Recently, I learned a lot about diesel starters. Thankfully, I didn't share in the financial aspect of the repairs. I suffered along with the frustration and may be able to save someone else a similar experience.
I will explain all of this in very simple terms so that anyone with no electrical or diesel engine experience can follow the discussion. A few steps in the process have been skipped or combined for the sake of clarity.
In mid November I met Bob O'Brien and his C&C Landfall 48 in Oxford, Md. Bob was waiting for a new starter that was due to arrive the next day. It seems that the old starter burned out when it failed to turn off and could not be heard over the sound of the engine.
A starter has two main parts. A high torque motor which engages to a flywheel to turn the engine to produce compression to start the engine and a solenoid to switch the motor power on and off. The motor is designed to produce a very high torque for a short time and will overheat and fail if run too long. A solenoid is an electrically operated switch. It consists of two main parts, a coil of wire that requires a relatively low current to create a magnetic field to move high current contacts and the contacts that apply power to the motor. Solenoids are also used in many other kinds of applications and they all have the same basic function of controlling a large load with a smaller control current.
When the new starter was installed and the ignition switch was turned, there was a click but no engine cranking. We thought that we had received a bad starter. The click was obviously the solenoid operating, but the motor did not turn. We checked all of the wiring to the engine control panel and ignition switch and found no problems. We took a piece of wire to bypass the ignition switch and the motor ran, but then it wouldn't shut off. Bob's mechanic contacted his supplier and told them that they needed to get him a different starter.
A couple days later the second starter arrived and acted exactly the same. The local supplier said that he had bench tested the first starter and could find nothing wrong with it. He recommended that we check our battery voltage, the voltage at the starter motor, and at the starter solenoid. We found that the battery voltage was dropping when we tried to start the engine. We took it in and found that the battery was bad. That was replaced, but we still had the same problem.
We were still measuring a drop in voltage at the starter and the solenoid. The voltage at the battery was good. This loss of voltage in the cables is due to change in the resistance of the wire and connectors as they age and corrode or become loose. The next step we took was to install new cables from the battery to the battery switch and battery switch to the starter. Now the starter motor would run sometimes and sometimes it wouldn't. It would also run and not shut off some times.
The starter supplier explained that if the solenoid voltage is low due to a bad switch or wire, the contacts do not come together properly and that can create a hot spot and electrical arcs that can weld the high power contacts together. So we installed a new ignition switch and we were still having problems.
After all of the failed attempts at making this repair, Dwayne Pasco, owner of the supply store said that he would come to the boat with a stater and work through the problem with us. When Dwayne arrived, he showed us a set of damaged contacts from the first starter we tried. Even though the solenoid is the lower current part of the starter, it can still require a significant amount of current. The cranking battery also has to be large enough to maintain voltage when the high demand of the motor is applied.
To ensure that we would have a large enough starting battery, we added a second one to the start circuit and to ensure that we didn't loose any voltage in the ignition wiring, an auxiliary start relay was added at the starter. Like the solenoid, a relay uses a low current circuit to switch a higher current circuit. However, the relay has a very low control current. So the ignition switch operates the relay which then switches the power to the solenoid right at the starter. This resolved all of the problems and the starter operated correctly each time.
There was a lot more to the story. Much of that was wrong theories we had and wrong roads we traveled. I omitted those because I don't recommend that you repeat our mistakes.
The biggest lesson learned is that if you only hear a click when you try to start your engine and then it does start after a couple tries, you are on the way to a failed starter.
I recommend the following:
1. Check the voltage at the battery posts while the starter motor is running.
Replace the battery if the voltage drops more than a few tenths of a volt.
2. Check the voltage at the starter motor when it is running. If it is dropping much below the battery voltage, replace the cables.
3. Check for any losses on the ground side of starter. Measure from the negative post of the battery to the ground side of the starter. Use an extension test wire for your meter, if required. A light wire can be used for the meter. There will be only micro amps of current in the meter circuit. Replace the ground cables if required.
4. Measure the voltage at the solenoid. If it is below 11 volts, replace the ignition
switch and wiring or better still, install an auxiliary relay.
My opinion is that you can't go wrong with installing an auxiliary relay. That will give you the lowest voltage loss at the solenoid.