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"surging" HVAC fan and random heat...

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

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clarkerussell
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"surging" HVAC fan and random heat...

Post by clarkerussell » Fri May 10, 2019 2:56 pm

Hey guys,

My HVAC has started behaving badly... After running fine for a while the fan will slow down, then go up 100%, possibly blow some hot air, and then drop back down to normal, all in a span of about 5 seconds. Any thoughts on what could be causing this?



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abscate
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Re: "surging" HVAC fan and random heat...

Post by abscate » Fri May 10, 2019 10:16 pm

Two things to explore

If the cabin temp sensors get clogged ECC will be erratic

If the fangordses k


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Sommerfeldt
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Re: "surging" HVAC fan and random heat...

Post by Sommerfeldt » Sat May 11, 2019 3:43 am

abscate wrote:
Fri May 10, 2019 10:16 pm
If the fangordses k
Covfefe?

- S
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'96 855 T5, R bumper and spoiler, Koni Yellows & blue H&R springs all 'round.

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abscate
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Re: "surging" HVAC fan and random heat...

Post by abscate » Sat May 11, 2019 5:38 am

abscate wrote:
Fri May 10, 2019 10:16 pm
Two things to explore

If the cabin temp sensors get clogged ECC will be erratic

If the fangordses k
THis is what happens when you post after being up for 36 hours post red eye flight..

If the fan motor is giving up the ghost and dying, it will get erratic, too. Sometimes you will notice it changing as you brake and accelerate the car


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clarkerussell
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Re: "surging" HVAC fan and random heat...

Post by clarkerussell » Mon May 13, 2019 2:47 pm

abscate wrote:
Sat May 11, 2019 5:38 am
abscate wrote:
Fri May 10, 2019 10:16 pm
Two things to explore

If the cabin temp sensors get clogged ECC will be erratic

If the fangordses k
THis is what happens when you post after being up for 36 hours post red eye flight..

If the fan motor is giving up the ghost and dying, it will get erratic, too. Sometimes you will notice it changing as you brake and accelerate the car
Thanks goodness, I thought you had stroked out our something... :)

The fan is a noticeable item, but I'm equally confused by the surge in heat when it's been set to cool. I'm going to have to test the cabin temp sensor...



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abscate
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Re: "surging" HVAC fan and random heat...

Post by abscate » Mon May 13, 2019 6:25 pm

Try vacuuming it out first. The sensor has a tiny fan that pulls in cabin air to feedback to the ECC control unit. It often clogs up with pet hair and other detritus.


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Re: "surging" HVAC fan and random heat...

Post by markone » Tue May 14, 2019 3:19 pm

Where is the sensor located?


1997 850 Sedan NA, manual trans, 268k miles
1998 V70XC AWD, turbo/auto 165k miles
1998 V70, NA/manual trans, 351k miles
2005 BMW X3 3.0i, auto, 135k miles
2010 Toyota Highlander AWD, 91k miles
2007 Malibu Response LXi, Ebony/white, 340 hp, 330 hours

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abscate
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Re: "surging" HVAC fan and random heat...

Post by abscate » Tue May 14, 2019 3:46 pm

In a 1997, up near the grab handles on each side of the car near your ears

In 1998-1999 , it transitioned to a dash position , next to the heated third rear seat button to right of steering wheel,
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