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Hard starting - Looking to diagnose electrical system

Sep 24, 2018
1,483
O'Day 25 Chicago
I have an OMC saildrive that sucks a ton of energy out of my batteries. I want to determine if the old 2awg cabling is suffering from corrosion, the starter relay has any resistance and if the 43 year old motor is operating as it should. What is the acceptable resistance I should see for the wiring and relay? Is there a way to determine if my starter is working as it should without taking it apart?
 
May 17, 2004
3,485
Beneteau Oceanis 37 LE Havre de Grace
It will be hard to tell quality of connections based just on resistance with a multimeter. A single strand of copper will show basically no resistance carrying the amount of current the multimeter pushes, but a very large voltage drop under starting current. Better to measure voltage to narrow down where voltage drop is happening under load as sail sfbay suggests.

What exactly do you mean by sucking a ton of energy out of the batteries? What are you measuring to determine that energy is being sucked out?
 
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Likes: Ken Cross
Sep 24, 2018
1,483
O'Day 25 Chicago
I didn't think about it but you guys are absolutely about needing to measure the voltage drops while cranking. I don't have shore power available while I'm away. I can get my hands on one of those big chargers that auto shops use to charge/jump cars. This would give me a lot more time to diagnose but would that affect the system voltage and throw off the voltage drop readings?
 
Feb 26, 2004
21,959
Catalina 34 224 Maple Bay, BC, Canada
but would that affect the system voltage and throw off the voltage drop readings?
Not really. VD is VD. If your house bank is at 12.5V and you read 12.1 at the end of a wire (under load), then it should be the same % if your house bank was 12.2 or 12.7. What you're looking for is a "culprit."
I have an OMC saildrive that sucks a ton of energy out of my batteries.
What exactly do you mean by sucking a ton of energy out of the batteries? What are you measuring to determine that energy is being sucked out?
David asked the same question I was going to. How can your engine be sucking power out of your batteries? What do you mean by this?

 
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Likes: JamesG161
May 17, 2004
3,485
Beneteau Oceanis 37 LE Havre de Grace
With a booster the voltages will be different, but the drop itself should be pretty close. The drop might be a little bit less when the voltage is higher (because amps can be lower), but close enough to diagnose the connections.

Having said that, even if you have lots of drop in a connection it shouldn’t really affect the state of charge of the battery. Starting is a pretty short term event that doesn’t really run a battery down very far, so even if you’re wasting energy as heat in a bad connection you’re still not draining many amp hours. Just don’t want to see you chasing voltage drop across the connections if the issue is really with the battery or the starter.
 
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Likes: JamesG161
Sep 24, 2018
1,483
O'Day 25 Chicago
The starter has a hard time turning over the engine unless it's been freshly charged. It's been like that with multiple batteries. Would the starter tester at an auto parts store tell me any useful info if they can connect it to their machine?
 
Feb 26, 2004
21,959
Catalina 34 224 Maple Bay, BC, Canada
The starter has a hard time turning over the engine unless it's been freshly charged. It's been like that with multiple batteries. Would the starter tester at an auto parts store tell me any useful info if they can connect it to their machine?
How old are the batteries?
You do realize that this info, coming in dribs & drabs, has the possibilities of pulling us all down a rabbit hole?
You need to approach this systematically. Power source, wiring, switches, devices. So far you've asked about wiring, now raise batteries...
:)
 
Jan 11, 2014
7,855
Sabre 362 113 Fair Haven, NY
The amount of power used to start a small diesel is trivial, an amp hour or less. However, that power needs to be there in a split second.

The symptoms you describe, won't start unless the battery is freshly charged and starter having a hard time turning over are symptoms of a poor circuit. Which could mean, poor connections, inadequate wiring, failing battery.

The solution could be as simple as getting rid of wing nuts on the battery terminal (if you still have them) or cleaning and tightening connections to as complex as rewiring the entire starter circuit.

How old is the battery? What kind of battery is it? How is it charged?

How is the battery cable connected to the starter? Are there additional connections on the starter, alternator lead, solenoid lead, etc? Make sure the battery cable is at the bottom connection stack so it is in direct contact with the starter.

FWIW my 1 year old start/reserve battery is exhibiting the same behavior. It won't start the motor unless it is fresh off a charging source.
 
Oct 24, 2010
2,399
Hunter 30 Everett, WA
I would start by alligator clipping meter leads across the starter itself. And I do not mean the starter solenoid. Monitor the voltage while the engine is cranking. If it's below about 10 volts I would then back up a little and connect it somewhere else and see what the voltage is there. Eventually you will find where you have probably multiple points of voltage drop. Most likely it will be connection issues or possibly a crimp terminal on a wire itself. I certainly wouldn't rule out a bit of corrosion at every connection. If you put a meter on the battery itself and crank you will probably notice that the voltage doesn't drop very far. If it does, yard that battery out and have a load tested. Or at least use a hydrometer so you can see the chemical status of the electrolyte. You can buy hydrometers relatively inexpensively at any auto parts place. Always do your checking with a fully charged battery. Hopefully this gives you some things to chew on.

Ken
 
Sep 24, 2018
1,483
O'Day 25 Chicago
I don't remember the exact capacity of the battery at the beginning of the year but it did pass. I pulled it to charge a few days ago so I'll try to get it over to an auto parts store for testing before re-installing it