Repairing The Cause Of The Voltage Drop
To make the long story short, it turned out that the Ground terminal for these circuits is on the frame rail itself. This Ground Circuit had become corroded and was the cause of the voltage drop.
The picture here isn't all that great but you can see that the entire frame rail is heavily rusted around the Ground terminal. The actual contact surface of the terminal was in the same bad shape. I cleaned it, re-installed it, and the problem was solved!!!
After the repair, I removed the jumper wire that the customer had attached to the right side low beam circuit. Double checked my work and road tested the truck. Everything worked. Case solved and cleared.
OK, now for a more in depth look as to why the multimeter gave the 7.5 Volts reading. Understanding this, and you'll be able to effectively use this test (and the info in this case study) on many different components' circuits to really test their Ground circuits or any other circuit. (has it ever happened to you that you replaced the starter only to find out that wasn't it and that it was just a corroded battery terminal!).
How It Works: The Voltage Drop Test
You're probably aware that there are two ways to check a circuit for an open-circuit problem. Testing continuity of the circuit with the multimeter in Ohms mode and the voltage drop test. The continuity method is not a fool-proof way of getting that YES or NO answer (YES this circuit is bad and IS the problem, or NO, keep looking). I'll explain why:
Let's say, for the sake of this conversation, that the wire itself is made up of 60 individual wires braided inside the wire's insulation. If only one wire of those 60 is actually transmitting the voltage (or signal or Ground or whatever) to its final destination, the circuit will fail under load (or cause some funny stuff to happen). But, it would still pass the continuity test that you'd perform with the multimeter. Why? 'cause that one wire (of 60) would transmit the small amount of voltage the multimeter passes thru' the circuit to check the resistance.
On the other hand the voltage drop test is a test done while the circuit is actually at work. The results from this test are guaranteed. Here's why:
If you were to place the multimeter in Volts mode and test the same faulty circuit I just described above, you would see a voltage reading around 10 Volts. The multimeter would be helping the rest of the voltage that can not pass thru' the one single wire reach its final destination.
If all of the 60 wires were doing their job, you would see a voltage under 0.1 Volts. All of the voltage would pass happily thru' the 60 wires and none would want to pass thru' the multimeter (the multimeter would now be the path of higher resistance and electricity HATES that). All of this stuff would also apply to a Ground terminal, like in the example in the beginning of this article.
My rule of thumb is that any voltage drop test that produces 1 Volt or above has some sort of unwanted issue that will adversely affect the component.
PS this article was written while TUI (typing under the influence) so if you learned anything it was unintentional!!!!! (just kidding, I've been wanting to say that forever).
OK, one last thing, if you're diagnosing a 4.6L or 5.4L car, van, pick up or SUV, let me just make you aware where you can find a complete list of 4.6L and 5.4L ‘How To Test’ articles that are located in this Web Site and at troubleshootmyvehicle.com and this list is found here: Ford 4.6L, 5.4L Index Of Articles.
If this info saved the day, buy me a beer!
