In need of guidance from someone who has a very good understanding of electricity

[bernk]

My boat has a marinized Kubota diesel hooked up to a new 12 V cranking battery. Until recently, this battery was starting this engine without issue. Now, when I turn the ignition key to the start position a single “tick” sound can be heard as everything hooked up to the cranking battery through the isolator switch goes dark. After this happens, the voltage reading of the battery is ~12.6 V while the voltage reading at any point after the isolator switch is ~5 V. As an example, I have a USB socket with a voltmeter on it which is wired to the distribution panel, and a handheld multimeter connected directly to the battery. The multimeter will show 12.6 while the voltmeter will initially display around 5 V. Over time the voltage after the isolator switch will slowly creep back up to match the voltage of the battery. This takes more than 15 minutes. Once it gets there, everything connected to the battery through the switch is once again powered and the process can be repeated. I have a limited understanding of electricity and find this very puzzling.

The issue first appeared when I attempted to restart the engine after it cut out when I idled the throttle too quickly. This was after motoring for more than 2 hours. The first time I attempted restarting, it sounded like the starter *very* briefly tried to turn the engine. Like it turned a tiny fraction of a full turn before the power abruptly cut. That one time the sound was more of a metallic thud or a clang. I don’t think the “tick” I’m hearing now is the solenoid, but I’m not absolutely certain. To me, it sounds like an arc pop, which makes me think there is a short somewhere.

Things that I’ve tried:

* Removing the solenoid and powering the starter motor directly from the battery using jumper cables. It works and turns the engine over just fine when I the pinion gear is manually engaged.

* Replacing the solenoid with a brand new one. Same issue persisted.

* Tested the voltage at the B and S terminals of the solenoid. Both showed 12.6 V.

I suspect something is wrong with the starter, but I’m not sure because the starter seems to work fine when connected directly to the battery, i.e., a lead going from the battery's + terminal to the motor strap on the starter, and a lead going from the engine block to the battery's - terminal.

All advice, ideas, suggestions, warnings welcome.

[Statistical]

It sounds like the solenoid is the issue even though you replaced it.

>* Tested the voltage at the B and S terminals of the solenoid. Both showed 12.6 V.

What about the voltage at the engine strap (the output of the solenoid going to the starter motor proper) when the starter button/key is pressed/turned? There is a switch inside the solenoid which only closes once the gears are engaged. If that switch remains open or has too high resistance meaning too low voltage then the starter motor itself won't spin.

[donradcliffe]

What are you calling the isolator switch? Does it feed power to both the distribution panel AND the starter solenoid? If so, either the wires to the switch may have bad connections or the switch itself may be bad. Measure the voltage on both sides of the switch JUST AFTER you have tried to start the engine. An alternative would be to take a jumper cable and jump across the switch.

[Fuss]

loose connection near battery or isolator

[S/V Illusion]

All the symptoms of a high resistance connection even more likely if you haven’t cleaned and maintained the start circuit wiring connections. When was the last time you did that?

[wholybee]

I assume "isolator switch" is a master power on/off switch? Or is it an isolator for 2 batteries?

In either case, there is a high resistance fault of that switch. It could be the switch itself, or a very poor connection of a wire to it. There may be an additional fault somewhere else (like a stuck solenoid) that is presenting a high load causing the voltage to take a long time to return to normal. Or, it might be that the fault at the switch is just incredibly bad. You need to fix the fault at the switch first.

I would test the switch with an ohm meter. It should read zero ohms, but it may be in the kilo or mega ohm range presently.

Can you post a picture of the switch so we know exactly what that part is?

[Reefmagnet]

The best way to locate bad connections in a circuit is to put your multimeter across the terminals of components and measure the voltage drop across them when under load. For example, to test the isolator switch put the leads of the meter across it's battery and load terminals and measure the voltage when attempting to crank the engine. If the switch is bad, you'll get a significant voltage reading (likely more than a couple of volts - and may even feel the terminals heating up). Usually the problem will a high resistance between a terminal and cable lug connection, however it is also possible for a cable or component to be damaged internally so don't hesitate to also measure across these, if suspected, as well



And the best advice I was ever given about troubleshooting a circuit is to start at the middle and figure out which side has the fault, then start in the middle of the fault side, test, and repeat.

[mrcarson]

As said. Jump the switch first and read voltages at both sides on and off. That'll at least tell you which side to start at.
BUT. From an old fart. EVERY yr, if you a sailor, Specially if going to sea. I clean/relube my main terminals and boxes. Starter Battery's Masters, etc. Also spray inside alternators and any moving electricals with Electrical spray. My steel boat in 1985. I've replaced the ring main Complete. .once. Mainly heavy weather sailing gives loose cracking lightweight wiring. Ring Mains the easiest. One heavy line with light feeders/fuses off to different things. Old fashioned and works Main terminals and cables still original.

[deblen]

Classic loose connection -probably in larger cables-start bat. Neg & Pos ckt.
To save time,get the correct size box end wrenches.
Start at the neg bat term.-loosen,clean & re-tighten.Move along neg cable to eng. connection,etc.
Next-start at pos.bat. term & repeat the above loosen,clean,re-tighten until you reach solenoid in+,sol.out+,starter in+.
If you haven't found the loose connection by now,short the 2 big bat+ in & bat+ out to starter terminals on solenoid with a "chunk of metal" such as a big screwdriver. If engine spins normally,the starter is ok.
Problem probably lies in solenoid ckt.
Thumb the starter button/turn key to start. If you get a solid clack from solenoid,but no turnover-it is probably bad contacts in solenoid.Repair or replace solenoid.
If there is no clack,or if there is a quiet click-it is probably poor/low voltage from start button/key/relay ckt. Check all connections in this ckt.
Note: When you turn the key/push the button for start-do the dash/gauge/etc lites dim severely? If yes-this indicates a possible poor/loose connection in the 12V supply to the start button/key.


Best of luck/Len

[MIRELOS]

Voltage is not an indicator of cranking power, how old is the battery?, over 5yr, could be you need a new battery with strong cranking power, diesel engines are harder to crank than gasoline engines

[jim King]

I had a similar problem with my starter and thought it was either the starter motor or the solenoid. My electrician boat buddy analyzed the able from battery to starter was not of sufficient thickness. We made a change to a thicker cable and the problem was solved.

[bernk]

Originally Posted by donradcliffe

What are you calling the isolator switch? Does it feed power to both the distribution panel AND the starter solenoid? If so, either the wires to the switch may have bad connections or the switch itself may be bad. Measure the voltage on both sides of the switch JUST AFTER you have tried to start the engine. An alternative would be to take a jumper cable and jump across the switch.

The isolator switch is one of the red Perko switches that allows me to choose between 1, 2, ALL, and OFF. Currently, I only have the one cranking battery hooked up. The switch does feed power to both the starter solenoid and the distribution panel.

I’m still very confused by the huge temporary voltage difference between the battery terminals and the circuit beyond the switch. What can cause it to suddenly drop like that, and why would it slowly climb back up to match the voltage at the terminals?

[donradcliffe]

Its bad cables, bad connections or a bad switch. I've seen them all. The voltage falls
when you try to start the engine, then will build back up because the bad part will let some current through, but not near enough.

[smac999]

If you have 12.6v at the battery (and input to the switch?) and 5v after the battery switch. (Using the same ground ref). Then the switch is bad.

If you are moving the ground ref point downstream then it could also be in the ground side.

[sailorboy1]

If the voltage is dropping to 5 while trying to start it is more than likely and wire/connection problem. Chased one for a year on e because most of the time the wire made enough of a connect.