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Ground Loops - Bad News

Some customers using the ECT module have had difficulty getting the gauge to operate consistently.  I know of at least two cases where, after some troubleshooting, customers verified the voltage on the ECT sensor would suddenly change when other electrical loads turned on and off.  Sometimes the change was only 50 or 60 millivolts, sometimes several hundred millivolts.  In particular the radiator fan and AC system seem to cause the ECT voltage to fluctuate the most, and that of course makes the temperature gauge display fluctuate too.

These symptoms are classic examples of a "ground loop".  If you are using the ECT module and your temperature gauge is inconsistent or changes from one extreme to another in less than a minute you may have a ground loop too.  The first test to make is to put a meter on the ECT sensor wires and measure the voltage.  Turn everything electrical ON and OFF and see if the sensor voltage changes suddenly.  It shouldn't change by more than a few millivolts no matter what is turned ON or OFF.  If it makes any sudden changes with electrical activity then you've got a ground loop.

Ground loops occur when current for one electrical circuit inadvertently flows through the ground leg of another circuit.  Have you ever seen the tail lights on a car blink funny when the turn signals are used, such as the left tail light goes on and off when the right turn signal is activated?  That's a ground loop, and in this case it's caused by a missing ground wire on the right side tail light housing.

If you have an S2000, here's a demonstration of how a ground loop can cause totally unexpected behavior of electrical circuits:  With the headlights off off, remove the fuse for one of your HID headlights (low beam), then turn the low beams ON and check out the front of the car.  Not what you expected, is it?  You probably figured one light would work and the other (with the missing fuse) would remain off.  But in fact you will see one HID light come on normally, the one with the missing fuse will flicker, and both high beams will be lit dimly.  How can the high beams come on if all you turned on was the low beams?  This behavior is caused by a ground loop.  Power from the working low beam circuit back-flows through the high beam filaments and makes it's way to the low beam with the missing fuse.  Because current passes through both high beams and one HID (one big series circuit) they each get less than 12v, causing the high beams to glow faintly and the HID to flicker.

With the ECT sensor wiring you can have a similar effect.  If the sensor ground wiring is not exactly perfect, ground current for other devices will flow in the sensor ground wire, causing a voltage change that the ECT module (and the ECM too) will interpret as a different temperature.  How does ground current get into the sensor ground?  Think about it - how does ground current get anywhere in a car?  It flows through the chassis.  We think of the chassis as "one big ground lug" but in fact it's "one big resistor".  OK, it's a resistor with very low resistance, but when you pass a large current through even a low-ohm resistor you will get a noticeable voltage drop.  That voltage drop will be added (or in some cases subtracted) from the normal ECT sensor voltage, causing the voltage to suddenly change.

Since the ECT sensor operates with very small voltages, even that "close by" ground bolt might not be the best place to maintain the ground integrity of the sensor wiring.  Even if there's only 1/100 of an ohm resistance between the two chassis points (original ground point and your new one), the radiator fan current (30-40 amps) could cause 300-400 millivolts change in the ECT voltage.  That 0.3 volts won't affect the radiator fan one bit, but it will surely screw up the ECT reading, as it can make the difference between a reading of 190 and 235!

Some questions to think about:

bulletDid you move ANY ground wires in your car? 
bulletDid you remove or re-route any wiring to the engine block? 
bulletDid you pull the engine for a rebuild and "forget" to reinstall all the original ground wires in all the original places? 
bulletDid you move a ground lug or terminal from one bolt to another one "close by"?
bulletDid you move the ECM and use a new ground for it?
bulletDid you ground the ECT module anywhere other than directly at the ECM or at the ECM sensor ground?

Some mechanics may reason "Why did the manufacturer put a ground wire on both sides of the engine?  Ground is ground, right?"  They may never think that there is a good reason for all those ground wires.  What the mechanic should be asking is this - "Why would the manufacture spend extra money installing a second ground wire?".  Or, "Why would the manufacturer spend the money to run a ground wire all the way over there when they could use this one that is much closer?"  The only logical answer is that it's really necessary.




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