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When I first got my S2 running it would get hotter and hotter with the engine running and the car not moving until it spat a load of coolant out of the header tank. During this eruption the fans stayed resolutely off.

Shorting the siwtch in the top hose in the engine bay switched them on so there was nothing wrong with the wiring or relay. I got a replacement switch from JAE but it made no difference.

For now I have wired up the rear foglight switch (which is, as far as I can make out, redundant on a US car) to switch on the fans manually but I would like the thermal switch to work. As I ma currently getting the radiator re-cored, I thought I would check the switch. I connected it to a multimeter and hung it in a kettle (the wife was out) and watched the resistance while the water warmed up. What I found was that just before the water boiled, the switch resistance dropped to about 550 Ohms. I was expecting a lot lower but, thinking that there must be some resistance in the circuit to stop fuses blowing, I checked what the relay's coil resistance was. It was only about 90 Ohms. This would mean that there is only about 2 Volts across the relay coil when the switch is on.

The switch is not the same as the original piece having Lucar connectors rather than round pins but it fits is in the pipe the same.

My questions are

At what temperature should the switch, switch on?

What should the resistance of the switch pp;be when it is on?

S4 Elan, Elan +2S, Federal-spec, World Championship Edition S2 Esprit #42, S1 Elise, Excel SE

 

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I have found these switches to be a bit of an Achilles heel. A correctly functioning switch should switch on around 80 deg. C - 80 to 90 is probably OK - but, as you say, the ON resistance should be close to zero ohms - otherwise you don't pass much current at 12v. The second day I had my Turbo, the switch failed and I have tried several aftermarket pattern switches and they have been very unreliable; the standard Lotus part, although horrendously expensive, has worked reliably for several years now. If you have an ON resistance of 550 ohms, I would recommend biting the bullet and buying the Lotus part - although I can think of no reason why the aftermarket switches don't last.

Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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I can't answer your question but looking at the circuit diagram it looks like it must be the the otter switch,

S2uswiring.jpg

See the bottom right of the diagram.

Hope that is some help.

Good luck!

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Trev, there are three Otter switches listed for your car.

82/72 Blue spot (hot climates)

86/76 (european climate?)

92/82 (cold climates)

The first temp value is on the second for off.

The same switch is used on MGBs with electric fans, the round pin switch is available here for $21.95 CDN

SUNP0003-1.jpg
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What should the resistance of the switch pp;be when it is on?

<{POST_SNAPBACK}>

The problem is over time any switch thats connected to a high current or inductive load will tend to get burnt and pitted, this is why you are seeing such a high resistance. Another point is that if your using something like a digital multimeter to read the resistance they only use a very small voltage and current during measurement which is good for not damaging electronic components but useless for really testing a high current switch as the low voltage doesn't break down the crap on the contacts. Idealy the resistance should be zero when its on but in reality it will get higher over time, however 550 ohms doesn't sound good. The best way to tell for sure is to connect a 12v 5W or so lamp in series with the switch and connect up to a 12v battery, then when you dip it in to the hot water if it doesn't light up the lamp is definitely knackered :) .

Martyn.

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I had the same problem on my Turbo.

The coolent dumping turned out to be a faulty pressure cap, however i replaced the Otter switch with an MG item and it has been reliable for 3 years now and half the cost of the lotus item, although the pins are round so a small adaptor connector was made up so that the genuine Lotus part could be fitted at a latter date if needed.

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I noticed that the S2 fan circuit isnt relay controled as on the S3 /Turbo cars.

It would take the load of the 3 fan motors off the Otter switch (prolonging its life ) to wire a relay into the fan circuit (just using the Otter switch to turn the relay on and off) :)

SUNP0003-1.jpg
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...however i replaced the Otter switch with an MG item and it has been reliable for 3 years now and half the cost of the lotus item, although the pins are round so a small adaptor connector was made up so that the genuine Lotus part could be fitted at a latter date if needed.

<{POST_SNAPBACK}>

Hi Andy

Out of interest, don't suppose you have a pic or a part number for the MG replacement part?

Think the Otter switch on mine's a bit dodgy, so looking for options to sort it out properly. And clean up the nest of wires and relays that live beside the switch

On a similar topic, does anyone know if the wiring loom to the front of the car is a separate item and can be split / unplugged? An auto electrician I've been speaking to wants to rip out the whole lot and replace it with something a bit more sensible.

Thoughts anyone???

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Found this in a catologue on my desk :)

MGBThermoSwitch.jpg

I found if the male and female bullet connectors and grounds were cleaned and re-assembled with dialectic grease , the wiring gave little to no further problems.

SUNP0003-1.jpg
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The switch is in the engine bay in the aluminium top "hose."

I do have a fan relay and a relay failure warning light.

The switch I replaced had the round pins. The one I fitted has the Lucar connectors.

Just ordered a new switch from JAE (Doh!)

The old switch was new (just re-read that bit. What I mean is that the switch I was testing was one I fitted about a year ago and has never switch the fans on) so I don't know why the connections are so cruddy.

Thanks for the temperature info, Wayne. Now I'm sure the switch is faulty.

I'll repeat the test when I have the new, new one.

S4 Elan, Elan +2S, Federal-spec, World Championship Edition S2 Esprit #42, S1 Elise, Excel SE

 

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Hi Andy

Out of interest, don't suppose you have a pic or a part number for the MG replacement part?

Think the Otter switch on mine's a bit dodgy, so looking for options to sort it out properly. And clean up the nest of wires and relays that live beside the switch

<{POST_SNAPBACK}>

The one that i've fitted is the one in the article from Wayne.

Thanks wayne saves me climbing underneath for a photo session. :)

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I noticed that the S2 fan circuit isnt relay controled as on the S3 /Turbo cars.

It would take the load of the 3 fan motors off the Otter switch (prolonging its life ) to wire a relay into the fan circuit (just using the Otter switch to turn the relay on and off) :)

<{POST_SNAPBACK}>

The S2 has a fused relay.

A common problem is the fuse to get corroded. Replace with a new relay with a newer blade type fuse ~$10

Graeme

Edited by GSimpson
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Err, I was looking at Angus's (Goose) wiring diagram posted above.

There is no relay for the fans on that one! (that I can see) :P

I presume its a wiring diagram for a Federal S1 then? :)

S2uswiring.jpg

Edited by WayneB
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Got my new switch. JAE didn't have one in stock, but Jeff there got Lotus Cars US to ship one directly to me. It looks different from the one that I took out. It still has blade type connectors but the part that sticks into the coolant has a bigger diameter. It also has a retaining clip that the other didn't. I will probably still use some lock wire to help hold it in, though.

Did the kettle test and the resistance drops down to less that 5 Ohms just before the kettle boiled. Got my re-cored radiator back yesterday as well. Gone from 2 to 3 rows of tubes so should be a lot better.

S4 Elan, Elan +2S, Federal-spec, World Championship Edition S2 Esprit #42, S1 Elise, Excel SE

 

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S2 has relay switched off otter switch. Also there is an additional relay for fan failure, essentially indicates if fuse is blown.

Diagram attached (hopefully)

If you need better detail let me know.

Graemewiring_s2.bmp

Edit

Can anyone see this attachment? Doesn't download correctly for me.

Anyone wants the diagram email me.

Edited by GSimpson
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Ahh... sorry I posted the wrong circuit diagram... :(

I should have realised that the 'S2' at the bottom of it was a bit of a give away... :D

Here is the correct one as provided already above but perhaps a little clearer. Sorry again!

s2diagram.jpg

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