Electric fan with ignition on or off?

[jeremyc]

The aftermarket electric radiator fan on one of my classics is wired to operate even if the ignition is off.

Is there any advantage (or indeed disadvantage) to this? There's no water circulating through the radiator/engine after it has been turned off.

On my two other classics the electric radiator fan only runs with the ignition on.

[Chris32345]

If the engine is really when tired off the residual heat could cause it to locally boil

I suppose running for a minute or 2 after shutdown when very hot would lessen the risk of that assuming some of the air flows of the engine as well


But just guessing then again post Cara do this nowadays so there must be a reason for this

[GreenV8S]

The main disadvantage I can see is that the fan may keep running for significantly longer than necessary to cool down the contents of the radiator, because the coolant isn't circulating past the temperature sensor. Once the radiator is cool the fan gives much less cooling benefit but will still be washing heat soak out of the engine bay. It's unlikely to run for a very long time because the coolant temp sender doesn't have to cool down very far to get below the fan on temperature.

(I combined this with an electric water pump and it was spectacularly good at cooling the engine down.)

[Belle427]

Just make sure you have a good battery.
Sticking on would be my only concern but lots of cars have it this way.

[tux850]

My wife's Fiesta has a little pump that can run with the ignition off to continue circulating the coolant, primarily to help cool the turbo if required. Looking at he schematics the fan is unable to run though in such a situation.

I think my only concern with an always-powered fan circuit might be the risk of it coming on whilst I was working in the engine bay. Of course, I ought to be always disconnecting the battery anyway but I've got to hold my hand up and admit that I quite often don't.

Edited by tux850 on Friday 13th October 09:53

[donkmeister]

Modern cars, even with an engine driven water pump, allow for the fan to continue running after switch off. Maybe there is still a small amount of thermosyphon action when the coolant is that hot?

Personally I like the idea of a small electric pump wired in parallel with the fan (and plumbed in series with the engine driven water pump)... could also be belt and braces if your engine driven pump fails for any reason.

Fans and small pumps are relatively low current and won't be run that long so a flat battery isn't really a concern provided you don't have it running until back down to room temperature.

[GreenV8S]

donkmeister said:

plumbed in series with the engine driven water pump

A small EWP in parallel with the mechanical pump is fine, but plumbing it in series with the mechanical pump is a very bad idea; it will restrict the water flow severely when the engine is running. Just look at the cross section area of the passage the water has to flow through within the EWP to understand the problem.

[donkmeister]

GreenV8S said:

donkmeister said:

plumbed in series with the engine driven water pump

A small EWP in parallel with the mechanical pump is fine, but plumbing it in series with the mechanical pump is a very bad idea; it will restrict the water flow severely when the engine is running. Just look at the cross section area of the passage the water has to flow through within the EWP to understand the problem.

That's fair, looking at electric secondary pumps they are small.

Not sure if parallel would be easy to plumb; the (admittedly small number of) water pumps I've replaced only had one pipe connector (for the inlet) with the outlet being straight into the engine casting. Maybe the easiest thing would be to drill and tap a hole into the water pump casting.

[Polly Grigora]

Peugeot were the dogs bks years ago when knocking out many versions of the 306 model that included a 1.9 diesel

My 1997 1.9D has fans that run on after parking up, no pump to circulate the water is fitted

There's no way that the above set-up would be used by Peugeot without good reason unless someone in the engineering department pulled a fast one for more over-time on a Sunday that paid 2.5 x the daily rate