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Old 25th March 2020, 16:27   #1
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Default How to Test Cooling Fan relay

I now have the front off my 2003 1.8 turbo, which has a habit of running the high speed fan most of the time and running on when switched off.

As I say, having removed the shroud, I am at the stage where I want to test the relays in the "control box" and ask the question, "how do I test the relays?" The resister is the original silver one, which has continuity.
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Old 25th March 2020, 17:08   #2
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Have a look here: https://www.the75andztclub.co.uk/for...=294123&page=9 post 83 SD1too

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Old 25th March 2020, 17:24   #3
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Have a look here: https://www.the75andztclub.co.uk/for...=294123&page=9 post 83 SD1too

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Many thanks for that Mike, I've put everything away now, so will try to translate this during this evening. Already observed colour differences, so will test tomorrow, thanks.
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Old 25th March 2020, 18:25   #4
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Thanks for this Mike.
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I now have the front off my 2003 1.8 turbo, which has a habit of running the high speed fan most of the time and running on when switched off.
Fred, if the fan runs on for more than a few seconds then it indicates mild overheating (coolant reaching 108 degrees). What was the result of the "demist test"?

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Old 25th March 2020, 19:00   #5
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Thanks for this Mike.

Fred, if the fan runs on for more than a few seconds then it indicates mild overheating (coolant reaching 108 degrees). What was the result of the "demist test"?

Simon
Thanks Simon. As I have the whole of the front off I am going to work backwards again, eliminating those relays while I am there.
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Old 26th March 2020, 08:27   #6
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... eliminating those relays while I am there.
You have already eliminated the fast speed relay since you've reported that it is turning on and off. You can test the slow speed relay by doing the 'demist test' which is why I asked about that. If you haven't already done it I recommend that you do this test now, before anything else.

The relays are very reliable. It's very rare that a forum member reports a failure but remember that if this does happen, the relay contacts stick together permanently which results in the fan running constantly. As I understand it, that isn't what you're experiencing.

Don't get hung up on the relays Fred. The first thing we need to know is whether the slow speed is working. The easiest way to check that is the 'demist test'. Just a reminder that the engine needs to be running.

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Old 26th March 2020, 10:05   #7
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You have already eliminated the fast speed relay since you've reported that it is turning on and off. You can test the slow speed relay by doing the 'demist test' which is why I asked about that. If you haven't already done it I recommend that you do this test now, before anything else.

The relays are very reliable. It's very rare that a forum member reports a failure but remember that if this does happen, the relay contacts stick together permanently which results in the fan running constantly. As I understand it, that isn't what you're experiencing.

Don't get hung up on the relays Fred. The first thing we need to know is whether the slow speed is working. The easiest way to check that is the 'demist test'. Just a reminder that the engine needs to be running.

Simon
Thanks Simon for your constructive advice. I understand your suggestion and will attend to it, when I am satisfied that with the amount of gear off the front of the car is not going to fly off in all directions. But, while I have those pieces off the car and everything open, I asked if someone could suggest a way of testing the relays. I do see what you say in respect of the fast speed proof, though when I'm looking at the two relays, there is nothing printed on them to indicate to me which is high and which is low.

I am back to the car in ten minutes and will again assess your advice, thank you.

Thankfully just found the wiring diagram in Haynes, though the picture reference earlier has different colour wires.

Last edited by planenut; 26th March 2020 at 10:08..
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Old 26th March 2020, 10:49   #8
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Hi Fred.
In addition to what Simon is saying have a look at the thread below, also change the silver resistor as par for coarse to a gold one.


https://www.the75andztclub.co.uk/for...ead.php?t=1371
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Old 27th March 2020, 08:06   #9
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I do see what you say in respect of the fast speed proof, though when I'm looking at the two relays, there is nothing printed on them to indicate to me which is high and which is low.
On a resistor system the large relay controls the fast speed and the small relay (located in the centre of the control box) controls the slow.

Regarding testing the slow speed relay, as you say that your silver resistor has continuity, running the demist test will prove that. It really is the easiest and safest way to check the relays but you can, if you prefer, remove them for testing on the bench using a 12v supply and your multimeter across the contacts.

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Old 27th March 2020, 08:15   #10
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Well as I am on "call out" for work, I have put the whole thing back together, and not wanting to sound big headed, but a lot of you will understand, I put it all back without resorting to the manual and with no bolts left over. Once you've done that it does make you realize that though there is a lot involved, it can be done. So, if I get called in, I can test the car.

As to the testing of relays, well it produced no result that I understood, and yes, I had looked at the need for replacing that resistor but could not see how one would extract it and connect it's replacement. I will research that when I'm stuck in work, on my own.

Whilst all dismantled, I had another look at my non-working n/s sidelight, establishing that the "break" must be somewhere between the switch and the multi-plug, in the loom I suspect. So, it's well hidden and not sorted but may just have to run a wire from somewhere, at another time.

Thanks to all for your suggestions and advice, it is appreciated, but it does require someone at this end with a bit more ability maybe. At least I know now where the go when I have done some more reading of the advice.

With the current situation regarding the virus, and the fact that all of my pre-arranged events up until September have been cancelled, I will have some time to get back into it.

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