To anyone else reading this thread trying to understand HOW to trouble shoot and locate charging system problems especially problems associated with excessive voltage drop. Here's a very good video from the "South Main Auto Repair" channel. Eric O goes through the step by step process a real
professional uses to track the problem down. Note how Eric O uses the voltage drop test from the alternator case to the battery negative. Also note the voltage drop test between the alternator B+ and battery positive. Most importantly note the results of the test showing excessive voltage drop (much greater than 250 mV).
Also note how Eric O used the voltage probe to further isolate exactly where the bad connection was. IMO it's important to note that none of these tests required fancy tools. The bulk of the work was done with a simple Volt-Ohm meter (VOM). Later in the video notice how Eric O cleans the electrical terminals before putting it back together. Shinny on both sides.
I'm a fan of the SMA channel and I think that Eric O does a lot to promote sound trouble shooting techniques. However I think that sometimes Eric O over does things for the sake of the video. I personally wouldn't test and re-test the amount that Eric O often does. But on the other hand, Eric O finds negative results (check out the bogus test result from the burnt test clip for example).
I'm a big believer in starting with the basics. I still stand behind the advice given in this thread. While cleaning everything may result in cleaning things that don't need to be cleaned it is a technique that can be explained in a forum post (could you image trying to explained HOW to do all of the tests that Eric O did?).
If this were my car I would start by getting the voltage drop between the alternator case and battery negative with all loads on under the 250mV limit. Then do the same thing to the battery B+. Once the charging system is known to be in tip top shape, continue the more difficult trouble shooting.
Note, IF the dash gauge still reads too high after the charging system is in good repair it may be necessary to perform a voltage drop test from the cluster ground (G203) back to battery negative. The cluster ground is behind the center console and is shared by multiple devices including the radio. This ground is frequently messed up during a radio install.
Ok. Some of you may be asking how this relates to the problem at hand? The reported test (the OP never actually did the tests in the manner requested) shows a possible 2.26 volt drop between the alternator and the battery. That is about a 15 percent voltage drop. Since the dash temperature gauge is driven purely off of a calibrated voltage drop having a system voltage that is 15 percent too low will make the dash gauge read 15 percent too high! This is pure Ohm's law. To put things into perspective let's say the motor's temperature is 220 degrees. This is within normal operating range. Adding 15% to the gauge gives a reading of 253 degrees! Clearly too hot!
IMO the OP benefited from additional cross checking. Otherwise the OP wouldn't have determined the motor wasn't really over heating but the dash temperature gauge was wrong. This clearly shows the benefit of cross checking suspected problems.
Hope this helps someone. Remember. Today's cars simply will not run right without a strong battery and charging system!
View: https://www.youtube.com/watch?v=fYjuP8FBUu8