Strange temperature / radiator fan issue

[Outrun944]

Hey guys,

My radiator fan seems to be kicking in a lot earlier than usual, and staying on after switching off for quite some time - even after short-ish runs. Even if driven really slowly, carefully and not overdoing it doesn't make any difference - an oil top-up to full seems to have made a very minor difference (hard to tell sometimes!).

Not a major issue - but (1) it's a bit annoying and embarrassing hearing the fan whirring away like an industrial hoover every time I pull and (2) bit worried it'll burn out the motor or drain the battery too much one day.

My garage so far has been unable to locate the cause of this, but temperature readinga indicate it's coming on a few degrees lower than it should.

Any ideas?

That's the quick summary, more in depth below...



Back story first -
In June 2012 there was an epic quest to fix a hot start up problem with it cutting out not long in (Topic here) - in the end it turned out to be the ECU and replacement fitted. Also replaced was the ignition coil, some wiring that had seen better days, battery connections, further cleaning on various sensors and air flow meter. It was then back on the road better than ever, though wasn't driven a great deal for a while and was put off the road over the winter. Early this year I was back in it and all seemed normal. A month or two later I noticed the radiator fan not coming on at all.

Another fairly epic fault finding quest started - the fan and motor was fine, temp sensor replaced twice, ECU seemed fine, lots of wiring replaced and points/earths checked. But no joy - mystery! We basically gave up - the next course of action would be to test/replace the ECU but had a nightmare with that last time - so we decided to fit in parallel a manual switch and relay to the fan (using one of the spare unused switches on the centre console in front of the ashtray) - which I can switch on when required (and works). After this was fitted - bizarrely - the fan was working again by itself!! Very strange, but with all the re-wiring maybe we inadvertently fixed something....!

The only thing is, it's kicking in at lower than usual temperatures and staying on for long periods of time after parking up.

Thanks guys!

 

[Riverside]

The fan isn't controlled by the ecu (or any kind of computer), just the thermostatic switch and a relay.

If it's coming on too soon I think it can only be because you have a different spec of switch.

 

[Riverside]

There are three temp sensors for the engine: One for the fan (in the rad), one for the dash gauge & one for the ecu. So you are right that the ecu takes the temperature into account, it just doesn't control the fan []

Can't remember which way round the fan switch works, whether it increases resistance (opens) with temp or reduces (closes), but it could be that your manual switch has changed the overall resistance, causing the fan relay to read low. I would also guess that the reason it started working again was because of a bad earth in the original circuit.

 

[Outrun944]

Yea the manual switch changing the overall resistance - that definitely sounds plausible.

Out of interest if the ECU is monitoring temperatures - what exactly is it measuring and then what does it do with it?

Thanks!

 

[GlennS]

If you are basing your diagnosis on the readings from the dashboard temperature gauge, I'd take the path of verifying that reading first. There are common faults which cause that gauge to read low.

 

[Riverside]

ORIGINAL: Outrun944
Out of interest if the ECU is monitoring temperatures - what exactly is it measuring and then what does it do with it?

AFAIK it just measures coolant temp and adjusts the fuelling according to the map. Air temperature is sort of measured by the AFM (Air Flow Meter, a flap in the intake that is moved by air pressure; more air flow = more movement, colder air is more dense).

It's a pretty basic system compared to a modern ecu controlled engine, but ime easier to diagnose than a modern car.
I've recently been working on my latest shed, a Mk4 Astra. It only has one coolant sensor, which is read by the ECU. The temp gauge & fan are therefore controlled by signals from the ECU. This sounds sensible but the fan wasn't coming on & it wasn't a broken fan or connection, so the fault diagnosis was much more involved. It transpired that the ECU sends a simple signal to a second ECU that controls both the coolant and aircon fans, and this second ECU had a few brittle solders on the board that I repaired (replacement would have been £120 and electronically keyed to the car requiring a dealer to plug it in). Couldn't even wire up a manual switch like you did because it screwed with the main ECU if it couldn't 'see' the fan working.

The Porsche is a pleasure to work on compared to more mass produced tat like the Astra.

 

[Outrun944]

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