blown formula

How do you test an EGT sensor electrically? I have twins & one EGT sensor refuses to work. I installed a new sensor & still no luck. The gauge is good as I switched engine leads to the gauge & it worked. What electrical test can you run on the sensor from the gauge end? What would one expect to see with a volt/ohm meter from the sensor wires (2) ??? I do no seem to read either ground or battery...????
Help!!

mcollinstn

A pyro sensor uses a thermocouple that changes in resistance with increasing temperature.

You should see "x" Ohms in it at room temp. At 1000 degrees, you should see "y" Ohms.

All I know to tell you is to check the resistance (Ohms) of the known "good" unit at room temp and compare (disconnect the probe leads from everything else or your reading will be bogus). Compare this resistance to the questionable probe.

Do the same after a hard run.

If the probes both show similar resistances at the temp extremes, then the trouble is in the wiring somewhere.

blown formula

...There are TWO leads from the sensor. I realize a normal sensor has a variable ground resistance, but what about the EGT??? 2 wires, so both cannot / should not be variable ground. Also, it takes a VOLTAGE to move the needle on the gauge/ so where does it get the voltage, if none is applied to the sensor??? (Not even on the sensor that works!!)
Totally confused here!!

Turbojack

I thought a thermocouple puts out a voltage. My guess ( & only a guess) if engine in running you will get a DC voltage when checking between the 2 leads. Since you said you took wires going to gauge & connected to the other pyro & gauge showed correctly Only problem could be bad pyro.

Mr Gadgets

Thermocouples do put out a small voltage.. there is a chart for each type of Thermocouple... J,K, etc...

You should see a reading with a good volt meter even at room temp.. Wires have to be compatiable with the thermocouple...

The gauge should do a conversion to make the needle indicate.. meaning special gauge.. Digital gauges are under $100 .. but a volt meter will tell you if the thermocouple is good.

Thermistors dont usually go that high in temp..

good luck

Dick

blown formula

So are we talking a millivolt or less??? .01vdc or less??? With all the "blower guys" on this board Someone must know!!


Turbojack- that could be true, BUT I repaced the sensor and same result. I guess I will just have to replace the wiring to be sure.....man oh man...what a hassle!!

Turbojack

Ok I I have reread your orginal post. Have you taken the wires from the engine that works & connected to the new sensor. Have you tried the new sensor in the engine that works? I find it hard the wire went bad but stranger things have happen. If this does not work maybe you have a water leak in manifold keeping the sensor cool.

Mr Gadgets

I cant remember what the voltages are.. If I remember correctly it is very low.. millivolts.. I have a chart at home. Do a search on the net for thermocouples and see if you cant find the charts.. that will give u an idea. It is a common thing has been around for lots of years, so it should be easy to find something..
LIke you said.. you replace the thermocouple and no luck. Have you tried that new on the good motor. Depending on the gauge.. if it starts to read at 800degrees or so., You can hook up the thermocouple to the good gauge at the motor and put a match or propane torch to it.. be careful .. safety is #1. You dont have to run the motors, just power up the dash gauges and get the thermocouple hot enough for the gauge to read.. Then read the voltage across the thermocouple or better yet at the gauge itself.. . Now take the good one and put it on the suspect bad gauge and see if you can locate the problem.. Just the regular subsition techniques.. see if that helps..

Good luck

Dick

mcollinstn

oooh, yep. Thermocouple does produce voltage. Thermistor is what I was thinking about (and was figuring the resistance was measured between both leads, not between a lead and ground).

After reading about thermocouples, it seems likely that you do have thermocouples and not thermistors (due to the temp range of available thermistors being pretty low in the world of egt).

see the following link:

http://www.picotech.com/applications/thermocouple.html

It does also say that it is troublesome to measure voltage across a thermocouple unless you have some kind of "cold junction". I'm thinking the ones you have already have the cold junction thing taken care of and I'm guessing that you should be able to measure the voltage with a digital multimeter OR your gauges...

blown formula

Thanks for the info....good info there!! Especially that part about a voltmeter cannot be used to read the voltage due to the meter leads & internal connection consist of MORE thermocouples themselves!! I have other fish to fry here, (so to speak), but I will try new wiring this weekend.