Thoughts on how to wire up a TEMP warning light?

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SuperEcono4x4
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My tercel:: 1983 Toyota Tercell Wagon 4wd

Thoughts on how to wire up a TEMP warning light?

Post by SuperEcono4x4 » Sat Sep 02, 2017 8:43 pm

I am just finishing install of a rebuilt 3A-C, plus lots of new parts...hoping the tercel to last another many miles. This all after the sad premature death of the original perfectly-running, no-oil-burning engine at only 265k miles due to the radiator fan not kicking on at idle. It was -15F at the time, but no airflow and thus the resulting overheating. It was only maybe 3 minutes in the red before my wife saw steam, looked at the gage, and shut it down. I was in the store at the time and came out to the sad news. I fixed the fan and thought he survived the injury, but alas... catastrophic engine failure 1500 miles later. :(

Anyway. I prefer not to make this costly mistake again! It seems unreasonable to constantly stare at a temp guage and if one coolant system component goes bad, that's it! Thus the dummy light was invented. However, MT tercels, at least of the '83 vintage did not come equipped with a TEMP warning or overheat light. Can anyone show me a diaagram or picture of where the sending unit was placed for those tercels that came with such a light? Or does anyone out there have any hints or thoughts on where to place a generic over temp light sending unit? This engine has the two sending units, one for the temp guage and one for the emissions, plus the temp switch at the radiator for the fan. Could maybe the block's coolant drain house a warning light switch?

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Petros
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Re: Thoughts on how to wire up a TEMP warning light?

Post by Petros » Wed Sep 20, 2017 7:06 pm

Hi superecono4x4,

I just noticed your post, i was out of the country (and away from the internet) until about a week ago.

Your idea has merit, I have thought about the same thing. The 3ac is real sensative to temperature and in my experiance you do not even have to get it into the red zone to compromise the head gasket. There are even unused "idiot" lights in the instrument cluster that might do.

There are three things that might work, the temp sensor for the emissions systems (the one on the coolant outlet) is just an on/off switch, but it might be possible to tap that line and run it to a warning light. It comes on before the overheat, only when it gets to a higher temp, which would be a good thing I think to give you an early heads up, before it overheats. Though in heavy traffic might just be a nuisance, but it would be something to get your attention so you can watch the gauge.

The same is true for the fan thermal switch, it is just an on/off switch, that comes on at a give temp. If you use that one it would light up every time the fan is running (or should be running). all it would do is light up when the fan comes on, catch your attention and you can see the temp drop and the light goes off. Might be a good plan. My fan rarely comes on, only when I leave it idling too long on a hot day, and sometimes in heavy traffic (which I do not drive too much where I live). I think the fan thermal switch is a good bet, it is reliable, already installed, and will come on before it is too late.

Another option is there are these after market temp sensors for after market radiator fans, they just clip or tape onto the coolant outlet hose (upper radiator hose). These would not combine with the cars systems, and would also be independent of them. You likely can get them in different "on" temps, check with a well equipped autoparts store. One that specilizes in RV and travel trailers would have those kind of kits to add to your tow vehicle, and would likely have the thermal switches.

than you either wire it up to your dash, or find an unused warning light location and connect a light there.

I like the idea, because I have done the same thing: not noticed until it ran odd, and than noticed the temp. By than the temp gauge is already pegged in the red...too late. But it means a bit of messing around, and right now I can hardly keep up with regular maintenance and replacing the odd minor stuff that goes out on a car this old.

Next time I have my instrument cluster apart I might look for a suite overheat warning light location and get back with you.

Good luck, lets us know if you get something rigged.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

SuperEcono4x4
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Re: Thoughts on how to wire up a TEMP warning light?

Post by SuperEcono4x4 » Sun Oct 29, 2017 3:44 am

Petros,

Thanks for the ideas!

I like the fan switch idea, though a separate overheat light would be ideal. I just saw your post and haven't check around for those aftermarket set ups you mentioned. I know I've seen the sender units for the aftermarket gauges, but all that stuff seems to be for race cars and the like. I just want a simple affordable dummy light.

I just finally got the old tercel all back together...after I had been way too distracted with other priorities. Tons of new parts...it's almost like a new car under the hood! Just need a dummy light so I don't ever make that mistake again!

I'll let you know if I rig something up!

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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Sun Feb 04, 2018 11:07 pm

I was thinking about how to add a warning light, and my brother found this.
It uses a thermocouple attached to a metal ring that is installed like a washer.
It has more features than I need, but it is interesting.
It is adjustable.

http://www.thermomart.com/engine-watchd ... arm-Sensor

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Petros
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Re: Thoughts on how to wire up a TEMP warning light?

Post by Petros » Mon Feb 05, 2018 3:52 pm

that is pretty nice set up. might look a bit too "tech" for our tercel, and I find digital instruments annoying, rather have a dial or needle to watch. the audible automatic alarm is with the price alone, very reasonably priced it seems to me.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

tercle
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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Mon Feb 05, 2018 8:56 pm

Is reading the temperature somewhere on the exterior of head or the block as good as reading the temperature of the coolant directly?
This thing was originally designed for another application and then modified to to be used on cars.
I originally was attracted to the fact that it didn't require a second automotive sending unit and a tee, but now I'm getting leary.

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irowiki
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Re: Thoughts on how to wire up a TEMP warning light?

Post by irowiki » Tue Feb 06, 2018 11:49 am

BTW the TEMP light you are talking about on a Tercel is actually for automatic transmission equipped cars and comes on when the transmission is too hot.
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tercle
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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Tue Feb 06, 2018 7:51 pm

irowiki wrote:
Tue Feb 06, 2018 11:49 am
BTW the TEMP light you are talking about on a Tercel is actually for automatic transmission equipped cars and comes on when the transmission is too hot.
Normally. that would be true.

But we are talking about adding an Idiot Light in addition to the Temperature Gauge, to remind the driver to look at the gauge (before it reaches the red zone)

SuperEcono4x4
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Re: Thoughts on how to wire up a TEMP warning light?

Post by SuperEcono4x4 » Sun Mar 04, 2018 11:29 pm

Thanks Tercle for that link! I really like the audible alarm idea and the super quick install. I wonder how reliable a surface temp sending unit would be in a water-cooled application though.

I already purchased another set-up... I found a cheapie gauge with a built-in adjustable dummy light (unfortunately digital and in Celsius) that fits a radiator hose coupling that I found in our size. I still need to install it though. Should work fine and is inexpensive, but I would liked to have found a sending unit just for a light and plug that into one of the blank spots in the dash. I didn't see anything online for the lower temps of the tercel. It seems most of the generic stuff out there is for cars with old American iron heads with higher heat tolerances.

tercle
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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Mon Apr 23, 2018 2:59 pm

I wired my project up to Ground, switched 12v and the wire that goes from the sender to the gauge.

Image

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Petros
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Re: Thoughts on how to wire up a TEMP warning light?

Post by Petros » Mon Apr 23, 2018 3:46 pm

nice addition. I have been considering this for my own car considering how temp sensitive these engines are.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

tercle
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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Mon Apr 23, 2018 4:11 pm

I am using a LM311 chip, A 2N3906 transistor and a relay*.
Setting the trigger point is done by adjusting a multiturn trimmer potentiometer (aka "pot")

I also wired up a 50 ohm trimmer pot in series with a 20 ohm fixed resistor.
To set the trigger point, I disconnect the sender connector at the engine head and connect my 20-70 ohm variable resistor and turn the ignition key to ON.
Put the needle where you want the light to come on using the 50 ohm pot, then adjust the trigger point with the first pot.
Then I disconnect the 50 ohm pot+20 ohm resistor and reconnect the sending unit.
The picture above shows where I set my trigger point.

*the relay turned out to be overkill, as the switching transistor easily drives the LED


Image

tercle
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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Mon Apr 23, 2018 8:27 pm

But after I started on this project, I found out that you can buy modules based on a similar chip.

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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Tue Apr 24, 2018 9:36 pm

Image

This is how the module is hooked up.
I'm using the triangle as a symbol for the whole module, which includes a LM393 comparator and some other components.

(see the description for op-amp voltage comparator)
https://en.wikipedia.org/wiki/Comparator

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Re: Thoughts on how to wire up a TEMP warning light?

Post by tercle » Sat Apr 28, 2018 11:22 am

This is a Comparator Module that I bought on ebay.
It is available from many sources, with a few types of connectors.
I got this one with terminal blocks (which barely accept 18ga wire).
The circuit board should say DT-110.

I already had a wiring harness hooked up for the one I made, and I connected this module up and tested it on my car.
note: if you are connecting a LED between the white wire and ground, add a current-limiting resistor.

It works if you hook it up this way:

Image

I have ordered a different type of module that is smaller, and I will test that when it arrives.

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