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mexicola

Hello,

 

I have a problem with my 9N in that last week the temperature warning light popped up on the dash and now I don't seem to be able to get rid of. I'll try and summarise here:

 - temp warning light came on - it was a hot day but temp gauge read 90 (as it should) and the coolant level was fine

 - pulled over and engine cooled to 70

 - started up again and all ok. then warning light came back on after 1/2 mile. Coolant temp and level still fine but I waited a little longer

 - started again, light came on again, but I just drove home anyway because I didn't believe what it was telling me. Got home fine and the temp gauge didn't get above 90

 

Since then the light has come on randomly. Sometimes not at all. Sometimes 10 seconds after starting. Sometimes without starting.

 

So, my thoughts were that it's either a problem with the temp switch in the stat or the coolant level sensor. Or maybe the ECU (I'll get to that)? I unplugged the level sensor, cleaned the contacts, and reconnected after getting a good resistance reading (not open circuit). Started the car and it was fine. Shut the drivers door and stood watched the engine for a few minutes. Went and checked dash, still no light. Job done. Then I opened the drivers door and the temp warning light came back on with the interior light. And strangely since then driver elec window is manual only (if i press the button all the way it doesn't open on its own - I have to hold it down).

 

This door business is making me think it's maybe an ECU fault. I'm happy that the car isn't overheating and that the coolant level is ok. There's no leak. There's no oil in the water. The temp gauge has never gotten above 90. Could it be that maybe it did overheat and the ECU is confused now - it thinks it's overheating but it's not? I have a new coolant temp switch to try, but I'm not convinced that it the problem. I think it's electrical but I don't know where to start with checking. Was tempted to get a VAG COM cable but I have a mac and don't really want to install a partition for windows just for diagnostics.

 

Sorry, that's an essay, but it's not a straight forward problem. I'd be very grateful if anyone else has experience of this sort of problem and has anything they could share. Is there anything else I could try myself before I concede and take it to a garage? Thank you kindly.

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Pete1

This tends to be the coolant level probes in the expansion tank.  Cheap to replace the tank but if you want to rule out other causes (the circuit that drives the probes is in the instrument cluster circuit board), then get yourself a couple of old-school resistors with wire leads.  You need a 56 kilohm one and a 62 kilohm one. With the 56k plugged into the loom connector instead of the expansion tank, the circuit should not go into warning state, with the 62k resistor it should.  What reading did you get when you measured the tank resistance as "not open circuit"?

 

You'll have to cycle the ignition to clear the warning between tests.

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mexicola

Brilliant idea. Thank you. I'll get me some resistors and try that.

 

Because of where the contacts are and the narrow gap they're in I didn't get a solid reading - it was jumping about around the 20-30k ohm mark. I was just pleased that it wasn't open circuit and left it at that.

 

Could you please clarify what you mean by "cycle the ignition to clear the warning between tests"?

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mexicola

I just tried harder. It was skipping between 200-300k ohms. Maybe it's the probes?

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Pete1
34 minutes ago, mexicola said:

I just tried harder. It was skipping between 200-300k ohms. Maybe it's the probes?

Yep, pop a new tank in there and that fault should be gone.

Not sure about the cluster of other things, but measuring battery voltage at the battery posts before and after starting the engine often reveals things that may be good to know.

How much usage has the car had lately?

 

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mexicola

New tank ordered. Thank you kindly.

 

The other issues might be a strange coincidence. I just thought it was worth mentioning as I could well understand that the problem could be some sort of electrical "confusion", for want of a better term. The central locking only works on two doors and I don't open the passenger window because the first time I did I couldn't shut it for three days. There's something odd going on but I don't get it. Or it could just be tired. I'll check the battery voltages later on. What am I looking for and what can I learn? I'm a domestic electrician by trade and car electrics is a very different beast, but it's interesting.

 

Old polo gets a run out most days, usually only a handful of miles locally as I visit customers for work. And then a couple of 50 mile round trips each weekend. She works enough, but not too hard. And she usually runs like a dream. Albeit with the aforementioned electrical quirks.

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mexicola

Intrigue beat me and I've delayed dinner to go out with the multi-meter:

Before starting: 12.69V

Running: 14.48V

Afterwards: 13.01V

 

Is this fluctuation telling us anything? To me it sounds about right - but this could be where I'm limited by my working with AC current.

 

Also, I haven't been driving about in it for the past few days in case my diagnosis is wrong. But, with what's been said here, would you agree the problem is the probes in the tank and that it's ok to drive while I wait for the new tank? I'm happy my temp gauge works because I got my IR thermometer out and it's giving a similar reading. So I figure as long as I have water in the tank and the temp gauge is on the 90 mark, it's all good? I suppose the resistor trick would tell us true. My other theory was water pump, but that wouldn't bring on the warning light and I don't think it was actually overheating anyhow. It has to be the tank probes, right?

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Pete1

All good there, battery is fully charged, and alternator is working well. 

Yep, drive with confidence. 

Whereabouts in England are you?

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mexicola

Awesome. That's good to hear. And I'm glad you agree on the diagnosis. It makes sense to me but the fact you see it that way too makes me much more confident. Thank you kindly.

 

I'm in deepest darkest Norfolk: where "next day delivery" for your new expansion tank means you'll see it on Monday. It's still 1983 here. Hahaha. Nowhere near as glamorous or sophisticated as Oxford.

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mexicola

I was being most unkind - next day delivery was only two days late. Anyway...

 

Changed the coolant tank and the light was still on. So i went and did the temp sensor in the stat as well. Again, the light came on when I fired up. Oh dear. However, my thoughts at the point must be that the problem is electrical one way or another, as I've changed the two things that trigger the overheat/low coolant sensor, and I know that the level and temp are ok.

 

So I tried disconnecting the battery for a bit. No difference. Then I tried the trick to clear the "service due" spanner indicator on the dash. That cleared the spanner and the overheat warning light as well. Hooray! No I've had the thing running for nearly an hour and it's up to temp ok with no warning light. Fingers crossed that's done it.

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