Yanmar 3GM30 27HP starting issues

[NorthCoastJoe]

Make sure all your ground connections are clean at both ends of the cables.
Clean up all positive connections, this includes the terminals at the switches.
Consider replacing the switches and the hot feed to switches and also the wire from the switch to the starter. Look at a table for voltage drop for 12 volts and size your wire one size larger than the table states (I do this to make up for future corrosion). Every connection needs to be tight and clean.
Also check your master switch, everything between the battery and the starter ( both sides of the circuit ) is suspect until proven otherwise.

Sounds like corrosion or a bad switch / wire to me. I have a Yanmar 3gm30f on my Pearson when I installed it I ran all new wire and switches.

[Wotname]

Originally Posted by Ecos

I'm sorry. I thought you could explain/show, why there is such a big voltage drop during cranking. Just saying there is a 2-3 volt does not mean anything to some of us. I've checked good working starter systems and the v at the starter is below 10 during cranking.
With a giant house bank it will be different.

Your post raises some good points and highlights the inherent difficulty in responding to electrical questions on the internet. To my mind, the two most difficult aspects are that we have to make assumptions about the boat's hardware and the electrical competence of the OP.

Clearly if one is an electrical professional, they probably aren't asking for advice on CF however for the rest who are asking - their knowledge of Ohms law, troubleshooting, using diagnostic equipment etc can vary from hardly anything to fairly clued up. One can make certain judgements about their electrical competence from the way they describe the fault symptoms but at the end of the day, it is still a guess. So that leaves the helper wondering if the help provided is too simple or too complicated for the OP to be assisted.

Likewise, we don't usually have hard data about say the age, condition, size of the OP starting battery.

Most of the time, people take voltage measurements and these aren't truly diagnostic without knowing the true health of the starting battery - so we make assumptions and these assumptions can lead to an incorrect diagnosis.

Current is king for a starter motor and it's torque is directly proportional to its current draw at any one moment. However as you most likely know, the starter motor RPM is also very significant element in starting performance and current (or voltage) measurements doesn't help us so much. As a point of interest and counter intuitively, lower current draw indicates higher starter motor RPM - assuming the supply source is close to perfect (and mostly it isn't).

The designer decides what current the stater motor should draw and at what RPM this should occur in order to get a good start. We usually don't have this information at our fingertips nor do we have the ability to measure it. So we are left with making decisions based on voltage only.

Here is an example of how voltage measurements can be misleading.

Situation A - The open circuit resting voltage of the start battery is say 12.7 V indicating it is fully charged. It's voltage while cranking drops to say 9.5V and the engine doesn't start after 10 seconds of cranking.

Situation B - The open circuit resting voltage of the start battery is say 12.5 V indicating it is 75% charged. It's voltage while cranking drops to say 9.5V and the engine starts after five seconds of cranking.

Without any other data my best guess would be the battery in A has sulphated plates and needs chucking out - I also think the RPM of the starter motor would be way too low and engine may have lowish compression but nothing is mentioned about these aspects.

My guess that battery B has 4 good cells and one bad cell and will fail soon if it was correctly sized initially but it may be undersized with all good cells and just needs a good charge. I also think the engine probably has good compression and with a good and correctly sized battery, it would start within a second or two. The delayed start (5+ seconds) was due to low starter motor RPM and some battery issue.

One thing we can sure of though, the voltage at the starter motor terminals should be close to the voltage at the battery terminals (when cranking). Any loss here is due to cabling / solenoid losses and should be kept to a minimum.

Again though, an oversized start battery can mask poor cabling while a very good cabling can mask an undersized battery. It is hard to be specific without knowing all of the details .

[Compass790]

Good post Wottie!
Yes you explained the difficulties of online diagnostics very well & educated us electrical newbies simultaneously.
Keep up the good work

[Lemsteraak]

I agree, you don't give us much info but it does sound like a voltage drop. This are very difficult to diagnose as it may show OK under a small load (meter) but have a large resistance under a large load. This video will help, I love this old guy, an unknown YouTube gem, he will show you how to fix things, old school.

https://www.youtube.com/watch?v=T0vRVSBnCj8&t=188s

Yes, my old 3GM30f had this same issue. Try this first ....