Testing cold start thermo switch.

I have a cold start issue which I assumed was  my thermo switch playing up. When cold, If I bypass the switch it starts, if I don’t it won’t. Opposite way around when it’s warm. So I ordered a new thermo switch. But I’m having exactly the same problem. Am I correct in assuming there should be continuity between the poles on the switch when cold and not when warm, is it that simple or am I missing something?

Thanks


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Have you a multimeter?

Can you give the VW and Bosch part numbers, generally they have the temp / duration in seconds also stamped if you can give that too.

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I'm assuming you've a MK1 1.6 / 1.8 from your avatar, if so that's the correct switch.

This is a cut'n'paste I made elsewhere, should hopefully translate to what you're wanting to do…



"Initially you don't use the connection at 'A' (between terminal 'G' of the switch and the postive of the battery) but instead with everything else connected as shown that in-line bulb should stay on indefinitely. This indicates the switch is earthing correctly.

You can then further test by including the connection at 'A' (you are now supplying power to the bi-metal strip inside the switch allowing it to heat up), doing this will cause the earth path to be broken after a short period of time and the bulb will extinguish.

If you want to repeat this part of the test you'll have to let the switch cool down for around a minute or so before retrying… lets the strip close once again, thereby completing the earth path.

The above test will only let you know the switch has a proper earth and that the bi-metal strip is capable of turning it off /on.

With all the switches I tested, the bulb turned off on each after exactly 3 seconds, no variation between them

If you've a multimeter you can then check the approximate points at which the bi-metal strip activates and that the ranges it does so are within spec.

Problem is I can't find anything definitive stating what those specs. are for a MK1 GTI, just those for several other marques that use the same switch.

Having said that the figures I've been getting, taken across a sample of four separate switches (all the correct to my car Bosch 028130214 / VW 043906163A) not only come in at virtually the same readings (maybe 0.2 ohms variation at most, which in itself suggests they are working correctly) let alone also tallying with the readings for those other marques I've found on line… if you've a switch of your own and can check, it'd be very useful for building up a bigger sample base, the more we can find with similar readings, the better it'll bolster the case that those we already have are accurate.

So… it's as simple as taking the ohms at cold, all 4 of mine were 35.8, then getting a cup of boiling water and immersing the thinner part of the switch entirely into it.

It'll take a minute or two, but you'll see the ohms suddenly jump from 35'ish to the high 60's / low 70's. All mine had higher readings that ranged from 68.9 to 70.03ohms… again very little variation between them.

If you then let the switch cool (or run it under the cold tap) it'll do the reverse and react by suddenly dropping back down to whatever point it started at.

All mine were 100% consistent throughout these tests in both bulb on / off time and their cold / hot ohms over repeated tests"

Thanks. I’ll get on and test them. Thanks again.


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When you get the Ohm readings for both your switches could you post them up... trying to build up a picture of the points that they switch from across as many examples as possible, cheers.