What Is The Advantage Of Having Fans Run After Ignition Is Turned Off

I noticed on another thread (about a relay and connections ) that 77 MGB fans would stay on after the ignition was turned off if the temperature in the radiator was high enough. Was also noted that 78-80 would not.

What would be the advantage/disadvantage of having the fans run after the ignition is turned off?

PS: The hubby has both my 79 & 80 wired so that "do stay on" when the ignition is off if the temp is high enough.



Your turn,

Running teh fan after turning the engine helps to cool the water that is in the radiator. For what it is worht most race cars do it. Now to REALLY be helpful you would also need and electric water pump so that the watyer in the block can be circulated too.

When I got my '78, there were many problems. One problem was that when stopping for 10 or 15 minutes, the car would be hard to restart and would not idle for maybe 30 seconds after starting. It acted like it had vapor lock. I wired my fans to stay on with the key off when hot, and that problem went away. I assume the carb just got too hot sitting there right above the hot cat without any air flow.
Since then I've fixed a lot of things on the car, new radiator, new (homade) heat shield, carb rebuild, etc. A while ago my alternator flaked out, barely keeping up with the drain and the fans were draining the battery too much, so I changed them to come on only when the key is on. My hot restart problem is still there, but to a much lesser degree.
I've since fixed the alternator, but left the fans in the key-on only configuration.

So there's an advantage and a disadvantage for you.

-Jim

The sensor / switch is mounted on the radiator. When you shut off the engine the radiator has no coolant flow so its' temperature rises and the fan comes on.
Heat soak happens. This why it is a good idea to idle for 30 seconds before shut down.

For a short period, a lot of manufacturers had the cars wired the way your's are - but I don't recall seeing any new car wired like that for quite some time. That tells me that the manufacturers must have decided that the negatives outweighed the positives.

Yeah, I'm with Joe on that. Lots of earlier electric cooling fans did that, none anymore, that I'm aware of.

JackMG Wrote:

Some new vehicles still use the cooling fan "on" strategy (e.g. Jaguar & Land Rover), after the ignition has been turned off. It all depends on what the engine control module sees for an engine temperature signal (e.g. overheating). Why some manufacturers use that strategy and some don't can all come down to the strategy used to run the engine cooling system and the engine type.

Why? I don't know. Mine are wired in the stock '78 configuration, only able to come on when the car is on.

Advantages I have experienced, I moved the white/brown wire to the fans from the third fuse to the fourth fuse to bypass the ignition switch, the engine compartment cooled quicker when my radiator was restricted, helping prevent vapor lock. After I replaced my radiator, a lot of my running problems I had that were caused by excessive heat went away, so I wired my fans back to the stock configuration.

Disadvantages, if the thermo switch failed in the closed position, the fans would run until the battery dies.

Japanese cars did it so why not ours. roflmao

My girlfriend's VW Cabrio has the same deal of running after you shut off the car, but it will only do this when it get pretty hot out side.

I wouldnt think there would be any disadvantage to having it keep running as long as you dont loose and juice when you have to start the car again.

My car has an extra fan in the front with a toggle switch in the console and is setup so it is "hot wire" all the time. If its gets hot out, I'll turn the car off when I stop and let the E-fan run a tiny bit.

I think the reason the manufacturers initially used to run the fans after the ignition was turned off was mostly due to simplicity. You don't even need an ECU - a simple thermostatic switch will do - if the temperature sensor shows hot, then run fan.

The problem with the strategy is that it doesn't accomplish anything to run the fan after the engine is off. First of all, it isn't very accurate - the fan cools the radiator water - but the sensor is in the head. So, the radiator could be cool and yet the fan still runs. Secondly, as noted by others, cooling the radiator has very little effect on the engine temperature once the water pump is off. But it is a simple, easy strategy. As noted, sensor failure can cause your battery to run down. There may also be liability issues as the fan can start while somebody has their hands in the engine compartment.

Never gave it much thought before, but perhaps the bit of air flow through the compartment might help prevent fuel "perk" in cars with carburetors, thus making hot starts easier after a brief shutdown.

It would be an interesting experiment to measure carb temp under various conditions to see if it actually accomplishes anything worthwhile.