S1: Otter switch temp ratings?

[jagwit]

Can anyone tell me at what temp a standard otter switch will be switch on and at what temp it will switch off?

[jjorj]

Can't remember but it comes in quite low and switches out low also so fan runs for a long time. I had problems in the Loire Valley last May with mine draining battery to the point we abandoned the car and have altered it for the Fosseway triangular adaptor and a switch with a higher and narrower band of operation the adaptor allow you top fit ordinary thermostat switches. IT has improved matters I also think the switch is in the wrong place.

George

[cactusman]

Not sure but given that normal temp is around 74 deg c I'd imagine the originals would have cut in about 85-90 and cut out again about 80. I had the same issue as George. The copy part simply refused to drop out. I dismantled it. Others may differ but mine had a cheap bimetallic strip thermostat switch that can be bought for a couple of quid from radiospares secreted inside the body. The thermostats have a huge hysteresis....up to 20 deg c....and IMO are not suited to the purpose. Well made bimetallic switches can have a hysteresis of just a few deg c but this did not apply to my copy otter switch. I scraped out the bimetallic rubbish and built myself an electronic thermostats switch with a semiconductor sensor in the otter switch housing. It has a hysteresis of 3 or 4 deg c and can be adjusted for set temp
My take on the otter switch position on top of the tank is that it is ok so long as the cool and level is high but if the level is a bit low it may not make much contact with the water....presumably why it was later moved to the side of the tank...

[Series1 Stu]

Hi
I have a long history in bimetallic switch design and used to work for the guy who designed the original E Type Otter switch. Even back then, they thought it was hilarious to use a wiper motor to drive the cooling fan.

While I don't know, or more to the point, can't remember the specifics of the switch operating criteria I can say that bimetallic switches do tend to have a large hysteresis curve. This is actually beneficial as it prevents the switch from cutting fans in and out too frequently.

Julian, I think you will find that your hysteresis of 4 degrees is too tight but it should be easy enough for you to change it in the electronics.

The location of the otter switch in the header tank was a mistake on the part of Jaguar as it didn't allow for coolant loss, although the user manual does tell us to check the coolant daily and top up as required.

Bimetal switches are still used today because they are so much more robust and compact than electronic switches with the equivalent current rating and are far more economical to produce in volume.

Regards

[cactusman]

4 deg is an estimate...might be 6 or 7... and it works brilliantly. Comes on in traffic. Runs maybe five mins or so depending on ambient then shuts off...and repeats if you stay still. I have the same design on my mgb which has worked fruitlessly for over 20 years. As you say, some hysteresis is vital or the thing is for ever switching on and off but bimetallic strips, while reliable and cheap, are pretty crude. For a one off solution an electronic sensor is much much better....IMO My original otter switch had been dead a while when I got the car. The replacement was hopeless. I tested it in a pan of water. It cut in at about 84 deg c or so but refused to cut out until the temperature dropped to below 68 deg C. On the car once on the fan would run until you had a chance of a good 3 or four mile run at 50 and even then on a hot day it did not always open....simply not fit for purpose. I would never go back.

[Gerry]

Hi Stu,I doubt that a drop in level makes a difference,as the pipe that the top hose attaches to extends into the tank and has a hole in it that the otter protrudes into,so the otter gets continuous flow even with a low level.

Regards Gerry 62 Ots.

[Series1 Stu]

Yes, Julian.

You're right about the aftermarket switches. They are dreadful. As a slight correction, without hysteresis the switch would only operate once. It would never switch back off.

You can get away with a wider hysteresis because the engine thermostat works in tandem with the fan thermostat, provided you have the correct one installed and it is working properly.

I find most aftermarket parts are complete rubbish. The more technical in nature, the worse they are. That's because the people providing (I refuse to say designing) them just don't understand what it is the products are meant to do and how they do it.

To list a few

Fuel tank sender units
Oil pressure sender units
Coolant temperature sensors
Voltage regulators
Suspension coil springs

The list is very long

[jagwit]

I also think the switch is in the wrong place.

If it was up to me, I would install a fan switch on the cold side of the radiator which I believe is much more effective at regulating coolant temperature exiting from the radiator - like modern cars (and the S3 e-type) have.

In principle, the temperature at the otter switch should actually remain effectively constant (while the thermostat is open to any degree). The way a thermostat works (or SHOULD work) is that if the coolant flowing over it is too cold, it should become more closed (not fully closing), forcing more coolant towards the bypass route, thus warming it up and if the coolant flowing over it is too hot, it should become more open(but not fully open), thus allowing more "cold coolant" from the radiator to flow into the engine thus cooling it down, unit its becomes too cool again etc.

In other words, a thermostat should never be fully open actually and should only be fully closed when the engine is cold or well below the operating temperature. This is why the otter switch in its original location should see very little temperature variation and hence very little "information" for the switch to act on. It is the fan switch's job to supply the engine with coolant sufficiently cold to ensure the thermostat never has to open fully.

At the cold side of the radiator, the switch would experience a much greater variation of temperature and thus have more information to act on. My 7.2L Jensen's fans will cycle 20secs on and about 40 secs off when I just let it idle in the garage (with its fan switch on the cold side of the radiator.)

[jagwit]

I find most aftermarket parts are complete rubbish.

I agree. I recently ordered a new oil pressure sender and gauge for my friend's S3 E-type. The items received were supposedly a matched pair that was tested prior to dispatch. It was not. They were completely mismatched and obviously not tested. The second pair received, were better matched, but still shockingly inaccurate when I tested the pair against known pressures. This pair basically don't offer much more info than the oil pressure warning light. Most disappointing. At least the dash looks more complete and original.

[cactusman]

I did some lengthy investigation into both the original and after market senders. They appear to work slightly differently although in my view neither is ever likely to provide an accurate signal proportional to the oil pressure....at best they will provide an indication of the pressure, probably plus or minus at least 10 psi....pretty hopeless. The consensus seems to be the the oil pressure sender particularly was never much good even when new. Quite why Jaguar did not use a mechanical gauge is a mystery....maybe the hand of Mr Whittaker! All MGs of the era use mechanical gauges and these are both extremely reliable and accurate. While there are suppliers who will provide a mechanical gauge I have yet to find one that visually matches the original dial. What we need is for the usual suppliers to engineer a mechanical gauge that looks like the electrical one. I for one would be happy to pay a couple of hundred for a well made kit. Mean time I treat the indicated pressure as a rough guesstimate. I have fitted a simple 15 psi oil pressure switch and bulb. If the meter goes to zero I am unconcerned. Were the light to come on then I'd know there was a serious issue.