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Intermittent fan fault


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The AC system has another temp sensor mounted on the intake of the heater box under the scuttle trim that shuts off the AC in the event of low temps.

Would this cause the heater fans to come on full even when turned off? I get an P0480 fault code. Which states cooling fan 1.

So far, fans, battery, ambient temp sensor, both coolant temp sensors, and fan relay have been replaced. Fusebox has been stripped twice and cleaned, all strip fuses have been replaced, earthing points have been cleaned.

Getting 12.4 volts to fan connector and have continuity on the earth side. Fans operate just they stay on all the time and it brings EML light on dash.

Edited by Mikek3111
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Anyone know what ohm readings i should be getting on iat sensor?

Not sure on that, but they're generally NTC thermistors, which go from high resistance at low temperatures, to lower resistance when warmer. What are you seeing?

 

Just been looking at current flow diagrams (at last!) and the fan control module in the main fan seems to be 'informed' by a single wire coming from the engine ECU.  Can you confirm that there are two connectors coming out of the main fan module, a four-way with two fat and two thin wires; and a two-way with slightly less fat wires  going to the other fan?

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There is a fat one with 4 wires coming from harness into larger of the two fans then a 2 wire link that goes from fan 1 to fan 2.

Both fans come on at full speed regardless of engine temp or aircon on or off

In regards it iat sensor I get 1.772 ohms with sensor disconnected. According to my obd2 reader it's reading 89F/31C with engine cold so I'm assuming that's well.

Edited by Mikek3111
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Can't help thinking that it has to be the wire going to the engine ecu that's causing all this grief. At first glance it's the only one that isn't simply a supply or earth. Just scoffing spaghetti bol., back in a bit.

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Found a secondhand fusebox for £11 so thought it's worth a try. Should be here Monday/Tuesday so will fit that to rule that out. Tomorrow I will have to try and get the scuttle panel off and check the Ecu. Would have been handy if skoda had built in an inspection flap :(

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Worth a try, but as far as I can see, only a 50A fused permanent live comes from there to the fans (thick red), and possibly/probably an 'engine running' 12V from the fuel pump relay (thin blue/red, turning to black at the four-way fan connector).

The one from the engine ECU is green on the fan side of the four-way, turning to yellow/lilac on its way to the ECU. I wonder if this wire has an intermittent short to chassis that is causing the fans to come on flat out. May be able to measure for this by disconnecting the four way while the fans are misbehaving, and measure volts on this (yellow/lilac) wire. If steady at zero, switch to Ohms and see if it reads low Ohms to chassis.

Edited by Wino
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Yep, looks like the engine ECU dictates what the fans do, with some sort of control signal down that yellow/lilac wire.

Think it's direct, ecu to fan module, but out walking dog now!

Edited by Wino
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Yep, looks like the engine ECU dictates what the fans do, with some sort of control signal down that yellow/lilac wire.

Would that not be a pulse signal though? There is a large red (perm live) large black (ground) then 2 thin wires (canbus signal wires?) Edited by Mikek3111
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They don't look like canbus wires. One's 12V whenever the fuel pump is running. The ECU one probably is a PWM signal, at a guess. Actually probably LIN bus whatever that is, cos Techie said so.

Edited by Wino
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Diffrent car but did have this on a audi a4 and fans were running all the time even with key out . The bulkhead drains had blocked and got water in the ecu and relay housing. Ended up changing one of the relays and that stopped the fans running. Cant remember which relay it was tho.

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They don't look like canbus wires. One's 12V whenever the fuel pump is running. The ECU one probably is a PWM signal, at a guess. Actually probably LIN bus whatever that is, cos Techie said so.

Ok so I have continuity on 3 of them but the yellow/purple trace on I have 14.2 ohms? So I'm taking it that there's a break in this line somewhere?

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Diffrent car but did have this on a audi a4 and fans were running all the time even with key out . The bulkhead drains had blocked and got water in the ecu and relay housing. Ended up changing one of the relays and that stopped the fans running. Cant remember which relay it was tho.

Yeah I e already replaced that relay. Made no difference though

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Sounds like a possible short?
Could do with someone with working fans doing the same measurement. Though I guess if theirs has pulses of some LIN bus type, it might upset the multimeter.

Edited by Wino
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Just bloody annoying in that everything appears to be normal, from what I can see. Fans obviously getting power as they work, they just stay on all the time wether hot or cold and brings this code up.

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Willing to bet a tidy sum that tracing that wire back to the ECU, looking for possible shorts to chassis, isn't a piece of cake.

There's only about 3 inches of wire from the fans then it's all enclosed into the main harness that runs under battery tray etc. Not the easiest of jobs.

May have to wait till tomorrow, strip the scuttle out, find the wire at Ecu end, disconnect battery and connect other end to fan end and do a continuity test and see if that helps? Do you know if that wire is direct or branches into a fusebox anywhere?

Edited by Mikek3111
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Second thoughts I could have been purely reading the ecm's signal to turn the fan on? The ohm readings suggest battery voltage, therefore, if the ecm is sending power to tell the fan to turn on I could be measuring that reading. With the other 3 wires having continuity it would then show wiring to be OK would it not?

Edited by Mikek3111
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