Thread: Water Temp Sensor Ground Shorted

What would cause my ecu pin 21 to short? It is the Water Temp Ground. I don't want to put a new ecu in until i find the problem with this. Thanks!

A bad sensor or a fray in the wiring that touched something metal along the way. The way it works is the ecu sends a signal to the sensor, the other wire off the sensor goes to the sensor ground which are the grounds located on the intake manifold. Pretty simple circuit. So somewhere between the ecu and sensor it shorted to ground.

To diagnose unplug the coolant temp sensor, use a multimeter on continuity setting, one lead at pin 18 and other lead to a chassis ground. See if your getting continuity. If not, sensor shorted, if you are, then trace the wire and find the short.

Simple as that.

Then to double verify if your not getting continuity to ground is to ohm test the coolant temp sensor. If it fails (out of spec) then thats another good idea that its the sensor at fault.

Why are you saying new ecu though. I dont recall that circuit causing an ecu to go bad. If there is no resistance it would just get a super high reading for coolant temp. All it is is a signal voltage put out by the ecu.

The ecu has its own potentiometer inside that keeps the circut from frying if something happens to the wiring or the coolant temp sensor.

Same way if you were to take an electronic aem or autometer gauge and instead of touching the signal wire to the sensor you touch it to ground, it will just peg the gauge.

The traces on the ECU are burnt out. I figured that was probably causing some of my tuning issues i have been having. I will give this a test in the morning. One minute my car will run rich and the next its running dead lean. Also the fans wont turn on even if i unplug the coolant sensor.

BTW pin 18 is your coolant temp sensor input (if this is on a b13 ecu), Pin 9 is for the cooling fan relay pull to ground low side driver.

Pin 21 is just a sensor ground (basicly just another ground for the ecu), its not sensor specific as all the ecu grounds go to the same place (the intake manifold) If you burnt the trace for pin 21 then you have other issues with the ecu.

Is this a chipped ecu? If so something may not be right. Obviously.

You need to figure out what caused that ground to fry. Usually those grounds are for multiple sensors/triggers/drivers

If you have idle air control still, i would almost bet its the idle air control motor that did it. Again every sensor can be tested to see whats at fault.

The coolant temp sensors does not function that way and is probably not the likely cause of the burnt trace. Again just test everything.

I know for a Fact that the IACV is the problem. I just ordered a new one yesterday. I will be going through every sensor this weekend to check this out.

Yeah the idle air control is one of the few circuits that the ecu actually puts out a 12v+ pulsed voltage to control the motor. If the motor fails which is very very rare, it can damage that circuit as its not protected in the ecu.

Older mitsubishi's had the same problem but magnified. Notorious for blowing ecu's up because of a bad idle air control motor. A long time ago we bought a 92 mitsu diamante w/ v6, no spark or fuel. Bad ecu, put a new ecu in it started for a couple seconds and then died and once again no spark or fuel. Did some research and whalla, idle air control motor causes the whole issues. Replaced it and the ecu and good to go.