Lotus Elan

Cooling and electrics

PostPost by: gav » Sun Sep 15, 2019 12:00 pm

Hi all

Running with two cooling issues at the moment. One is straightforward and the other I think I need help with.

Background is that I am running a large electric fan which has a 14.5A initial draw. I fitted it to manage spirited driving and it is spectacularly efficient.

When driving, the rad needs a little help from the fan which I think is due to air bypassing the rad. The rad is in good condition and the cooling system is fine. This is the straightforward fix - I need to sort out a rad shroud I will also look at helping the air to escape the engine bay better.

The other issue is with the electrics that control the fan. I have used a 30A relay triggered by (originally a signal from the ECU but I abandoned this when it was dragging the alternator when it triggered because I didn’t want to risk ECU damage). The switching part of the relay is triggered by an earth connection so it has a permanent feed into the relay and when the switch is turned on it closes the relay to earth and triggers the power to the fan.

I fitted an otter type switch (from Burton Power) at the top of the rad and wired it to close to earth when the temperature triggers in the otter. In addition, I have a dash switch that also closes to earth should I want to override the otter. The trigger on the relay share the same terminal on the relay (otter and override both going to earth)

So here’s the thing - The override switch works fine but the otter doesn’t trigger the fan even though I checked with a meter. My sense is that the otter is designed to run a positive signal rather than a negative one but don’t want to break anything electrical because I am a mechanical person not an electrical one and I don’t trust my ability to diagnose electrical problems.

The parallel issue is the drag on the alternator when the fan initiates at tickover. It a 40 ACR and I wonder if it needs to be be bigger to cope with the increased draw from the fan and various other modern electrical gizmos.

Thoughts gratefully received.

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PostPost by: billwill » Sun Sep 15, 2019 12:13 pm

It seems unlikely that an otter switch would be electronic, and mechanical temperature switches would not be polarity sensitive.

So it is more likely that the otter switch is not closing, either being set for too high a temperature or broken.
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PostPost by: Craven » Sun Sep 15, 2019 3:05 pm

If the signal from the otter switch is a positive feed then don’t operate the manual override at the same time = dead short. On it’s own a positive signal from the otter switch just results in a Positive on both ends of the relay and it won’t work.
Normally the battery should provide any reserve power required so suspect a poor feed from the battery to the fan relay.
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PostPost by: 69S4 » Sun Sep 15, 2019 3:42 pm

Are there any electronics in an Otter switch? Given how far back they date from I'd have thought you'd find valves rather than chips inside if there are. :lol:

I've never taken one apart but always assumed they were a simple mechanism - a wax pellet or a bimetallic strip type arrangement - a bit like a better packaged version of the temp compensators to those familiar with Strombergs. The diagram in my workshop manual shows a reversible two pin plug and doesn't have any warnings about getting the pin direction wrong.
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PostPost by: gav » Sun Sep 15, 2019 3:57 pm

To clarify - it is a modern switch that screws into the radiator. I am calling it an otter although it isn’t one really - its a modern equivalent.

I wired it to trigger from the earth side of the switching part of the relay (the low amperage side) so that when it makes the connection, it closes the earth side and makes circuit (positive into the switching part of the relay and negative to earth at the switch and ‘otter’. When either the switch or the ’otter’ close they complete the circuit to earth and switch the relay to provide power to the fan.

I might try thicker cables to see if that’s the problem although when I liven the circuit and short both sides of the ,otter’ cables, the fan does trigger (although I haven’t done this with the engine / alternator running).

Am I right in thinking that the suggestion of undersized wires is on the power (30A) side of the relay?

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PostPost by: Craven » Sun Sep 15, 2019 4:27 pm

You say the ‘ thermal switch ‘ came from Burton do you have a part number, nothing wrong with your thinking re relay switching the earth side of the fan. There will be a live feed to the ‘ hot ‘ side of the fan, as this will be carrying high current this feed should be taken from as close to the battery heavy cable in the engine bay.
In your setup the ‘ thermal switch ‘ is handling a very small current no obvious reason it does not work.
Otter is a trade name.
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PostPost by: gav » Sun Sep 15, 2019 4:42 pm

I’m not doing well at explaining this so I am enclosing a sketch.

I replaced the ECU trigger with the thermal switch.

Does this make more sense?

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PostPost by: Craven » Sun Sep 15, 2019 5:32 pm

Nothing wrong with that circuit arrangement, it’s possible the ‘ thermal switch ‘ has relative high effective contact resistance not providing a suitable path to earth if the internal switch is a semi-conductor of some type.
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PostPost by: gav » Sun Sep 15, 2019 6:02 pm

With that in mind, 2 questions if I may:

1) is there merit in reversing the switch wiring so that it closes positive rather than the current negative?

2) what might be causing the alternator to labour when the relay switched the fan on (only initially) - is that the size of the 30A circuit?

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PostPost by: Craven » Sun Sep 15, 2019 6:31 pm

Conventionally current flows from positive to negative so a reverse of current flow needs the battery connected in reverse but I understand what you mean. Until the type of thermal switch you have is known there would be no point in switching the feed to terminal 86. I think you have already tried but if you test on the ‘thermal switch’ terminal connected to the relay pin 85, you should measure, below temperature = 12volts, above temperature the voltage should drop close to zero. It may however sit somewhere between 12v and 0v, if it is not close to 0v then this could be your problem.
Until the fan reaches full running speed it will draw a higher current, but this higher current under usual condition is met by the battery.
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PostPost by: nigelrbfurness » Mon Sep 16, 2019 8:16 am

If you live somewhere so hot that you need an enormously powerful fan, you are coming at the problem from the wrong starting point. Fit an aluminium full-width radiator and a low current fan for when you're stuck in traffic. Even on the hottest days, my fan rarely comes on and I'm running a 1760cc block and big valve head.
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PostPost by: Bigbaldybloke » Sun Sep 29, 2019 1:09 am

I’m assuming that from your diagram the ecu has been replaced by the otter switch (or whatever make it is). Looks like it should work, one simple error that can be made is when people crimp a terminal to a wire they either crimp it on the insulation or the wire breaks just by the terminal and it’s only the insulation secured in the terminal. Check the continuity of the wiring from the relay to the otter switch and the connection from the otter switch to earth. If both are ok, short the two wires together at the otter switch and the fan should start, if it doesn’t, your earth wire from the otter switch is not going to earth. Can’t be the relay or the power to the relay as you have already proved this is ok with your override switch. Otter type switches are usually based on a bi-metallic strip so are purely mechanical so not polarity sensitive.
As for high fan current draw, you may be better off with two smaller fans, one controlled by the otter switch with the second as a manual selection if the single fan is not cooling it enough. They are rated at 80 watts each so draw approx 7A each.
I’ve fitted two smaller fans and the current draw is not a problem, I’ve not actually measured it, but have blanked off under the radiator to ensure as much air goes through the radiator when on the move.
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PostPost by: gav » Sun Sep 29, 2019 8:59 am

Thanks for the assistance

I cheated and took off both terminals to the thermal switch and joined them together and the fan immediately started so I’m pretty sure I’m down to a dodgy thermal switch and too high starting current.
Talking to an electrician, the starting current can be 3-4 times the duty current albeit only initially. If it draws over 40A in start up then I could see why the system would struggle. When it is running it is fine.
It moves a significant amount of air (around 1250 cfm which is double my previous 10” fan and I have concluded a little much for my electrics so I am going to downgrade to a 900 cfm version which will be 50% more than the previous fan but with a 6.8A draw, will be similar to the older one and therefore much more manageable.
Also, I want to revert the wiring to the relay so that it is controlled by the ECU because it is much more accurate and I can programme the on and off temperatures.

Version 3.......

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PostPost by: Bigbaldybloke » Sun Sep 29, 2019 9:26 am

Glad it’s all working now.
Just a couple of things, how do you know the otter switch is seeing a high enough temperature that it should switch on the fan, have you measured the temperature at the switch with something like an infrared temperature gauge? Is your switch adjustable, do you know it’s setting? Are you relying on the car temperature gauge, do you know it’s correct, there are more than one sensor and gauge combination and if they are not matched the reading can be incorrect. Is the thermostat opening correctly to let the hot water through to the radiator and hence to the otter switch.

Sorry if I’m telling you basics you have already done, I was just thinking of any other possible causes late last night and thought I’d pass them on. My background is in instrumentation and controls and I used to find everyone blamed the instruments when it was often something else.
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PostPost by: gav » Sun Sep 29, 2019 9:42 am

I put a volt meter across the terminals of the otter switch when the engine was cold and when it was running.
It worked properly in both scenarios which is what flummoxed me and really started the thread - I was wondering if the switch was broken and had sufficient capacity to close the circuit on the switch but not enough to run the relay which is probably a metre away. Ill take it out when I do my next drain down but having the fan run by the ECU before was much better and really where I want to be.
I have a water and oil temp gauge in the car and I’m not convinced that the dual gauge is particularly accurate which is another reason why I want to revert to the ECU taking control of the cooling.
Funnily enough, I think my oil temp gauge under reads so between the two I am working in averages.
The thermostat opens at 82 degrees and seems to be working fine so I think Ive covered the bases.
My rad is a Caterham competition one and is really efficient. It lives in the front of the car about 6” behind the front grille and I am currently re making the shroud to direct air into the rad rather than around the sides.
I dont have a problem with temps when running - its only in traffic and because I live in an area that always has traffic (and pathetic drivers) I am covering the bases to prevent problems in the future
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