Speedwagon Posted July 8, 2011 Share Posted July 8, 2011 Today, I decided to see why the volt meter always wants to read low. I figure if it is reading low, other gauges might be off too. But after spending a good hour on it, I'm not really any farther ahead than I was. Took the cluster out, traces the lines, and put a 12V power supply on the 2 pins. Volt meter read dead on 12V. Dropped it to 10V, meter read 10V. Up to 15V, meter read 15V. Ok, so the meter works great, back to the car. Find the appropriate pins, and check the voltage. Right around 11.8V(battery terminals read 12.7V). Great, all should be fine, but it's not. When the ignition and ground wires show 11.8V, the volt meter shows less than 10V, it looks like it is seeing somewhere around 8-9V, and the low voltage light comes on. When the car is running, the volt meter usually reads right at 12V, though it should be showing me 14V. Why is it doing this? What piece of the puzzle am I missing, that is causing the volt meter to read so low? I even put the DMM across the 2 posts that the volt meter is bolted to, and got around 11.8V, while the VM was reading under 10V. Link to comment Share on other sites More sharing options...
ruparts Posted July 8, 2011 Share Posted July 8, 2011 hi , my 84 wagon had this same thing, i finally found i had a bad ground wire connection. it was under the inst cluster and was a heavy gage black wire with a ring terminal. it is a " made to be ground " , one of several built into the wire harness, on my car it comes off of the main harness just above the floor/bulkhead under the dash. i had to remove the fuse panel and the ecm to access the area , once i saw this loose wire i put it to ground on the bulkhead and my various electrical issues finally were fixed. my guages are steady and consistant and volts read correctly , something they never did before i found this unconnected ground lead. you did not say what car you have , but these symptoms sound like the same thing, a bad or not connected ground in your wiring . the fsm electrical section has a page showing all the main harness ground locations , this is my best suggestion. Link to comment Share on other sites More sharing options...
Cougar Posted July 9, 2011 Share Posted July 9, 2011 (edited) Good info by Ruparts. Some difference in voltage readings between the battery and the dash voltmeter are expected due to wire losses between the two areas. From experience about a 1 volt difference is fairly normal. Since you saw over a 1.8 volt difference in readings from you meter and the dash meter it means the dash meter has a problem or the connections to it may be dirty. You may need to replace it but clean the connections to the meter first to see if that helps. Another thing that can affect the reading is the internal impedence of the meter. Whenever the meter is tied to the circuit, current is used from the test circuit which loads down the circuit under test. Your digital meter most likely has a high 10 megohm input impedence which places very little load on the circuit you are testing so very little voltage difference from the actual circuit voltage is seen. The cheap dash meter may only have a 10K ohm input impedence which can cause a error in the voltage reading depending upon the resistance of the circuit it is measuring. Edited July 9, 2011 by Cougar Link to comment Share on other sites More sharing options...
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