I mentioned a few weeks ago that I had the parts to realise a delayed shut off of the fans on my S3.
I have now hooked it all up and it works as follows:
Once the ignition is turned off, the fans will run for up to 15 minutes (adjustable) if the otter thermo switch is shorted due to the high temperature of the coolant. If the otter switch opens, the fans stop. If heat builds up and the otter switch closes again within the set time (max 15 minutes) the fans will run again.
It can be modified to run for the set amount of time, regardless of the otter switch.
Its basically a Hella timer relay, a diode and 4 short wires and connectors.
In addition, I have run a wire for a “fan on” control lamp if I decide to install it.
Here are some pictures.
Disconnect these two:
I like it. On my S2 I made a short jumper to connect my fan circuit to one of the other fuses that is hot all the time, I think it was the hazard lights. This allows my fans to come on after the car is off or to run on until the otter switch opens, but I would be lying if I didn’t say it worries me that they could run until the battery goes flat if the switch sticks or whatever else. So for that reason I am going to look into this as an add-on to the mod I already made.
What you really want is water circulation rather than air circulation as the only water moving after shutdown is that due to any syphoning effect.
After shutdown, the water sitting in the cylinder heads continues warming up and will easily reach 95’c after five minutes. By contrast the huge volume of water in the radiator, which is insulated and well away from the metal of the engine block and served by having lots of cooling fins, has dropped in temperature by a similar proportion.
From looking at the many temperature sensors around my engine, what appears to work best is to wait five minutes and then run the engine for about 90 seconds. This is just enough time for the water in the engine block and cylinder heads to be displaced with the cooler water from the radiator. The temperature of water exiting the heads drops to high 70’s 'c and there is then plenty of headroom for it to absorb more heat from the heads without getting truly hot. Running the engine longer than that serves no purpose as the reservoir of “overcooled” water is used up and is replaced with freshly heated water and thus the thermostats will close and remove any benefit of further running.
Running the fans on their own can last much much longer and simply use up battery power.
The same principle ought to apply to the six cylinder engines.
Dear Marek,
I agree with you wholeheartedly, and off course see this as the next best thing.
However, unless some makes have electric water circulation and have that running as well, this is what many car manufacturers are doing today.
Regardless, it will get rid of some of the under bonnet heat and help cool down the engine as the air is passing on either side of the engine block and heads.
There is room for an inline water circulating pump in the “plumming” to and from the oil cooler in the oil pan, I just haven’t done my research yet and don’t know what is available.
For those interested, here’s the wiring diagram and product details for the Hella relay and the diode used to enable control of the fan relay without sending 12V back to the rest of the car when the ignition is switched off. I tested it live yesterday when the engine was started for the first time in 21 months.
The delay is set by the dip switches on the relay in combination with an adjustment next to the switches.
Hi Ole
We set up the Hella relay to the wiring diagram you have. Running into an issue. After we shut off the engine we cannot get the delay to work more than 30 seconds… we would like to get 5 minutes. We have tried the adjustments by the dip switches and adjustment next to the switches. Everything we tried and we still get 30 sections. Any suggestions on what that could be due to? How long a delay do you have
Sandy
Ole
On your opening paragraph…you say “It can be modified to run for the set amount of time, regardless of the otter switch”. How is this done?
Sandy
Hi Sandy,
The dip switches determine the amount of time that fans can run, but the otter switch has to provide the ground connection to the other side of the relay for it to switch the fans on. If however, you have an otter switch override switch to keep the fans running, then the dip switches will set the amount of time.
You can try to connect the two leads on the otter switch with a piece of wire to see if the fans will keep running as you’d expect, bas d on the dipswitch settings.
Any help ?
