dtcom Posted May 26, 2011 Report Posted May 26, 2011 On a 1984 M20J the factory oil temperature guage seems to read slightly high. Then when you add an electrical load, landing light, pitot heat, even the panel lights, the temp jumps up, seemingly in proportion to the increase in load. It goes back to where it was when the electrical load is reduced. Does anyone have any suggestions?
danb35 Posted May 26, 2011 Report Posted May 26, 2011 What happens to your bus voltage when you add load?
Barry Posted May 26, 2011 Report Posted May 26, 2011 Sounds like a high resistance common ground, somewhere ...
dtcom Posted May 27, 2011 Author Report Posted May 27, 2011 The bus voltage stays the same on increasing load. We've tried adding grounds from the guage cluster frame directly to the engine, and have not had any success.
Barry Posted May 27, 2011 Report Posted May 27, 2011 If you are measuring the bus voltage through a good ground, you will see the voltage stay the same. If the sensor ground is bad, the drop will be on the ground side. It's not the bus voltage that is varying but the voltage drop across a bad ground on the ground or return side of the system. Try adding a better ground directly at the sender or directly at the meter terminal and see if that cures the problem.
dtcom Posted May 27, 2011 Author Report Posted May 27, 2011 Quote: Barry If you are measuring the bus voltage through a good ground, you will see the voltage stay the same. If the sensor ground is bad, the drop will be on the ground side. It's not the bus voltage that is varying but the voltage drop across a bad ground on the ground or return side of the system. Try adding a better ground directly at the sender or directly at the meter terminal and see if that cures the problem.
Barry Posted May 27, 2011 Report Posted May 27, 2011 Well, if lower resistance = higher temp reading, you are right. It is barking up the wrong tree.
dtcom Posted June 4, 2011 Author Report Posted June 4, 2011 Just found a new symptom. The CHT was inop for some time - we have an Insight GEM which shows CHT. Now that the sensor for the CHT was replaced, it exhibits the same behavior as the oil temp guage. I checked various grounds, including a temporary grounding cable between the engine and the battery to eliminate any high resistance ground, but no change. When the alternator breaker is pulled, the displayed temps go down significantly, but I can't see the alternator affecting the guage readings. Anyone have any more ideas??
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