piscespokerbrat Posted March 13, 2013 Report Share Posted March 13, 2013 So, I just got this CS, and the temp gauge pegs. I pulled the ECT to check for resistance between the yellow/black wire terminal and the black wire terminal. The resistance seemed to check out fine. The temp gauge however, that pegs full hot, all the time. I unplugged the ECT sensor while the car was running and the gauge dropped down to C. Am I looking at a faulty ECT sensor here, or a faulty gauge? I appreciate the help, thanks guys! Quote Link to comment Share on other sites More sharing options...
White93z34 Posted March 13, 2013 Report Share Posted March 13, 2013 That is the gauge for the computer, the sending unit for the gauge is a single green wire sensor on the corner of the rear head. Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted March 13, 2013 Report Share Posted March 13, 2013 since it's a 96 3100, it sounds like you have the single 3-wire coolant temp sensor. that combined both sensors into a single unit, if disconnecting it caused the guage to switch sides, it sounds like it's internally grounding. in 96, both the 3100 and 3800 got the single 3-wire unit, but the LQ1 still used two seperate sensors. or at least that's what it looks like in the diagrams. Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 So, there is 2 sensors? The 3 wire I understand is a yellow/black, black, and green wire unit under the throttle body. Since unplugging that drops the gauge, does that mean the pcm sends the temp signal to the dash? Or is there a second unit in the head that works in conjunction with the ECT under the throttle body to signal the dash? I have the GM service manual for the years, I could trace wires I suppose.. I was just hoping someone else has had this same issue and could shed some light. Quote Link to comment Share on other sites More sharing options...
cutlassdude96 Posted March 14, 2013 Report Share Posted March 14, 2013 96 3100 has only 1 sensor Quote Link to comment Share on other sites More sharing options...
White93z34 Posted March 14, 2013 Report Share Posted March 14, 2013 Oh, ok. In that case I stand corrected. Quote Link to comment Share on other sites More sharing options...
alec_b Posted March 14, 2013 Report Share Posted March 14, 2013 If you have a 3-wire, you only have one sensor. You say you measured resistance between the BLK/YEL and BLK wires. That would be the feed for the PCM. You need to measure resistance between the GRN and BLK wire pins. That's the circuit that feeds the gauge. Most likely you have a bad sensor. Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 If you have a 3-wire, you only have one sensor. You say you measured resistance between the BLK/YEL and BLK wires. That would be the feed for the PCM. You need to measure resistance between the GRN and BLK wire pins. That's the circuit that feeds the gauge. Most likely you have a bad sensor. Howdy neighbor!! I feel like an idiot now, but that does make sense. If the green and black are shorting internally in the sensor, then that would peg the gauge, I think. Does the resistance between the black and green follow the same chart that I am posting below? Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 Ok, I pulled the sensor.. and measured across the yellow/black and black and did the test again under running water.. so 50 degrees or so cold was about 5000 ohms and 110 or so hot was about 1300 ohms. I did the same test with the black terminal and green terminal, and the entire time the meter read open. I think that means a bad ECT sensor.. and the guy I got the car from put a new one in. IS that common to have a defective ECT from the get go? Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted March 14, 2013 Report Share Posted March 14, 2013 to be more accurate to how the circuit operates.... measure between the green wire and the engine block/head/intakes as a ground, since the guage circuit is grounded that way. Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 (edited) to be more accurate to how the circuit operates.... measure between the green wire and the engine block/head/intakes as a ground, since the guage circuit is grounded that way. I just found this tidbit a little bit ago.. there is 2 thermistors (sp?), anyways, inside the 3 wire ECT. So, that would mean that there SHOULD be resistance between black and yellow/black and black and green. I also found out how to test the gauge.. "To test the gauge, You can unplug the sensor.... and install a 44ohm resistor ( you can get one from radio shack for a dollar or so) between the dark Green wire and the black wire on the coolant temp sensor. Turn the key on and the gauge should go to hot (max) or 280 deg. A 240ohm resistor should show 194deg. If it gauge doesn't move, you can ohm that wire from the sensor to the back of the cluster, if ok, replace the cluster. You can ohm the pin on the coolant sensor where the green wire plugs into it to the base of the sensor. At 68deg it should be 3520ohms or 2238ohms at 77deg." Edited March 14, 2013 by piscespokerbrat Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted March 14, 2013 Report Share Posted March 14, 2013 while the black wire will work when it's connected to the harness(since it's a constant ground), the green wire is supposed to be grounded through the shell of the sensor. you could test to see if there is a connection between the shell and black wire, but there isn't supposed to be. Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 Well, I just did tip of sensor to green pin, and i got readings, YAY! At all temp spectrums, the sensor reads correct.. Now im gonna get a resistor and see what happens there. What a chore to chase! LOL Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 14, 2013 Author Report Share Posted March 14, 2013 Ok, so I was impatient and didnt go get a resistor.. instead I powered the ignition up, verified the gauge was on C (it was), turned the key off, jumped the black and green terminals with a piece of speaker wire (1 ohm of resistance.. lol), and I hit the key on and the gauge pegged. So, gauge checks out, wiring check out, all thats left is the ECT sensor or PCM (have not verified voltage at the yellow/black wire yet). But voltage should not affect the green wire as that is resistance based, am I right? Quote Link to comment Share on other sites More sharing options...
RobertISaar Posted March 14, 2013 Report Share Posted March 14, 2013 PCM has absolutely no connection to the coolant temp guage circuit. i don't feel like writing a book explaining how the guage works, but more or less, yes, it is resistance based. Quote Link to comment Share on other sites More sharing options...
piscespokerbrat Posted March 15, 2013 Author Report Share Posted March 15, 2013 Slapped in the new sensor, and the gauge works perfectly now!! I also sold the car tonight for $1500 Quote Link to comment Share on other sites More sharing options...
PHANTOMKID Posted October 24, 2013 Report Share Posted October 24, 2013 this thread is not as old, sorry if youve already fixed your concern. Hope this information will help..... Gauge operates based on resistance. The ECT changes resistance, inversly related. When temperature rises resistance lowers. The gauges ground through the ECT, that being said this is how the ECT affects the gauge when resistance is high((temp is low)) power through the ECT is lower due to ECT resistance. When resistance is low((high temp)) power through the ECT increases.. When unplugging the ECT the resistance not only raises, the circuit is now open on the ground and now power passes throught the gauge, thus leaving it at C... less resistance= greater current to move the needle. Im dealing with a similar problem. By jumping the wires to the gauge with 1ohm, you cant really deam the gauge as accurate, but as operating, due to the fact that we dont know where the needle should sit at what resistance value. What ive found out is at about 226 when fan 2 should come on, the gauge is in the red. Im wondering if this is due to a gauge being out of calibration. Does anyone know where the gauge should sit at 226???? Also if the vehicle does not have heads up display the overheat light seems to be wired through the gauge, so my guess is a inaccurate gauge can through a false overheat light???? hope to help and also looking for help. Any help is appriecated. Thanks guys. Quote Link to comment Share on other sites More sharing options...
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