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Rad fan, still not working, diagnostic approach?


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I'm still struggling to get my radiator fan on.  I've had the car up to 108c according to my OBDII tool, though Derek @ CC disputes this will be accurate.  I'm not entirely convinced with that, in my experience if a protocol can be negotiated and read, it's going to be as intended.

Assuming Derek is correct and my reading is too low I should let the car get hotter, however, I'm just not comfortable with that, its nearly at the red on the analogue gauge anyway!

I'm therefore left to check for electrical issues.

I've checked the loom plug which connects to the radiator, its grounded on the black wire, open on the other, which seems correct.

I've checked the relay operation with a bench supply and its switching OK.

I'm therefore considering the connection between the low and high current side of the relay might be an issue.  Is there a trick to test continuity of relay connections within the fuse board..

I'm thinking I need to check continuity between;

  • Relay low current side to ground and ECU signal pin (not sure which it is yet)
  • Relay high current side to ground and also fan connector positive.

I don't want to insert a round probe from my meter into the flat receivers on the fuse board, is there a special diagnostic insert I can use for this?

Sorry never done this sort of work before.

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Most blade fuses have small contact points on the top that'll enable you to measure there without removing them. You can then measure across to see if they're blown and you can measure to earth to see it's live.

Which ECU does your car have? I am not sure I have the correct wiring diagram - or would you be able to email the correct one to me? I appreciate that the format nowadays is a bit different but as wires have numbers etc I trust I can navigate across it.


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RJ, I'm confident the fuses are fine, its the relay circuit I was looking at, but I guess you are saying check the fuse to the plug right?

Sorry not sure on the ECU, and I don't have a wiring diagram.  I was going to try and look at the wire colours....

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CC only sent me the Ikea manual which is useless as it only covers Sigma cars, and I have a 420.  I have sourced an electronic copy of the 2015 manual but I don't believe it is entirely up to date.

Derek sent me the following diagram in response to one of my queries, it suggests I have a 620 loom... who knows.


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Beaten to it. Does the relay have some sort of numbering on it?

30 would normally be the +ve feed and 87 the output - there's a chance that the +ve is always on and that it's a swiched earth in wich case I'd expect 30 to be earth in the socket.

The output of the ECU would be switched earth.

Can you post a picture of the ECU or at least the connectors to give me an idea of what we're talking about?


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Can't help you with a picture as I don't have a 620R to hand and here there's no such thing as a 420(!)

That said it sounds like the black / green has to go in as marked with a red arrowImage21-02-2019at13.30_0.jpg.6d153619d49bdf1b146e779575c53a53.jpg

I would also expect the one marked N 29 to be a +ve feed and therefore if you short this and the black green the fan should come on.

I doubt it's an airlock in the rad as I believe the rad is controlled via the ECU - or is there a thermoswitch in the rad?

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My bet is the connection between the ECU and relay.  I suspect the software / ECU is fine, or we would have 420s over heating all over the place.   It's been know for the pins not to be seated properly in the ECU connector, I wonder if that the case for your fan control wire from the ECU. 

If you have tested the relay, fan, fuse  and the continuity between the fan and the relay, the continuity with the fan and earth, then it would be next on my list.     

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