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High Oil Pressure for a 340 - What's Your Problem?

Stay Grounded
Mike Tafaro,
Douglassville, PA: I'm building a street/strip Nova for my brother while he's away making license plates, and I can't get the engine to start all the time. Usually, she just gives me the starter click and won't turn over. The motor is an SBC 383, around 10:1 compression. The starter is a Powermaster Ultra Torque, has been bench-tested, and works as it should. The battery is a brand-new Diehard Gold rated at 795 cold cranking amps (CCA) and is fully charged. I have a kill switch mounted to the back of the car and a junction block under the hood to provide power for the fuel pump, ignition, fan, and a small air compressor for the rear shocks. The battery ground goes to the body near the passenger side rear wheelwell. The battery positive connects to the switch, and the hot side of the switch runs to the starter and the junction block. The alternator is wired directly to battery hot, and I have one strap from the engine block to ground and another from the fender to the subframe of the car. Everything works: the lights, compressor, pump, and so on-but the car just won't start. Electrical has always been my weak spot, and any help would be appreciated.

Jeff Smith: The good news is this is an easy fix, Mike. I had the same problem with a '55 Chevy that I built way too many years ago. The problem is a weak ground circuit. You cannot rely on the body of the car to act as a suitable ground for the starter circuit. What makes this problem a head scratcher is that all the other circuits in the car-like the electric fuel pump, lights, and small compressor-appear to operate normally. This is because these components require far less current to operate-the most might be the electric fan at around 20 amps. According to Kevin Bennett at Powermaster, the instantaneous current flow before the starter begins to turn can be as high as 800 amps. Once the starter begins to spin, the current demand stabilizes between 200 and 250 amps. As you can see, the instantaneous current demand is so high that without a decent ground, the resistance is excessive and the starter just clicks. Also, Bennett says, "Keep in mind that as the input voltage to the starter drops, the amperage draw goes up. (Yes it's counterintuitive, but it works that way.) Therefore, bad cables and/or weak batteries create a situation in which the starter will pull more amperage."

To diagnose this problem, round up a digital voltmeter. First, let's test something easy like the electric fan. Turn on the fan and with it running, read the voltage at the battery. Next, read the voltage at the fan again with the fan running. You will record a significant difference in voltage at the fan compared with the reading at the battery. Electrical resistance is present in any circuit, and the voltage at the load (the fan) will decrease with resistance. For example, a 0.60-volt-or-more loss directly at the fan compared with the reading at the battery indicates excess resistance in the circuit; voltage was lost somewhere in the circuit from the battery to the load and then back to the battery on the ground side. If the original power cable is large enough, then the voltage drop is probably located in the ground circuit, since resistance can occur on either or both the power and/or ground sides of the circuit.

Next, let's isolate the circuits. Install a dedicated, fullsize ground cable that's the same size as the positive battery cable. Connect this ground to the engine and battery with another cable over to a common ground that will include the electric fan. With an improved ground circuit, the voltage at the fan should increase. If the voltage difference between the fan and the battery is now 0.40 volt or less, the circuit is close to ideal. Keep in mind that resistance increases with the length of the wire. This is why a larger cable is required when distance between the battery and the load increases. With a dedicated ground cable now in place, the starter motor will crank over just fine.

Now that the starter motor works, if your digital voltmeter has long enough leads, you could test for a voltage drop over the entire length of the ground cable. This can only be done while the engine is cranking. Place the positive lead on the case of the starter motor and the other on the negative terminal on the battery. While the engine is cranking (disable the ignition to allow enough time to read the meter), you should read no more than 0.50 to perhaps 0.60 volt. This is a great test for the internal resistance present in the battery cables in any car. Both cables can be tested this same way. Even brand-new, inexpensive cables can have enough internal resistance to cause a problem. A friend of mine once purchased a brand-new battery, cables, and starter for his small-block Camaro but experienced hard starting issues when the engine was warm. I checked the voltage drop on the cables and discovered numbers in excess of 1 volt. We installed new 1/0 welding cables and the engine cranked over like it had a 24-volt battery. The voltage drop number plummeted to roughly 0.38 volt.

More Info
Powermaster Electrical;
Systems West Chicago, IL; 630/957-4019;

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