High Water Temeprature Light switch

Loom, lights, gauges, indicators & wiring

High Water Temeprature Light switch

Postby wold racing » Wed Dec 14, 2016 12:35 am

I have U.S. 1967 Morris Minor 1000 that I owned almost since new. I was the second owner car and I bought it with only a few miles on it. I have had it a very long time and have done all the work on the car since I had it.

The US version has a high water temperature light in the speedometer. We call them idiot lights in the US... This light comes on when the car temperature gets too high. I am only guessing, but believe that at about 220F to 230F the idiot light comes on. It has worked over the years just fine.

There is a temperature switch that screws into the cylinder head and has a single wire that connects to the switch. This is the switch that goes to ground when the temperature gets too hot and causes the high temperature light to come on.

Everything worked great until the connector on the temperature switch broke off. No big deal I thought. I will just buy a new switch and screw the old one out of the cylinder head and screw the new one into the cylinder head. I knew that the old switch worked well because over the years I had the temperature light come on a couple of times and sure enough, I was low on water or had a leak in the rad hose. So I threw out the old switch and long after the old part was hauled away from the trash I started the car and went for a drive. Easy fix, right??

But no, nothing is easy.

With the new temperature switch installed, now when I start the car, the idiot light comes on almost right away, but at a very low level of brightness. As the car warms up, the light gets brighter and brighter. I bought two of them at the time, so I put the other one in and the same thing happened. I measured the engine water temperature and the temperature was less than 130F and the idiot light was shining very bright.

The idiot light mounted in the speedometer is a two wire light. One wire captures power from the back of the speedo and goes to the light. The other wire from the light goes to the temperature switch. When the temperature switch in the cylinder head closes due to high temperature, the switch goes to ground and the idiot light comes on. What is happening with the temperature switches I bought (they are not switches by the way, they are temperature senders that were sent to me) is that the temperature light begins to comes on right after the car is started (very faint) because it is a temperature sender unit for a gauge that was sent and not a switch. With an ohm meter, you can read resistance across the switch (sender) right away which is why the light comes on almost right away when the engine temperature is still low. As the car heats up, the light gets brighter and brighter because the sender resistance gets greater and greater.

I looked for my old switch so I could get a part number, but it was gone with the trash. Damn.

What the light should do is remain off until the switch goes to ground, then it comes on bright...

After a great deal of conversation with the vendor of the temperature "switches" that they sent to me, they finally got back and said that indeed the "switches" were not really switches for an idiot light but they were temperature senders for temperature gauges. Yea!! I wasn't crazy.

But the vendor can't obtain the proper switch.

The vendor has a part number specifically for switches for idiot lights and another different part number for senders for gauges. They sent me what was thought to be the correct part, but as it turns out, they really don't carry the temperature switch for an idiot light and only carry the sender for a gauge.

I have zero interest in installing a gauge, so installing a gauge is not an option. Period. Not going there for many reasons. And I don't want to install any electrical circuit boards with converters to make a light work. I just want my temperature switch back!!

So, the question. Does anybody have a temperature switch or a part number or something that I can use to buy the correct part and make my idiot light work again??

wold racing
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Re: High Water Temeprature Light switch

Postby Plin » Wed Dec 14, 2016 12:47 am

:? sorry I can't help with your sensor light query, hopefully others will pop along later to offer their advice but until then :D welcome to the forum!
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Re: High Water Temeprature Light switch

Postby BrianHawley » Wed Dec 14, 2016 3:32 am

If you can find someone with the knowledge and time (not me unfortunately), it should be possible to build a small electronic card to make your existing sender drive the existing bulb the way you want. I would guess only a few dollars in parts, but it would take an electronics nerd.

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Re: High Water Temeprature Light switch

Postby Custard » Wed Dec 14, 2016 8:15 am

Welcome, no numbers I am afraid, but certainly over here some of the basic models of cars like the Ford Fiesta had a warning light only and that may be your best bet.

Though you may have to check the thread size.

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Re: High Water Temeprature Light switch

Postby wold racing » Wed Dec 14, 2016 1:39 pm

I was hoping that somebody with a MM could look for numbers on their temperature switch in their car and the numbers could help find the correct item. Thanks.
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Re: High Water Temeprature Light switch

Postby emjay » Wed Dec 14, 2016 3:56 pm

You won't find the answer with original Minor owners. The light in the speedo was used to detect a blocked oil filter. If you still have the original canister type oil filter there will be a tab for a wire. Obviously the original owner (or dealer) thought it would be better used as an overheated indicator. The options mention above are good. Finding any vehicle that has a similar function idiot light would yield an appropriate switch, but I'm surprised the manufacturer couldn't identify one so try another manufacturer. Adding a simple circuit shouldn't be too difficult for a sparky. Put the sender in water at your desired threshold temperature and measure the resistance of the switch and then your sparky can built the electrical equivalent to a pressure valve.

Come to think of it my 86 Cherokee had that light. Rockauto lists the sender as http://www.rockauto.com/en/moreinfo.php ... 81&jsn=378 You could search for any coolant sensor and when you find choices that state "with light" or "with gauge," you want the former. The worse you have to do is make an adapter for the thread, but they are fairly standard and change the connector.
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Re: High Water Temeprature Light switch

Postby BLOWNMM » Thu Dec 15, 2016 4:05 am

I believe a Morris Minor has a 4 PSI radiator cap. If this is so then at this pressure the coolant will boil at 225 deg F. (107 C.) See link. Having an idiot light operating at 220 to 230 deg F (107 to 110 C.) would be above the boiling point at 4 PSI, a much higher temperature than I would consider desirable. I believe such a warning light should operate at more like 210 deg F. (99 C). You may find a suitable 'Tridon' Thermo Fan Switch to do the job in the link provided. They start on page186 and although they are meant for switching fans on they would be suitable for a warning light.
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Re: High Water Temeprature Light switch

Postby cardiffrob » Thu Dec 15, 2016 9:59 am

I made a dashboard for a 1930's car from a Arduino. It was the first thing I made so it isn't too difficult. For maybe $30 US you could make one up or find a 10 yr old kid to do it for you? :D

http://www.mig-welding.co.uk/forum/thre ... 518/page-5

I have the program for rpm, water temp, oil pressure and they can all be set with warning values to illuminate LEDs or to sound buzzers.
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