HEVAC: fans "hunting"

[prelude]

Gents,

For starters, it's winter down here, usually around 0c lately. Like most climate controls I know the range rover seems to not start blowing it's fans until the coolant temperature reaches at least the set temperature. Since a V8 is highly efficient [bighmmm] the coolant get's up to temperature in no time and the fans kick in. When driving away from the house I need to make a few turns and slow down to the point that the car reaches idle again. I can understand the drop in voltage since the alternator can not keep up with power demands so the lights dim slightly and fans turn a bit slower, however, the fans go almost indiscernible. When I drive off again it takes a while for them to pick up again. Under certain circumstances the fans cut out when driving but usually only when the car has had a cold start. If I drive for a day (like yesterday, 850km) the problem never returns.

So I am puzzled. The behaviour seems logical, perhaps the internal radiator cools to fast for the engine to provide more heat (seems unlikely) or the reduced flow cools the coolant in the radiator faster. Does anyone have / recognize this behavior or do I need to worry that my HEVAC unit is beginning to go?

Cheers,

-P

[PeterH]

I would be checking the battery voltage, battery terminals and alternator as a first point.
P38's need a very strong battery to operate all the systems properly, from what you describe, it kind of sounds like not enough voltage present.
Sometimes these things start happening and you barely notice them getting worse over time.
Do your fans kick in when you switch the ignition to the second position without starting the engine?
On mine, the the fans stop during engine start (as far as I can tell over the engine sound), then kick in again shortly after that, where they stay constant.
If the battery and alternator check out ok, the next thing I would be checking the the fan relays, they may possibly be affected by the cold temps, then work normally once the engine bay heats up.
That's what I would do first anyway!
Hope that is of some help,
Cheers, Pete.

[prelude]

It's been a while but I have been observing and testing the hevac.

I tried: testing the voltage. my faultmate reports 13.something volts, it varies a bit between idling and making sufficient rpm for the alternator to work when driving of course but it seems to be within acceptable parameters. I measured with a digital voltmeter across various points in the hevac system but I can see no discernable difference. Being digital it is slow to respond to small, temporary changes so I can not rule that out, I would have to take the scope out for that.

My fans never kick in when turning on the ignition (at least not noticeable to me) but I think that is normal with the current temperatures down here. They are nearly always below the set temperature and the hevac only kicks in when it detects the water temperature of the heater to be sufficient to at least match the set temperature. However, once the engine has run for a little while or I have started driving the fans kick in mostly in a fairly high setting. Whilst driving the fans switch down to their lowest setting and within, say, 10 seconds they ramp back up to their required speed. At first I thought this was with a fairly cold engine only, but recently we had 14c temps and I drove for 20 minutes and the fans kept doing their thing :(

What I have noticed: both fans respond equally, this makes me suspect the hevac itself. Setting the fans manually instead of full auto they keep their speed, except when coming to a halt. The drop from the alternator is discernable but only just.

My only conclusion thus far: Perhaps the water temperature sensor has a fault in it or does not make sufficient contact. I'm not sure how to test that as of yet but my first thought is to place a resistor over the thermistor to fool the hevac into thinking it always has at least, I dunno, 50c water so it should never adjust the fans. Only downside is the fans will start in the cold blowing in cold air, but for testing this is acceptable :)

Keep you guys posted, any thoughts always welcome.

-P