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I need a switch that will activate at a certain temperature. The sender should be able to be in coolant and the trigger temp should be adjusable. Sender should be accurate at around 95 degree fahrenheit.

The switch will drive a relay so the power requirements are low. At the moment Id guess trigger temperature to be around 85 degrees.

Off topic, while searching for this I found someone who drives a vegetable oil 98 Jetta turbo deisel. Interesting read http://ctbiodzl.freeshell.org/votdi.html

I posted a reply in your other thread about the intercooler water pump. I guess I'll expand on it here.

The best sensor to use would probably be a GM coolant temperature sensor. On naturally aspirated GMs, it's also used as an intake temperature sensor. It costs like 10 bucks new, or can be pulled off pretty much any GM car in a parts yard made in the eighties and early nineties. They also thread into a standard 3/8" NPT fitting.

I presume you want it to activate above a certain temperature?

Connect one end of the sensor to ground (some versions already ground through the body). Connect the other end to a regulated 5-volt source through a resistor. This makes a voltage divider; the voltage in between the sensor and the resistor will vary with temperature.

Use Ohm's law (V=IR) and the info in this page to figure out the relationship:

http://www.diy-efi.org/gmecm/component_ ... nsors.html

Then figure out the threshold voltage, and set up another voltage divider that will output that voltage. This is most easily done with a potentiometer, with one end grounded and the other end at the regulated 5 volts. Then you can adjust the voltage at the wiper by turning it.

Then, get a comparator. Radio Shack carries the LM339 which actually contains 4 of them. A comparator has two inputs, inverting and noninverting. When the voltage at the inverting input rises above the voltage at the noninverting input, it will ground its output. Connect the inverting input to the wiper of the pot and connect the noninverting input to the middle of the voltage divider that includes the sensor.

So now the output of the 339 will be grounded when you need to activate the relay. Unfortunately, the 339 can only sink about 16mA, so you can't drive a normal automotive relay. You can drive a reed relay (Radio Shack carries some appropriate ones) though, which can drive bigger relays if necessary.

The only issue would be if the GM coolant temp sensor doesn't react quickly enough. The intake air temperature sensor used on supercharged GMs has an open element so it responds faster, and would probably be okay when immersed in coolant, but I don't know for sure. Any other thermistor would do though, from the electrical side of things.

Looks like that's more or less what's in this kit:

http://www.mpja.com/productview.asp?product=5289+RB

Marlin P. Jones & Associates is a cool supplier.

If you'd like me to come up with an exact circuit give me a couple days and I'll draw it up.

Okay, here's what I was thinking of:

http://www.graphics.cornell.edu/~v/pics ... ontrol.gif

I just drew it last night, and of course haven't tested it... but it seems right. :) It shouldn't be too hard to build and test. The trigger temperature when the potentiometer is all the way to the top is around 45 degrees, and at the bottom is around 190 degrees. I figured that range would allow for some tolerances in all the resistive components while still getting you the range you wanted.

A GM coolant temperature sensor, with its closed element, will probably be okay. GM does use them in their fuel systems to measure intake air temperature to compute the right amount of fuel. Also, this circuit has no built-in hysteresis, so it'll rely on the hysteresis of the sensor and system, so it's actually better for the sensor not to respond instaneously.

Ill try it. So the circuit is providing the 5 volts you mentioned before? Will I need a heat sync? I will probably try to encase the thing in epoxy when its done.

Anyone know the best source for the temp sensor and perhaps a part number?

The LM2937 provides the regulated 5 volts. It should have a heat sink attached to it to be safe, but it really won't get too hot in this circuit.

If you do encase it in epoxy, be careful... Epoxy is normally not very heat-conductive, so things could get hot in there. I did pick components rated to at least -40 to +125 degrees Celsius, but still... The LM2937 and the 2N2222A both will get a little warm, so let their heat sinks poke through a little or something.

There are a bunch of different GM temperature sensors you could use... I believe the commonest one is GM part number 12146312. It has a 2-pin connector (so it doesn't have to ground through the body) and 3/8" NPT threads. The Wells part number is SU109. If you go to a junkyard you should be able to pull one off almost any GM car made in the 80s or early 90s. Most non-supercharged GMs of those years also used the same part as an intake air temperature sensor. You're gonna have to go to a parts yard for the connector anyway, so you may as well grab a sensor too, at least for testing. But they're cheap new; I think NAPA has an equivalent to the GM part for like $8.

If it's more convenient, there are some with a single terminal which ground through the body, and at least some Saturns have ones with smaller threads, but I don't have part numbers handy.