Ventilation fan varies?

liuleichan · Oct 15th, 2002, 08:46

The fan speed in my 98 S70 (manual aircon) seems to vary, sometimes.

If I have it on position 2 or 3, it can seemingly sound as if it's running at normal speeds, or sometimes seems to make less noise.

Doesn't seem to matter whether I've got aircon on, or not - in fact seems most noticeable with it off.

It's not related to the re-circ function, as it's very rare I use that.

I'd like to think it's most apparent when waiting at junctions, or lights - but perhaps that's merely where I notice it, perhaps with attention and noise it's either not noticeable when moving - or possibly doesn't happen them.

It doesn't seem to occur at the higher fan speeds (ie 4 or 5).

Anyone any suggestions?

Lei Chan

sheerwater · Oct 15th, 2002, 09:25

Could be due to voltage variation or load on battery. This used to happen a lot on dynamo cars. I'll be corrected on this but... the car battery is 12 volts and the alternator puts out 14.4 volts. The alternator supplies power to the electrical system but obviously the faster the engine runs the more powwer it puts out. As you sit at traffic lights the engine ticks over, the alternator probably does not supply enough to power all the lights/radio, fuel pump etc so uses power from te battery to back up the shortfall. We have a situation where the voltage iin the system is running at anything from 14.4 v at speed to 12 volts at tickover. The fan being a motor reacts accordingly. At heavier loads i.e. starting the voltage drops well below 12 v and this is why cars sometimes have difficulty with starting iif the battery is low. Cheers Nigel

pettaw · Oct 15th, 2002, 09:37

I would say that electrical voltage doesn't vary that much now because alternators are so much better now than they were before.

I agree with your comment about battery voltage. You need enough volts to crank the car over, but you also need enough volts to actuate the ignition and engine management systems and tell them 'I'm trying to start my car now, please turn on the fuel, thank you!!' Irritatingly enough, the point you need the most volts is on compression which is where the battery and starter is working hardest.

I would possibly go for a resistor pack problem, or possibly a dirty connection either in the resistor pack or fan.

I dunno what others would do, but its such a job getting at these things that it might be better leaving it until it packs up completely.

Oct 15th, 2002, 08:46	#1
liuleichan Premier Member Last Online: Apr 17th, 2019 22:01 Join Date: Dec 2001 Location: .	Ventilation fan varies? The fan speed in my 98 S70 (manual aircon) seems to vary, sometimes. If I have it on position 2 or 3, it can seemingly sound as if it's running at normal speeds, or sometimes seems to make less noise. Doesn't seem to matter whether I've got aircon on, or not - in fact seems most noticeable with it off. It's not related to the re-circ function, as it's very rare I use that. I'd like to think it's most apparent when waiting at junctions, or lights - but perhaps that's merely where I notice it, perhaps with attention and noise it's either not noticeable when moving - or possibly doesn't happen them. It doesn't seem to occur at the higher fan speeds (ie 4 or 5). Anyone any suggestions? Lei Chan

Oct 15th, 2002, 09:25	#2
sheerwater Master Member Last Online: Feb 24th, 2019 17:59 Join Date: Dec 2001 Location: Northampton	RE: Ventilation fan varies? Could be due to voltage variation or load on battery. This used to happen a lot on dynamo cars. I'll be corrected on this but... the car battery is 12 volts and the alternator puts out 14.4 volts. The alternator supplies power to the electrical system but obviously the faster the engine runs the more powwer it puts out. As you sit at traffic lights the engine ticks over, the alternator probably does not supply enough to power all the lights/radio, fuel pump etc so uses power from te battery to back up the shortfall. We have a situation where the voltage iin the system is running at anything from 14.4 v at speed to 12 volts at tickover. The fan being a motor reacts accordingly. At heavier loads i.e. starting the voltage drops well below 12 v and this is why cars sometimes have difficulty with starting iif the battery is low. Cheers Nigel

Oct 15th, 2002, 09:37	#3
pettaw Forum Support Team Last Online: Jul 23rd, 2023 15:29 Join Date: Jan 1970 Location: Glasgow, London	RE: Ventilation fan varies? I would say that electrical voltage doesn't vary that much now because alternators are so much better now than they were before. I agree with your comment about battery voltage. You need enough volts to crank the car over, but you also need enough volts to actuate the ignition and engine management systems and tell them 'I'm trying to start my car now, please turn on the fuel, thank you!!' Irritatingly enough, the point you need the most volts is on compression which is where the battery and starter is working hardest. I would possibly go for a resistor pack problem, or possibly a dirty connection either in the resistor pack or fan. I dunno what others would do, but its such a job getting at these things that it might be better leaving it until it packs up completely.

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