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1996 850 Low speed radiator fan not turning on [Bad ECT sensor] Topic is solved

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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Re: Low speed radiator fan not turning on

Post by abscate » Wed Aug 14, 2019 12:17 pm

It sounds like it just defaults to "I don't know the temperature so lets turn the fan on high to cool the motor just in case its hot"


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Re: Low speed radiator fan not turning on

Post by ZionXIX » Wed Aug 14, 2019 12:54 pm

abscate wrote:
Wed Aug 14, 2019 12:17 pm
It sounds like it just defaults to "I don't know the temperature so lets turn the fan on high to cool the motor just in case its hot"
I wish that was happening. As long as the infinitely resistive sensor is plugged in, the ecu reports a temperature and the fan does not come on. If I unplug it, it runs the fan on high in case of overheating. Anyways the part should be here soon and then we can see of it works.



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Re: Low speed radiator fan not turning on

Post by Chuck W » Thu Aug 15, 2019 5:53 am

Also check your AC high pressure switch. It's what allows the ECU to signal the low speed fan to turn on.

Still not sure why using the app didn't trigger the fans. It's independent of all of the sensors/switches.
Did they even act like they triggered at all? It doesn't turn them on to run, it just pings the relay, and all you hear is a quick engagement for just a second.



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Re: Low speed radiator fan not turning on

Post by ZionXIX » Thu Aug 15, 2019 7:13 am

Chuck W wrote:
Thu Aug 15, 2019 5:53 am
Also check your AC high pressure switch. It's what allows the ECU to signal the low speed fan to turn on.

Still not sure why using the app didn't trigger the fans. It's independent of all of the sensors/switches.
Did they even act like they triggered at all? It doesn't turn them on to run, it just pings the relay, and all you hear is a quick engagement for just a second.
My AC is working. Does that suggest a functioning high pressure switch?

I was expecting a quick turn of the fan, should I be listening for the relay to click?

Am I just checking the AC high pressure switch for corrosion or is there an ohm value I need to check too?



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Re: Low speed radiator fan not turning on

Post by Chuck W » Thu Aug 15, 2019 8:58 am

ZionXIX wrote:
Thu Aug 15, 2019 7:13 am

My AC is working. Does that suggest a functioning high pressure switch?

I was expecting a quick turn of the fan, should I be listening for the relay to click?

Am I just checking the AC high pressure switch for corrosion or is there an ohm value I need to check too?

Nope. The switch was bad on the wife's car. The high speed fan would come on when the car got hot enough, but it wouldn't kick on when the pressure on the high side of the AC system got high. The AC worked just fine, aside from that. The switch sends a signal to the ECU when the pressure in the system gets over 3 bar.
Someone posted a pinout of the switch, but I can't remember the thread, It's a 3-pin switch. power, ground, signal.

Basically all I heard with the test was a loud click of the relay trying to start the fan, then just stop. You will not hear the fan run.

If you retest with the app, and the relay signals are good, it's most likely the high pressure switch.



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Re: Low speed radiator fan not turning on  Topic is solved

Post by ZionXIX » Sun Aug 18, 2019 7:23 am

Today is the day people!
I changed the coolant temp sensor and what an incredible difference.

My old sensor would read about 206F when the coolant was expanded to the top of the reservoir and then begin seeping out at 208F.

The new sensor reads 208F and the coolant is barely expanded above the the MAX line. At 215F the coolant was only up to the middle of the word "MAX" on the reservoir. At 217F the low speed fan kicked on!

This information leads me to believe the coolant was running much much hotter before when it was at the reservoir cap. I think I will sleep better not knowing.

Anyway problem is fixed. I did not have to replace the high pressure AC sensor switch luckily. That thing is expensive. Of course if I have cooling fan problems in the future I might replace that next.
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Re: Low speed radiator fan not turning on

Post by abscate » Sun Aug 18, 2019 12:08 pm

Anyway problem is fixed. I did not have to replace the high pressure AC sensor switch luckily. That thing is expensive.
Very cheap and highly reliable PnP part - never buy new.

I harvested a couple from my parts cars, check part number if anyone ever needs one.


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Re: 1996 850 Low speed radiator fan not turning on [Bad ECT sensor]

Post by Chuck W » Sun Aug 18, 2019 12:43 pm

I'm pretty sure the low speed fan is supposed to turn on with the AC (when required) and the high speed with the engine temp.

If you get NO (low speed) cooling fan when the engine isn't "hot" and the AC has been turned on, your high pressure switch is most likely bad. (This assumes that you have a good Freon charge in the system)



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Re: Low speed radiator fan not turning on

Post by ZionXIX » Sun Aug 18, 2019 3:20 pm

abscate wrote:
Sun Aug 18, 2019 12:08 pm
Anyway problem is fixed. I did not have to replace the high pressure AC sensor switch luckily. That thing is expensive.
Very cheap and highly reliable PnP part - never buy new.

I harvested a couple from my parts cars, check part number if anyone ever needs one.
I like new shiny parts and the lifetime warranty though, and as far as reliability my sample is 1 so I haven't experienced the reliability factor. :D



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