re-circulate mode had not been enabled.

Thanks for the feedback


There is nothing wrong with your system- it automatically goes into recirc mode when head pressure hits 350 psi--this makes it sound louder wshen there is less air flow restrictions-- system is performing as designed. We call this "poor man's climate control"

Per the service manual-- recirc door shut when system pressure returns to 250 psi or below- ther is a timer involved as well, see if when you slow down, (head pressure rises) that recirc door opens- then closes after a given amount of time after driving at highway speeds-- You may also have a fan clutch problem- or air flow across condenser problem-- dirty condenser would definitely cause this.

The service manual specifically states the recirc door is enable with head pressure greaterthan 350 psi--

It is a manual system that automatically goes into recirc- and probably does not turn recirc light on. C/K trucks have been doing this since late 80's- popping into recirc mode when they experience high head pressure on highest and next to highest fan speed, and coolest temp setting- I have experienced this complaint for 20 years- folks just like you telling me the fan goes into high high fan all by itself- for no reason-- and they have manual a/c systems-- please read your service manual if you have difficulty believing what I am telling you- I would cut and paste it for you- but Mitchell on demand is copyrighted and I respect their position.....

wierd issue...

Bought my K2500 Sub used...noticed that when A/C was on high fan / cold (without recirculation enabled) the the A/C blower fan would increase speed at random times.

Had the circuit board (reheostat) replaced...no change. Turn on A/C normal mode, fan high.. at times the fan will increase in speed purely at random...it does not maintain this speed, it will maybe have a dealy of 10 to 15 min where it slows back down, and then it runs faster for while.

Only guess now is that the fan motor potentially has an issue. Only applies to the front A/C, not to the rear. No issues noted with ALt output.

Anyone experienced this ?



I still believe your problem is the blower motor ..upon removal you should be able to determine if it is in need of replacement...IT SHOULD ROTATE FREELY WITH NO DRAG...

Since you recently have had work done to this system It is very possible you have too much refrigerent in your system...I would check out the hi/low pressures to determine if this is so...

when the system is overcharged this reveals itself when the air temps rise ...If the system was charged during cool weather and the charge WT. was not carefully installed this can occur..