[V6-12v] Radiator Fans not switching in correctly...

Tom Christiansen tomchr at gmail.com
Sat Sep 9 17:08:45 EDT 2006


It's a simple switched circuit. There's a thermo switch in the
radiator with two switches in it. One for low speed, one for full
speed. When the temperature in the radiator reaches the trip point for
the "low" switch, the fans get power from the battery through the
resistor pack, through the thermo switch. When the "high" temperature
is reached, the second switch in the thermo switch kicks in and shorts
out the resistor pack. There's nothing to it. There are no timers or
any other funkyness (that was the case on the 4000/5000 models of the
80'ies, but it does not exist on my 1994 90S). The fans are controlled
by the temperature of the water in the radiator only. The reason the
fans stay on for a while is that the thermo switch has some built-in
hysteresis. Otherwise the fans would kick in, turn off, kick in, turn
off... at a rather rapid rate.

Re expansion tank cap: Mine ran about $5. I'd just get a new one. I
don't know at which condition the valve in the cap is supposed to
open, but I do know that the pressure has to exceed 15 psi (about 1
ATM) for it to happen.


On 9/7/06, James Whitehouse <james_whitehouse1 at yahoo.co.uk> wrote:
> From testing this morning, the rad seems to heat up fully when getting up to
> temp from cold, then after the fans switch in once, it develops a cold spot
> near the top. Does that indicate a thermostat issue rather than a rad?
> The thing is that none of the cold spots are near the bottom where the
> thermo-switch is, so I still don't understand why the fans aren't on long
> enough to cool the engine down. What controls how long the fans run for? Is
> it a relay, or the actual radiator thermo-switch itself?

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