10.4 volts should be acceptable. 9.6 is the normally accepted minimum voltage at the starter. Since a small amount of voltage is always lost in the cables and especially in the mechanical connections, it's good to have a little more at the battery.
I assume the starter relay is still chattering. Now, move one of your meter probes from the battery post to the cable clamp and have your helper crank the engine. You should again see 10.4 volts. Then move the second probe from that battery post to its cable clamp. You should see 10.4 volts again. It is common for the voltage to drop a little, but 0.2 volts is the maximum allowable per connection.
This can be a hard way to read the voltage because it will bounce up and down a lot. There's an easier way to do this.
The other way to come up with the same measurements is to place one meter probe on one of the battery posts and the other probe on the cable clamp. Logic says you should read 0.0 volts because both probes are in the same place in the circuit. The problem is there will always be a small amount of resistance in that connection. Any resistance impedes the current trying to get to the starter. The normal resistance in any connection in this circuit is WAY too small to measure, but you CAN measure the results of that resistance. That's what you are doing now.
The proof there is too much resistance in this circuit is the slow cranking starter / chattering relay. This is a hard concept to understand and I won't bore you with the details, but that excessive resistance can be inside the battery. You normally see, again, the results of that resistance, and we call that a "dead battery".
The easiest way to find where excessive resistance is in the circuit is to find where it causes an excessive voltage drop. Just as you can add resistance by squeezing a garden hose, there will be no drop in pressure, (voltage) unless the nozzle is turned on and water is flowing, (current). When resistance is too high, not enough water, ... Ah, ... Current, can get through.
Ok, back to your circuit. Place one meter probe on one battery post. Let's start with the negative one. Put the other probe on the cable clamp right next to the first probe. Put the voltmeter on the lowest scale, then have your helper crank the engine. The highest voltage you want to see is 0.2 volts. Ideally there should be 0 volts, but most circuits aren't ideal. With the symptom you're having, I expect you're going to find at least a couple of volts at the bad spot.
Leave the probe on the battery post. The next step is to move the other probe from the cable clamp to the next accessible point in the circuit. In this case that would be where the negative cable is bolted to the engine block. Since the cable is new and tight, we can jump over multiple points and go right onto the engine itself and bypass the terminal and bolt on the end of the cable. Those are mechanical points where two pieces are joined together. So, one probe on the negative battery post, and one on the engine. Have your helper crank the engine. The maximum acceptable voltage now is 0.4 volts. 0.2 is the maximum for any one connection; 0.4 is the total allowable for the entire negative side of the circuit regardless of how many mechanical connections there are.
Holler back after you take these measurements. I'll start typing already for my next reply so it won't take so long.
Thursday, March 25th, 2010 AT 10:37 PM