Starter motor and wiring

[Airscrew]

Hi to all, and especially those of you with thorough knowledge of starter electrics.

In the autumn, our engine was gradually taking more seconds to start.
Audibly, it appeared that the engine was cranking more slowly prior to starting.
Suspecting the starter motor or wiring, today I took down a brand new, fully charged 540CCA battery destined for one of our 'student' cars.

As usual in the spring, I checked oil (ok), turned over by hand (all ok and compressions as usual), 'opened' the 2 decompression levers, and was expecting to spin the engine without starting.

The result was dramatically different to last november: very sluggish and laboured turnover, perhaps 1 rotation per second.
I immediately abandoned, and I suspect either the wiring, or the starter motor.

How do I check for defective wiring and defective starter motor ,and how do I test and differentiate between the two.
A detailed description would be most helpful.
Note that I will be single handed and mid-river.

Many thanks.

 

[earlybird]

The earth connection between the battery -ve and the engine is often an out-of sight and neglected area. Worth checking as a first, low cost precaution.

 

[Lon nan Gruagach]

Feel for something getting hot, very hot so feel carefully.

 

[superheat6k]

The vast majority of electrical issues like the one described are due to a poor connection somewhere. Start at the battery +VE then follow the cable around the entire circuit stripping and cleaning every assembled joint, and closely inspect every crimped or soldered joint for corrosion or looseness, right back to the -VE at the battery.

This could also be a poor contact on the motors commutator.

 

[William_H]

You can after checking for hot spots as above use a volt meter to check for voltage loss. Best with clips on the meter leads. Connect one lead to negative battery post and one lead to starter motor body. Try cranking should be no discernible voltage or less than .25 volt. Now fit meter lead to battery positive post and the other lead to the bolt on the starter motor with the heavy wire on it. Again voltage measured while cranking should be very low less than .5 volt.
Finally measure the voltage pos to bolt on starter motor to negative battery. This should measure 12+ volts but when you start cranking might drop to 9 volts. (normally) If voltage does not drop then starter motor and attached solenoid should be removed for service. Probably bad commutator. You could try cleaning it and check brush length. As it has become worse from a period of inactivity it is likely to be corrosion on copper segments of commutator.
If voltage drops dramatically down to 6 or less when cranking or you get a huge voltage drop from second test (pos of battery to starter terminal) you might have a bad battery isolation switch or bad wiring/connections. ol'will

 

[Airscrew]

Thanks William, thats a very helpful explanation.
I will get some long leads and croc clips.

I also found some useful diagrams at
http://www.tb-training.co.uk/MarineE06.html#STARTERS & CIRCUITS
from Tony Brooks.

I will use your advice and his diagrams, and will work my way through the connections and wiring, probably starting with the engine block earth wire.